Triumph Repair: 77 Spit Diff ratio, speedometer gear, gear set

Question
Jim,
 I have a 77 Spit which I believe has had the differential gears changed.  The differential whines and after looking in the manual that came with it they had notes about setting the clearances.
 I checked the speedometer with a GPS and it shows to be good at low speeds but off at higher speeds.  The speedometer is new so I know it is correct.  The chart I used said that the speed in 4th gear at 1000 rpm should be 18 mph if the ratio is a 3:63 to 1 which is what I am getting. Both on the speedometer and the GPS.

  So I have 3 questions.

1. Is this ratio equivalent to adding overdrive?

2. If it has been changed would this cause the speedometer to be correct at low speeds but off at higher speeds?

3. Is there a replacement speedometer gear available to correct the speedometer?

Thanks again for you expertise,
Mark

Answer
Mark,

The typical overdrive unit ratio is approx. .79.  So adding overdive to the original 3.89 would be equivalent to dropping the overall ratio to 3.07

As to the speedometer, if it was spec'd as a US 1500 then it is designed to work with the 3.89 gear set.  It should read about 5% low.  You won't notice the difference until you're up higher speeds.

There used to be different gearing available depending upon final drive (3.27, 3.63 and 3.89).  Rimmer Bros in the UK list them as available there if you can't find a vendor in the US (see www.VTR.org and look at the links pages).

Cheers,

Jim