>I still don't get it!
>What do the tires have to do with the ratio of engine RPM and
>the speedo? The drive for the speedo is taken off of the back
>end of the transmission. You could remove the back wheels,
>back axels, the diff., and the drive shaft. Jack the car up and run
>the engine and put the car in gear and the speedo would tell you
>the ratio between engine RPM and drive shaft RPM.
>I just don't get the bit with the size of tires.
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It has to do speedometer accuracy with different size tires, and that
changes with the circumference traveled. With a larger tire, the car
actually travels farther for a given rotation. it may be inches for one
tire rotation but when you add them up over several thousand rotations it
adds up quickly. That said, if the speedo is calibrated to read correctly
with say a 165/80 size tire, at 4000 rpm (for comparisons sake) and you
replace the tire (and nothing else) with a larger diameter 215/80 tire, as
you state, the speedo measures the same rpm/speed, but the car is actually
traveling FARTHER/FASTER than is indicated. Since the rpm indicated is
the same at the speed indicated, but as I stated earlier, the tire is
covering MORE ground that of the smaller tire the speedometer is now
reading incorrectly. That is why if you change tire diameters, you have to
re-calibrate your speedometer to read correctly. As you point out, if you
jack the car up and run it you will get a speed reading on the speedometer,
but it will be incorrect, since you aren't going anywhere :-)
Does that answer your question or did I miss the point entirely?
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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