View single post by NigelK
 Posted: 10-05-2015 09:58 pm
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Joined: 07-16-2014
Posts: 188
Dear forum users

After the W58 gearbox swap in my GT, I've noticed that my speedo is reading about 15% low. I've done some calculations, as follows:

Circumference of standard 185/70/R13 wheel is 72.88". This equates to 63360 / 72.88 = 869.4 wheel revolutions per mile. Which in turn equates to 869.4 x 3.44 (diff cwp ratio) = 2990.9 prop shaft revolutions per mile. This means I need a 3:1 speedo driven gear ratio to match the Smiths speedo 1000 tpm rating. The standard speedo driven gear ratio in the W58 gearbox as fitted to the Lotus Excel is 3.5:1.

This difference in speedo driven gear ratios could account for why my speedo is reading low. The question is what to do about it? Some internet research suggests the nearest available alternative speedo driven gear ratio for W58 boxes of the correct vintage is 31:10, so I'd need to recalibrate the speedo as well as swapping out the driven gear at the gearbox.

How have you guys with W58 conversions handled this issue?

Many thanks in advance.

Best wishes,