Speedometer Precision Test

Charles drives a US-spec Integra Type-R in Canada. Recently he contacted me to inform me of an innovative 'experiment' he did. Like all of us, he was curious about two things of the speed of the ITR. 1 - How fast is it, ie what is the top-speed; and 2 - How accurate would be the speedometer be when we have reached that speed ? Charles' experiment attempted to answer these question.

Charles took advantage of a flat stretch of Highway that is equipped with kilometer marker-stones (ie stone markers that are positioned at each km of the road) plus a well-thought out procedure to take into account all factors that can be practically accommodated for. What he did was to first establish the error of the speedometer reading at a 'practical' speed of 100kph (practical in the sense that he can execute the procedure for accuracy check properly and without endangering his life !) From there, he proceeded to gun the car to its top speed. Using the speedometer accuracy calculated from the 100kph run, he then applied it as a correction factor to calculate the real top speed of his car.

The error rate that Charles obtained for his 100kph run stabilized quite consistently at about 4.2% (he also took tyre wear into account, actual computed error was 5.8% but tyre wear was calculated to contribute about 1.6% out of this total rate). In his top-speed run, Charles reached an indicated 256kph using his stock speedometer. So, by applying the original computed error rate of 4.2%, Charles was able to compute his real top speed to be 246kph (or 152mph for readers used to the imperial measurement). This proves that the Integra Type-R is very fast indeed !!

Full documentation of Charles' testing is available at his personal homepage. Readers who prefer to link straightaway to the speedometer accuracy test procedure can follow this link instead.

We had some discussions about the validity of his test. Although Charles couldn't take into account the effects of centrifugal force and heat build-up which increases his tyre pressure (both will bloat up his tyres and thus will contribute to a higher actual top speed), normal enthusiasts like us will not able to such factors into account due to lack of proper equipment. Another interesting fact is to note that Charles' test results corresponds quite close to the page_number=1">JARI results I quoted from Japan's MOTOR magazine which uses an optical wheel to measure actual speed very accurately (the JARI results were tested on Mitsubishi cars and produced an error rate of not more than 3%. We can safely assume the same error rate should apply to our cars too). This means that the two factors above probably do not contribute much more than 1 to 2% to the error readings from the stock speedometer, so Charles' results are most probably very accurate.

Thanks Charles for sharing your test with TOVW !