Simanaitis Says

On cars, old, new and future; science & technology; vintage airplanes, computer flight simulation of them; Sherlockiana; our English language; travel; and other stuff


HORSEPOWER IS motorized fun—so, governments say, why not tax it? Indeed, the idea of extracting cash from motorists is as old as the automobile. Some of the techniques of doing this have been downright bizarre; automaker and owner responses have been equally so.


Two of my favorites are the Brit’s RAC Horsepower dating from 1921, and the current situation in the home of the exotic car, Italy. Missouri is included in the group, just to show that Americans don’t miss out on the fun.


An original RAC badge. Image from

In 1910, His Majesty’s Government gave the Royal Automobile Club the responsibility of devising a means of soaking that portion of the rich who were driving “motors,” as cars were called back then. The actual RAC Horsepower tax didn’t go into effect until 1921.

RAC Horsepower was defined as (b2 x n)/2.5, where b is the bore of the engine, i.e., the diameter of its pistons, measured in inches; and n is the number of its cylinders.

For example, the Model T Ford, its British production starting in 1911, was rated at 22.5 RAC Horsepower based on its four cylinders, each of which had a 3 3/4-in. bore.


Ford Model Ts were built in England from 1911 into 1927. Image from

Kudos all around to the RAC fellows, because the Model T’s 177-cu.-in. (2.9-liter) engine actually produced 20-22 horsepower, as measured by technology of the day. By the way, the engine’s 4-in. stroke gave it a slightly “undersquare” bore/stroke ratio of 3.75/4.00 = 0.94, not atypical of the era.

However, it didn’t take long for automotive engineers, mindful of stroke’s omission in the RAC formula, to design engines with ultra-long stokes. Thus, engine displacement could be large without invoking a high tax.

Small bores and long strokes minimized taxation, but they also compromised engine performance. Small bores limited the available room for intake and exhaust valves. Long strokes increased frictional losses and favored lower revs. On the other hand, such engines displayed plentiful low-end torque.

Often the RAC rating and actual horsepower were used in car names.


A 1927 Talbot 14/45 open tourer, “a lively, amusing and utterly reliable motor-car.” Image and quote from Classic Cars in Profile, Volume 2, Anthony Harding general editor, Doubleday & Company, 1969. Both and list it.

For example, the 1927 Talbot 14/45 was powered by a 1.7-liter inline-six with a 61-mm bore, 95-mm stroke and hence an ultra-undersquare 0.64 bore/stroke ratio. Its RAC rating worked out to 13.9; actual output was 41-48 hp, hence the 14/45 moniker.

The RAC tax remained until 1948, after which a flat tax was applied. Long strokes survived even longer.


2000 BMC Mini. Its 1275-cc inline-four had a bore/stroke ratio of 0.87, still undersquare, though not extreme.

The BMC A-Series engine, powering everything from the 1951 Austin A30 to the 2000 Austin Mini, had an initial bore/stroke ratio of 0.76.

Several years ago, during one of Italy’s financial crises, the government decided to go after horsepower Big Time. Atop the bollo, an annual tax, came the superbollo, yet another annual hit of €20 per kilowatt for anything in excess of 185 kW.

Put into good ol’ ’Merican, this works out to $20.40/hp for anything exceeding 248 hp.


A 2010 Chevrolet Camaro SS in Italy would not be immune to its superbollo.

One’s 2010 Chevrolet Camaro SS? Its 426 hp would invoke a superbollo of $3631.20 each year for the first five annual hits. It drops with age, €12/excess kW for cars between five and ten years old; not disappearing until the car is more than 20 years old.

Apparently the superbollo has been encouraging exotic car owners to register their machines in Switzerland.

My Internet prowess revealed that a Missouri car’s annual registration fee depends on “taxable horsepower.” See for an overview. No explicit formula is offered, but shows a bunch of recent cars and their taxable horsepower.


Our hypothetical 2010 Camaro SS has a Missouri taxable hp of 53 and a 1-year fee of $39.25 plus $3.50 for processing. The least powerful cars, less than 12 tax hp, start at $18.25; the 10-tax-hp Smart ForTwo is an example. Cars beyond 72 tax hp pay $51.25. Nothing on the Taxable Horsepower Chart is this high; a 2002 Mercedes-Benz SL600 tops the chart at 59 tax hp.

Unlike the Italians, it seems hardly worth the bother for people in K.C. to maintain a residence across the river in Kansas. ds

© Dennis Simanaitis,, 2013

6 comments on “TAXABLE HP

  1. carmacarcounselor
    October 25, 2013

    I wonder what the purpose is of Missouri’s basing vehicle tax on horsepower, and then not telling how it is determined. Ignoring for the moment that laws are seldom logical, it would seem that if it was to encourage an outcome, say reduced oil consumption, or discouraging people from driving powerful cars, they would provide the means for automakers to design a car to the standard, and thus make progress toward their goal. I expect that somewhere, buried in the legislative record there is a formula. Alas, the internet has made me lazy, and if it’s not in Wikipedia, I probably won’t bother digging deeper.

  2. kenny
    January 16, 2014

    ONe benefit I see is “choices”. You can get ultra high gas mileage, lighter weight vehicles in Europe. Which, of course, helps with the $7+ more a gallon cost of gas. Wish we would subsidize less at the pump/let folks just pay the $8 a gallon, tax high HP cars, and encourage more small displacement cars. Like Diesel VW Lupos that will receive 75 MPG on UP.

  3. Onaroll
    May 17, 2014

    Sorry to dislike your suggestion, Kenny. Everything you buy and use has motorfuel baked into its cost. So you like the idea of poverty, perennial economic stagnation, rampant dissolution of our industrial base and commerce? So does our White House, unfortunately, and they have shown the clout and chutzpah to bring it on.

  4. carmacarcounselor
    May 18, 2014

    Dennis, You might want to consider moderating replies to your posts, and asking your contributors to refrain from political polemics. Onaroll obviously has a political agenda, which seems out of place in a technical discussion.

    • simanaitissays
      May 18, 2014

      Hi, Carma,
      I pondered the political aspects of that reply, something that I tend to eschew in my own writing. Indeed, it was in/out and finally in again, mainly because its author had straightfoward replies to other items.
      Know that I am given opportunity to moderate all replies (and intend to do so with moderation).
      Thanks for your vote for apoliticality (if I may invent this term).

      • carmacarcounselor
        May 19, 2014

        One possible compromise. My occasional letters to the editors that get printed are usually edited in some way (other than my short “I don’t believe it! A Car that lip-synchs!” in horror over the BMW M5’s in-cabin synthetic engine noise). Onaroll’s first two sentences could have been left on their own without the rest, and I’d have agreed with him.
        On the other hand, it’s your blog. One of the things I like about blogging is the freedom from editorial interference, although it does have a downside, as this post demonstrates – I tend to run off at the keyboard.

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