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MG MGB GT V8 Factory Originals Technical - Another Rover SD1 box/turns per mile question

I know you all have talked about turns per mile, etc. with respect to speedometers. One of the posters had noted that an SD1 box spits out 1000tpm into the speedo cable. But this isn't very helpful, unless we know (i) what ratio diff an SD1 used and (ii) what size tires a Rover SD1 had (anybody know, by the way?)

Can someone simply tell me this: forgetting everything else, for every revolution of the driveshaft, what fraction of a revolution does a Rover SD1 5-speed gearbox turn the speedo cable?
Tim Samuels

Can't tell you that, but I can tell you that an MGC 3.3 to 1 axle with 185/70 14 Goodyear NCT tyres at 24 psi requires the speedo to be calibrated at 1120 tpm.

A 3.07 axle would require it to be 1042 tpm.
Chris at www.octarine-services.fsnet.co.uk

Chris, that is very helpful info. Just to confirm, I assume you have the most popular SD1 5-speed box with .833 overdrive, correct? How did you calculate this stuff, by the way? (Oh, also, that tire pressure seems really low. Most fo1lks I know run 32/28.)
Tim Samuels

Yes the LT 77 19A series box from an SD1.

Calculated by pushing the car along and counting the speedo cable revs and the distance travelled by the car for 6 complete turns of a rear road wheel.

This info was used by the speedo recalibrators and the speedo came back calibrated at 1120 tpm. You really need to do this on your own car because whilst all else is constant the rolling diameter of the tyres is a critical factor. I changed the rear tyres for new ones and found I was under recording on the speedo by 5% whereas it was spot on before the tyre change.

The 3.07 ratio calculated by taking 3.07 divided by 3.3 times 1120.

Re tyre pressure - I don't know why people don't stick with the recommended pressures, 21 front and 24 back.
Only use 28 back if loaded or high speed cruising.

I too was recommended to raise the pressures by my rolling road tuner.

At 26 / 28 psi the handling was destroyed, the ride was rougher and the balance upset. Much more skittish and prone to ride the ridges in the road surface.

The main guide to tyre pressure is how the tyre wears.

At 21 / 24 psi all four tyres wear evenly across the tread - not more at the edges or in the centre.
Chris at www.octarine-services.fsnet.co.uk

Tim the internal gearing of the gearbox is irrelevant since the speedo pick up of the output shaft and so is only subject to the rear axle ratio, tyre size and of more importance the actual nylon gear that drives from the output shaft to the speedo cable or transducer. There are I recall 5 gear with between 20 and 24 teeth and they are colour coded.

Rog
Roger Parker

Further comment: there are two different drive gears in SD1/TR7/TR8 boxes depending on the model and the diff ratio. Part no:s TKC 1273 (TR7/SD1 2000 ?)and TKC 1274 (TR8 and other SD1). Are all 5 gears that Roger mentioned interchangeable with either of the drive gears ?

Jukka
Jukka Harkola

Just to answer the original question the final drive ratio on the SD1 was 3.08 : 1 and the wheel size 6J x 14. The standard tyre was 185HR14/195/70HR14 or195/65HR375 for the Denovo type.
Philip Barton

Tim,
The speedo drive gear TKC1273 (white) was fitted to the 2000 and 2400TD SD1's and the TKC1274 (black) to the 3500, 2600 and 2300, I don't know about the TR's and I don’t know how many teeth the white or the black gears have – can anybody tell us?
The driven gears come in five mechanical types plus six transducers for the electronic speedo (‘82 SD1 onwards). Mechanical drives are:
Orange gear = 20 teeth
Green 21
Red 22
Black 23
Blue 24
White 25
Part numbers are 219001 to 21906.
All the carb 3500 and manual Vitesse had 3.08:1 diff's, the 2600 and 2300 had 3.45:1, the 2000 and 2400TD had 3.9:1 and the VDP EFI and auto Vitesse had 2.84:1
Wheel sizes were mostly 5.5J or 6J x 14 steel or alloy but the Vitesse and VDP EFI had 6.5J x 15 alloys.
As Roger says the gearbox gear ratios are irrelevant but the permutations of wheel/tyre size, axle ratio and speedo drive and driven gears are almost endless. Geoff

Geoff King

This thread was discussed between 22/12/2000 and 02/01/2001

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