Talk:Tax horsepower

The article Packard Model 30 (Series U) states that the car had 30 hp (N.A.C.C.) Does this mean National Autocycle and Cyclemotor Club and is this the US equivalent of Britain's RAC? How was NACC horsepower defined? Biscuittin (talk) 12:03, 19 November 2012 (UTC)

I can't help much; I first encountered the NACC horsepower while writing about a Cunningham I photographed: The Volney-Lacey designed 442 cu in (7.2 L) continued, with 45 hp according to the outdated NACC measuring method - for 1923, 90 bhp were stated.^[1]  Mr.choppers | ✎  07:02, 20 November 2012 (UTC)

NACC horsepower was named for the National Automobile Chamber of Commerce, which later became the Automobile Manufacturers Association. It was indeed the U.S. equivalent of the British RAC horsepower, and it used exactly the same formula. It was originally referred to as ALAM horsepower (for the Association of Licensed Automobile Manufacturers, a predecessor of the NACC), and was sometimes referred to as SAE horsepower. (This should not be confused with more modern meanings of SAE horsepower, based on actual power measurements.) I've seen it frequently quoted in old automobile publications that you can find through Google Books, Hathitrust, or the Internet Archive. Some later writers not familiar with this figure's meaning (example: John Gunnell in his Standard Catalog of Chevrolet) have misinterpreted it as SAE net horsepower, even though it is something completely different. --Colin Douglas Howell (talk) 10:54, 10 March 2016 (UTC)

The Italian interlanguage link goes to "potenza fiscale", but this article talks about tax horsepower as essentially historic ("Although tax horsepower was computed on a similar basis in several other European countries during the two or three decades before the Second World War"). Potenza fiscale was apparently introduced in 1976. Should the link be removed, or the English article altered? — Preceding unsigned comment added by 93.58.18.114 (talk) 12:20, 15 September 2015 (UTC)

Anyone happen to know how this is executed? Seeing as it's apparently different from the others, but no more detail is given? 193.63.174.115 (talk) 15:28, 1 July 2016 (UTC)

I think this is somewhat misjudged. There's no reason an undersquare(?) engine with a long stroke and an emphasis on torque should be any less reliable when motorway cruising than an oversquare(?) one that focusses more on top end power. In fact, in the modern age it seems the longer stroke and lower rpm setup is becoming more popular again as it's easier to make an efficient yet tractable motor using that design.

More likely, it's the state of the roads and general culture in the country of manufacture, effect of taxation based on power/capacity as a whole in an age where specific output was quite poor, and gearbox technology that had more of an effect. Let's look at cars from the three mentioned countries from around the same time, IE when the Beetle was becoming popular in the USA.

First up, the Beetle is maybe an unfair reference point in the first place. It was designed and first built under a dictatorship where it was to be one of the only available cars, doled out in waiting-list fashion via a social savings scheme. If any tax was levied at all, then it would have been effectively a flat rate as the government had a fairly big say not only on taxation but vehicle design and manufacture also. The car was made pretty much as a device for travelling along the great propaganda tool that was the Autobahn, and so was provided with an engine of slightly more than the requisite power for cruising at the promised speed of 100km/h on the flat (25hp, from an 1100cc unit, vs about 21hp needed to overcome drag), to give it some measure of gradeability and also the guts to cope with a transaxle incorporating an inherently "overdrive" top gear (IE, it can't quite rev out to maximum power, let alone the redline, in top, and cruised at something like 3300rpm vs its 3600rpm peak power; the internal ratios and final drive ended up being kept for many future models as they proved to still be practical decades later, even when mated to smaller wheels, whereas a lot of other old cars now seem disasterously under-geared). As the Nazis weren't having to pay market rate for either materials or brainpower, and the car was being built as a public relations weapon, they could afford to equip it rather better than most other contemporary economy cars, too - for example, fitting a 4-speed gearbox where most others only had 3 (and some just 2), meaning that it was possible to have a comfortable "high speed" cruising gear as well as good hill-start ability in first, without the gaps between each gear being too large for easy driving, and stealing a general layout and rather aerodynamic body shape from an existing motor manufacturer in occupied Czechoslovakia which reduced the amount of power needed for fast cruising, so the engine didn't have to work as hard either (plenty of other old cars which struggle to beat 65mph with 30+ horsepower just because they're shaped like a brick, and an over-large one at that because of poor packaging). Not to mention the fan-forced air cooling setup instead of the more common water cooling, which did away with boiling radiators and was somewhat self regulating in that when the engine started to get too hot, it wouldn't run quite as well, and so rev slower and make less power. The final puzzle piece being the flat-four layout which, much like the 2CV, balanced out some of the forces on the crankshaft and made the engine as a whole run rather smoother, greatly reducing bearing wear even at continual high speed. Germany is also a more spread out country that saw a lot more ancient influence from the Romans, so, like France, there were a lot more good quality, fairly straight, not-too-steep roads, even before the building of the autobahns, so problems with high speed running became apparent much more readily.

In the USA, there was no such thing as taxation based on engine size or power, and during the development of early motoring there wasn't really anything much resembling decent roadways in a lot of states, nor any particular requirement to register vehicles at all. Most long distance travel and freight went by rail. Cars, at least in the Model T era, were built as utility vehicles rather than speedsters, and were either rather luxurious or ultra-basic, but not particularly fast (the quickest 4-cylinder Model A was about as rapid as a Beetle; the T itself tapped out around 40-45mph) and, with no artificial limits on capacity and there not being too much of a difference in how much fuel it used - or fuel being at all expensive, or used at a fantastic rate given the speeds available and how far people generally travelled - equipped with comparitively large, torquey, but lazy-revving engines, all the better to slowly but surely grind their way up a muddy, rutted back lane whilst carrying a load of supplies back from the nearest general store 20 miles away. So was born the pattern of the American car, even through the rapid development that then happened in terms of national routes, the nouveau-turnpike and then interstate system, and of course, even bigger and more powerful engines, particularly the seminal Ford V8, which suddenly put enough BHP to hit 100mph within the reach of basically everybody with a mind towards buying a car. The power meant that big speed was possible (though still hampered, in the main, by bad aerodynamics), and although the gearboxes were often rudimentary 3-speed manual or 2/3 speed epicyclic semi- and full-automatics, the sheer amount of torque available (and the ability of their 6- and 8-cylinder engines to rev super low without stalling or shaking) meant they could be geared high and wide without the increasingly large and heavy cars they sat in being difficult to launch or bogging down too much when changing gear whilst going uphill. Plus what decent roads there were in the USA, both regular and motorway style, were usually reasonably wide, straight, and not too steep except where a mountain climb was unavoidable, as they were ploughing through mostly virgin territory, with no existing townsteads, patchworks of farmed fields, old historic routes and the like getting in the way or environmental groups complaining about the damage being done to the pristine environment. And, like the Germans, they got an early start on the motorway game, and boy did they have some distances to cover. So not only were they cognisant of the benefits of not overloading the engine if you want it to survive a long, high speed trip (and although it was more comfortable and prudent to keep to 60-75 most of the time simply because the tyres of the time weren't good for much more, this was an age where out-of-town speed limits were rare, so you COULD blaze cross-country at 100mph for hours on end if you really wanted to), but they had the ability to engineer them to suit without making them unattractive to the target market either through being overly expensive to manufacture (cheap fuel meant they didn't have to finesse the design TOO much for economy, and knocking out thousands of very similar units day after day also kept costs down, plus there was no purchase tax based on CO2 or whatever), or expensive to keep and run (either no registration fee, or one based purely on vehicle class, and again single-figure MPG wasn't really an issue when the country had its own extensive on-land oilfields).

Poor little old Britain, on the other hand ... cramped, full of market towns, hills, a quilt of fields with zig-zag roads between them, and generally not the largest distances to cover, with speed being neither needed, regularly possible, or, a lot of the time, desirable (bad brakes, bad tyres, bad suspension, top-heavy bodies, etc)... and, until relatively late vs almost every other country in the world, heavily legislated against, to the point that we were well into the 20th century before first a blanket 12mph, then 20mph limit was lifted from private cars and motorcycles (both of those remaining in one form or another for e.g. tanker trucks and certain types of bus for quite some time). Space was at a premium in basically all things, materials were expensive, fuel was expensive, and particularly immediately post-war, both were difficult to get. People weren't quite as affluent as in the USA, and nor did we have a centrally planned economy at any point (granted, Germany became democratic after WW2, but the Beetle was a product OF the wartime government), so there was less money to spend on cars anyway. And, of course, we had the FHP based registration tax and, I wouldn't be at all surprised to find out, probably an equivalent that applied at point of purchase too. The roads were in a rough state because the improvements being planned and embarked upon in the 30s were abandoned at the outbreak of war and the money remaining in the late 40s had to go simply on rebuilding and patching at first, hence it was mid-late 50s before we started making our own motor-roads with any conviction. But the traffic volumes still inexorably rose... So, economy was the order of the day - small, low powered engines (the Austin 7 originally did only make that much, getting up to maybe 35, and by the end of its run was maybe up to 13hp and able to reach the heady speed of 50mph), generally under 1 litre but still 4-cylinder inline because that was an easy way to increase capacity in a narrow-bore engine without making it impractically tall, and also spread out the combustion forces somewhat. Small bodies meant that low power wasn't too much of a handicap for actually *getting* places, as a small car with a certain amount of power will climb hills about as well as something twice the weight with twice the power (if not better, because it will be necessarily lower geared), but it did hamper top speed because drag quickly became a limiting factor. Three-speed manual was the almost universal order of business (or 2-speed in some very basic models), and wide but low in order to give best starting performance plus optimal, slightly underdriven top speed (so the frequent hills wouldn't necessitate constant downshifting, a bit of a problem if you don't have synchromesh), but even so you wouldn't be able to take it to the redline for very long even if pushing hard, because there'd always be a hill, or a bend, or a junction, or a village with narrow streets, or traffic, all conspiring to slow you down again. It was also the age of the specified "safe cruising speed", which you were advised to not routinely exceed on stretches which actually were straight, open, and lightly trafficked (we do have SOME roman roads...). However, the coming of the motorways kind of showed the weakness in that setup, and would have done so with a small, low powered, cheaply made, undergeared and generally overwrought engine of any bore:stroke ratio, with marginal cooling capacity and lubrication system quality, running with poor quality single-grade oil. Suddenly, many miles of uninterrupted tarmac, with plenty of passing space, gentle bends, sympathetic gradients, no speed limits, and a general lack of the usual in-your-face reference points that gave you an idea of just how fast you were going. So you head out from Lutterworth to take your Seven out on the M1 to whiz down to your aunt in St Albans who you haven't seen for ages because of the cost of rail travel and the difficulty of the drive, get into third, put your foot all the way to the floor as per usual to make the most of all thirteen horses, and fail to pay proper attention to the safe cruising speed or indeed either the speedometer or the engine noise at all. Before too long the water (probably not "coolant" at that point) in the radiator is nearing boiling point, the oil has thinned out to a dangerous degree, the carburettor may well be struggling to provide a suitably rich fuel:air mixture (increasing cylinder temperatures even further), and the little engine which might have been good for a sustained speed of 4500rpm, reached its maximum output at 5000, and was already doing 5500 at normal maximum road speed, has had enough of maintaining revs in the low 5s and occasionally exceeding 6000 downhill, and one of the cheap pot-metal parts lunches itself in catastrophic fashion... either a bearing fails, and/or the crankshaft cracks or warps, or a conrod goes wrong, or a pushrod bends or escapes, or a piston or piston ring burns through, or it straight up seizes. It's not particularly a fault of excessive force due to the bore:stroke ratio (after all, each piston is likely not putting any more force on the failed parts than its equivalent in the larger Beetle engine or much larger Ford one), it's a combination of things, most particularly the need to maintain absolute maximum engine power (and thus keep the rpm very close to the allowable maximum, from where it's easy to end up exceeding it) to maintain a reasonable rate of progress (50+ mph), particularly with the relatively high drag coefficient, the higher rpm at which this was achieved due to the small engine size and the assumption that this would only be required on a transient basis (pulling away, climbing hills...) and that normal cruising speed would be much lower (which also factored into the marginal cooling and lubrication), and the low gearing employed to make the best of the engine's characteristics. Even in the 60s, Morris made much of how the Minor could still pull from 5mph in top gear, something which you wouldn't much want to try in any gear that's also suitable for motorway running in a modern car. All in all, it wasn't the RAC rating system's bias towards narrow bores that was the problem, it was that the taxation structure attached to that heavily incentivised making cars with engines better suited to use in light motorcycles and heavily penalising anything with what we'd nowadays recognise as a "comfortable" amount of power, and that people likely couldn't have afforded to buy or run anything that much bigger or more capable anyway. Even in the age of the Ford Anglia, absolute Vmax was about 65mph, and it was far more sensible to stick to about 55 instead for fuel economy, comfort and engine wear... and general safety, thanks to the rubbish, frighteningly narrow crossply-bias innertube-based tyres and drum brakes that more resembled bottletops than something designed to bring a three-quarter-ton lump of metal to a halt and lack of seatbelts or other crash amelioration measures. Although we were pretty good at luxury models and expensive sporty cars, everyday British runabouts simply weren't built for speed, in any way that you could measure. If they had widebore engines instead of narrow, but everything else remained the same, the situation would only improve by a narrow margin; the biggest difference would probably be that we would have had, like the Italians, a lot more shaky, putt-putt two-cylinder engine designs - which were, despite their smaller capacity and inverted B:S ratio, generally lower revving than their UK counterparts and still more focussed on torque than absolute speed, because a small change there could have a noticeable effect on performance in mountainous areas, but a relatively large change in maximum power would only have a minor effect, on a top speed that might not be often reached anyway... (and, again, as they had some REAL mountains to climb, they bit the bullet and fitted 4-speed gearboxes which allowed you to grind uphill at 10mph in first, but happily blast down the other side at 60 in top).

(If you've read this far, give yourself an internets medal; this point seemed very simple and well formed in my head but remarkably difficult and long winded to explain in full) 193.63.174.115 (talk) 16:47, 1 July 2016 (UTC)

Good stuff: well done. 86.148.153.168 (talk) 22:15, 25 November 2016 (UTC)

On talk pages there's room for individual's theories! Eddaido (talk) 01:28, 27 November 2016 (UTC)

One skilled in the art of adding references could use this

http://www.britishtaxdiscs.co.uk/tax-disc-history.php

to add to the article the fact that the rate was £1 per horsepower (so running a Model T would indeed have been expensive). — Preceding unsigned comment added by 86.148.153.168 (talk) 20:16, 7 December 2016 (UTC)

An old source states that the RAC hp was devised in 1910. However, the June 1909 issue of MotorBoating refers to the RAC formula and its shortcomings, as well as its underlying assumption of piston speeds - all of which means that RAC hp must have been around for a while longer. Best,  Mr.choppers | ✎  03:32, 17 December 2020 (UTC)

Yes, this article contains quite a bit of unreferenced material some of which is complete nonsense including the "formula" for calculating taxes as well as its origins. I'm sorry I am unable to contribute further for another day or two. Eddaido (talk) 12:56, 18 December 2020 (UTC)

I think the confusion over the date of introduction in the UK is because the British government adopted the system in 1910 into legislation. Quite when the RAC devised the formula remains unclear, I can't find a source at the moment, but it was certainly in use in 1904 when Rolls-Royce introduced their 10hp car. Earlier than that Belsize had a 12hp in 1901 and the 100mm bore 2-cylinder engine comes out at 12.8 RAC hp which seems to fit the bill. However, trying to tie up Edwardian car names, which were usually numbers, with either RAC or actual hp seems to be impossible. As Mr Choppers rightly says elsewhere the name/number was something largely dreamt up for marketing reasons so could mean anything. Malcolma (talk) 11:32, 28 December 2020 (UTC)

Slightly off topic but here is a note of the introduction of the same formula and for tax in both New Jersey and New York. The Motor World, New York, January 25, 1906 p 1015 . It remains in force in Missouri This value is still used for taxation and license fee purposes in the State of Missouri for passenger vehicles, with electric vehicles assigned to the 12-23 Horsepower bracket by statute.^[1][2]

Eddaido (talk) 21:06, 28 December 2020 (UTC)

Whenever the formula was created, HM Government appears to have adopted it in 1910 to penalise the large-bore, short-stroke Model T Ford, the principal predatory foreign competitor and possibly the only car whose tax horsepower (22, requiring an annual £22 payment at that time, which would now be worth more than 100 times that with inflation) was the same as its actual mechanical horsepower. In the 1920s, a 'humble' Model T still incurred about 50% more road tax than a far more powerful Bentley 3-Litre, with similar displacement but smaller bore and longer stroke (and four valves per cylinder, twin-spark ignition and dry-sump lubrication), which would stiff you for only about £15. It is somehow surprising that the Model T had a big 3-litre engine, but it did. It just wasn't a very efficient 3-litre, more of a smelly gloppeta-gloppeta gadget that enabled you to run a donkey cart with no actual donkey. Arguments about the demerits of British tax-enforced small-bore long-stroke engines appear a little inconclusive, and the claims made to that effect in the article are a bit iffy. If such engines were in fact unreliable over long distances at high speed, Bentley wouldn't have kept winning the Le Mans 24 Hours. Khamba Tendal (talk) 18:37, 13 August 2024 (UTC)

Could someone show a sample calculation for the 2CV? I've tried it myself but I cannot get the formula to yield a reasonable figure. (not signed)

Quoting from the article:

It was originally defined using the following formula:

${\displaystyle CV=n\times D^{2}\times L\times \omega \times K}$

According to this page the first engine's dimensions were 6.2 x 6.2 centimetres

So we have: 2 cylinders x 38.44 x 6.2 x engine speed in rpm x 0.000117

I need an explanation of the engine speed. Where would I find that? Eddaido (talk) 10:48, 21 September 2021 (UTC)

I believe the existing image and caption are not fit for purpose. Currently, the caption reads as follows

1912 Wolseley 16-20 see also 1913 Vauxhall 30-98 and many others

The actual image shows one vehicle (the Wolseley), mentions another, and then refers vaguely to 'others' - surely not the function of an image caption. Furthermore, there is no article for the Wolseley 16-20, and hence no explanation of what the numbers mean. In the case of the Vauxhall 30-98, even the specific article dedicated to that type is undecided as to what exactly the numbers refer to.

Presently I will try to find one, or more, better examples, and see if I can construct a more helpful caption.

• The Wolseley 14/60 from 1938 seems a better candidate, rated as 14hp for tax purposes, but offering 60hp on-the-road.
• The Lagonda 14/60 from 1925 offers the same numbers
• The Vauxhall 20/60 from 1928 specifies that the Tax Rating was calculated as "19.82 hp", which rounds off (up) nicely to 20.
• Possibly best of all is the Vauxhall 23/60, complete with an inline citation from The Times, No. 43146. 26 September 1922. p. 16. stating that the tax rating for this vehicle was 22.4 hp. The tax-man always rounds UP of course!

WendlingCrusader (talk) 00:20, 21 February 2026 (UTC)

Agreed. Any of those proposed images are good.  Stepho  talk  01:12, 21 February 2026 (UTC)

Your change is good. I made a small change of @ to at, which is easier for those who speak English as a second language.  Stepho  talk  01:52, 22 February 2026 (UTC)

Thank you - your feedback (and further improvement) is much appreciated. WendlingCrusader (talk) 10:54, 22 February 2026 (UTC)