Hello List,
The local AAA branch had their speedometer cal truck there and I had the
TR4A tested. Wow, was it off! At 60 MPH on the speedo, they clocked it at
53 MPH. The slope progressed at just over 6% low.
In the cars past life, prior to restoration, it had Michelin 165R15 ZX's,
and I don't believe the speedo was that far off. Now it's shod with
Michelin Pilots at 205/65R15 variety (on wider wheels of course). When
selecting the tires I used several tire calculators from the WWW, and these
seemed to match in circumference (less than 1% off).
I believe the TR4's were made with different rear ends (4.1:1 or 3.7:1),
which I also belive was based on whether the car had overdrive or not (4.1:
for O/D cars?). Does this mean that different speedos were used (i.e.
different gears inside), or was the transmission speedo gear changed?
Couple of questions:
1. Can the speedo be calibrated? If so, how? Where to get parts?
2. How do I tell which rear end I have?
3. Is there markings on the speedos for the correct ratio?
4. Is there anything else, short of changing tires, that I can do?
Thanks!
John in San Diego
'67 TR4A IRS no overdrive
'71 Volvo P1800E
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