The speedo reading has nothing to do with George's improved gas mileage -
other than I gather he is driving slower than he used to, which no doubt
helps. The new tires just eliminate the error he previously had in the
speedo reading.
As for the odometer reading, he's correcting the new readings by the ratio
of diameters of the new (taller) vs old (shorter) tires, respectively. This
gives a corrected miles traveled with the same accuracy, or inaccuracy, as
previously (with the OEM tires). So the math is correct, and the improved
gas mileage impressive, but it's still not clear to me how much of the
improvement is the result of the taller tires and how much is the result of
a more conservative driving style.
If a 5% reduction in final drive ratio nets an improvement of several mpg's,
I question what the manufacturer's were thinking in not taking advantage of
that.
Alan
VO wrote:
> It sounds as though speedo inaccuracy is the helpful factor in your
> experiment, George. In a sense you've corrected an error in the car. That
> by itself would be a postive step.
>
> I've confirmed several times in my 92 SHO that the speedo UNDERstates actual
> speed by exactly 2 MPH. I've noticed the speedo on our Crown Vic OVERstates
> actual speed a lot. The last time my wife followed me on the Interstate,
> she in the Vic and I in the SHO, she really gave me the dickens, thinking
> we'd been doing 85 MPH in a 70. We'd been doing exactly 75 -- I'd confirmed
> that using the stopwatch function in the dash clock, 10 miles in 8:00
> minutes. My speedo had been showing 73 MPH.
>
> VO
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