On Thu, 16 Mar 2000, Jeff McNeal wrote:
> Hi everyone. I inquired yesterday if 4,000 RPM at 55 MPH sounded right for
> my non-overdrive '67 Spitfire -- and received several replies stating that
> it seemed high, even with the original rear end for my car in place.
>
> It seemed high to me too. I seem to recall from my first Spitfire that the
> needles on the speedo and tachometer were more or less in parallel with each
> other while in 4th gear.
That IS normal, as I recall.
> Does tire (not wheel) size make a difference? The PO installed 145 SR 13's
> on my car. Would changing tires bring the ratio back into synch so my
> engine doesn't have to run so hard?
As I noted privately to you earlier, 145 SR 13 was the original radial
tire optional size for the Mk3 and is fairly close in circumference to the
5.20 13 bias ply tires.
[Oh, yeah, regarding that other message: I do know the difference between
"optimistic" and "pessimistic"; I just had all my other numbers mixed up.
Sorry! :-)]
Thing is, first you need to determine the relative accuracy of both
gauges. See if you can't borrow (if you don't have one of our own) an
engine anyalyzer that will show engine rpm. Compare those readings with
your tach at various engine speeds to see what's what. Similarly, go out
and find a measured mile on a highway, or find a highway with mileposts.
Try holding the car at approx. 60mph and see if it takes you 60 seconds to
reach the next mile. Do it several times and average out the results. Then
you know roughly how accurate the speedo is.
Of course, it is possible that you have a 4.55:1 rear axle ratio. Check
out <http://www.vtr.org/maintain/gearbox-ratios.html>, especially the last
two columns showing final drive ratios and mph/1000 rpms. Note that, other
than the Mayflower (which really doesn't belong on that chart, but then
it's my chart and I can do what I want with it :-) ), most of the cars
listed used a tire size in the 5.20 or 5.60 13 bias ply, or 145 or 155 x
13 radial size, so you can -- broadly - use all the numbers compared to
each other.
Once you get up to something like a 175 (80) 13 radial or a 6.50 x 13 bias
ply, you start getting big enough differences in circumference to really
be noticeable, as in effectively changing the rear axle gearing (not to
mention handling, inevitably for the worse). On the other hand, for all
practical purposes, a 145 R 13 and a 165/70 R 13 are close enough to the
same size, as are 155 R 13 and 175/70 R 13.
So maybe your gauges are both reasonably accurate (hey, it happens) and
you've got that 4.55:1 "race" diff!
--Andy
* * * * * * * * * * * * * * * * * * * * * * *
* Andrew Mace, President and *
* 10/Herald/Vitesse (Sports 6) Consultant *
* Vintage Triumph Register <www.vtr.org> *
* amace@unix2.nysed.gov *
* * * * * * * * * * * * * * * * * * * * * * *
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