On Mon, 12 Jul 1999, Shapiro - Kenneth J. wrote:
> Anyway, I just got a 1970 GT6+ that has no overdrive. The tires
> are 175/70SR13 instead of the roiginal 155R13. I'm assuming
> that's the reason why at 55 MPH the speedo reads 70 and the
> tach is hanging at over 4,000 RPM begging me to chill out. Am
> I right? The diff. and tranny have been rebuilt, and I'm
> also assuming the same ratios were put back in, but can't
> be sure.
First, 175/70 is close enough in circumference (on average) to the 155 13
originals that it could not possibly cause that much difference! Normal
condition for an original non-OD car would be a 3.27:1 diff, giving one
roughly 20 mph / 1000 rpm in top gear. In other words, a tach reading 4000
would ideally indicate a road speed of around 80 mph. Even if someone
somehow stuffed a 4.11:1 gearset in your differential, you should still
see an actual 63 +/- mph at 4000 rpm.
> Watcha all think...go to higher tires, buy an overdrive tranny,
> ...what's your experience.
Maybe you need to confirm both speedo and tach readings. The tach reading
should be easy enough to confirm with a simple, basic engine analyzer (or
any mechanica could hook your car up to his/hers and tell you in seconds
what your rpm's really are. As for the speedo, find a measured mile and
see how long it takes you to cover that distance at an indicated 60 mph
(should be right about one minute, of course). Maybe it's just your gauges
that are in bad shape (or even wrong for the car)?
If the gauges are correct, you'll need some really big tires to counteract
this setup. The original diff ratio of 3.27 would be the easiest way to
cut it back down.
--Andy
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* 10/Herald/Vitesse (Sports 6) Consultant *
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