>>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 ...
This is a stupid newbie question from someone (me, actually) who bought a 75
TR6 last summer to learn how to fix cars:
Is it possible to re-calibrate the speedo in these cars? If so, how?
TIA,
Wendy Hart,
Erie, PA
----- Original Message -----
From: Barry Schwartz <bschwart@pacbell.net>
To: Brian Borgstede <borgstede@umsl.edu>; <triumphs@autox.team.net>
Cc: <triumphs@autox.team.net>
Sent: Monday, October 04, 1999 11:35 AM
Subject: Re: Excuse my o/d gloat
>
> >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.
> ********************************************
> 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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