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Originally Posted by Bill
Did you ever ...
... measure your vehicle against one of those "Measured Miles" on the Interstate or the turnpike? In every vehicle I've owned for 30 years, the speedometer has always indicated 5% more distance than I traveled (1.05 miles) and the speedometer, of course, 5% faster than I was actually going. I don't actually know that this is a conspiracy by the automakers to make my car feel SPEEDY, but it has been remarkably consistent. Putting on larger tires should bring it back toward reality, by making it actually go farther per wheel revolution, and therefore actually go faster, without changing the odo/speedo readings. If I have all that worked out right.
Bill
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I haven't seen any of those "measured miles" stretches in literally years, maybe a decade or more.
But after the tire changeout I did check my truck's odometer and speedometer against both regular highway mile markers and a GPS over a 10 mile stretch of level road with the speedometer set at exactly 60 mph. I chose 10 miles to even out the minor variations of distance from mile marker to mile marker, to get enough distance that the .1 mile resolution of the odometer became less of a factor, and to minimize the human factors issues of exactly when the stopwatch and odometer reset were done relative to the markers.
The results of both the mile marker and GPS measurement methods were essentially identical (the mile markers were very close to accurate IOW) and showed the speedometer was low by about 3% and the odometer was low by 5% (it's not uncommon for the errors to be different for the speedo/odo). Keeping in mind this was after a 3% increase in tire diameter, it means that at least Toyota makes a very accurate speedometer and an almost accurate odometer. I should note that speed/distance measurement system in my truck is completely done by digital electronics involving a pulse generator on the transmission output shaft and a pulse counter in the odometer/speedometer.
I again did another very long test using the GPS only after the 10% change in axle gearing and found the speedometer was now 7% high
and the odometer was 5% high after 200 miles of travel. Since both the speedo and odo had a 10% change commensurate with the 10% change in gearing, I'm quite comfortable with my numbers.
Since I had the axle regearing done, I've purchased
this electronic speedometer recalibrator but haven't yet installed it. A tad pricey at about $190 shipped but worth the avoidance of mental gymnastics (and any tickets) from the 7% speedo difference. Plus it'll bring my odometer mileage back to true so I won't be needlessly using up warranty mileage.