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MG MGB GT V8 Factory Originals Technical - speedo problem

I have recently become the proud owner of a GT V8 (factory standard) which seems to be reading about 20% over, both on the mileometer and the speed. I cannot check whith the previous owner (deceased) but I understand that the gearbox has had an expensive overhaul, as has the rear axle. Could some one tell me what the problem might be, and also how to remedy it?
Mark Bushby

Mark,

Speedy Cables tel 01639 732213 can adjust speedo.

The formula is and calculations for a B
may be
MPH = driven wheel circumferance x 60 x rpm
-------------------------------------
1000 x gear ratio x final drive ratio

MPH = 6.339ft x 60 x 1000 = 23mph
-------------------
1000 x 0.802

For a V8 should be about 27 to 28mph per 1000revs.

You can always check with motorway posts as well.

Paul
Paul

Mark,

Could the speedo drive gears in the trans have been changed during the rebuild? The percentage sounds about the same as the difference between a 3.90 & 3.07 rear. When I installed a 3.07 rear in my converted '77 I achieved a proper speedo reading by swapping drive gears in the O/D trans. I don't remember the particulars but think I used an early gear in a later trans.

Joe
Joe Collins

Joe

I wandered about that 'cos I'd seen that there are two different 'thingys' that go into the overdrive unit. How do I tell if I've got the wrong one?

Mark
Mark Bushby

Mark,

I'm afraid I don't remember the particulars of which gears I used in the MG O/D box. I was just working out the numbers this evening for the T5 I'm about to install. The ratio of speedo drive pinion teeth to trans output shaft worm gear teeth should be about 2.7 : 1. This ratio would be correct for a 3.08 rear axle ratio, speedo @ 1000 turns per mile, and tire rolling radius of about 11 3/8". Later MG's (mine is '77) seem to have gone to the 1000 tpm speedo. Earlier models I have seen (4 cyl) had different ratios. The turn per mile spec is noted as a four digit # at the bottom edge of the face on Smith's speedo's. If I recall the # is usually in the 1000 - 1300 range. For the T5 the ratio is 19 to 7.

In order to get the right ratio in the MG box I had to change the worm gear on the output shaft as well as the pinion. If I get a chance to pull the gear from my old box I'll let you know at least what the pinion gear is.

Joe
Joe Collins

Mark
I agree that the diff is the most likely culprit but wheels and tyres are also a major contributor - what wheels and profile tyres are you using ?
Roger
Roger

Mark,

Got a look at the trans gear. The pinion is 21 teeth, couldn't get a good look at the output shaft gear but it seems to be 8 teeth which would produce a ratio of 2.63 : 1. This is about right for a 3.08 rear, 1000 tpm speedo and rear tire radius of about 11.8".

Joe
Joe Collins

Roger

The tyres are 175 HR 14, on the standard V8 wheels.

Mark
Mark Bushby

Joe/Mark,
more fun with numbers: the 2 speedo drive gears in my T-5 trans are 7 teeth (Dark Blue) on the output shaft and 19 teeth (Light Grey) on the speedo drive shaft. This gives a ratio of 2,714 turns of the trans output shaft to 1 mile read on the Jaguar's 1000 turns/mile speedo that I plan to use. So, with a rear end ratio of 3.9, 1 mile read on the speedo would work out to being the equivalent of 695 turns of the drive wheels.
That's going to be about a 29" diameter tire....yikes.
Looks like I need to change the plastic drive gears in the trans for greater accuracy of the 1000t/mile speedo through a 3.9 rear end ratio.
Using 8t x 20t drive gears for the speedo drive would result in 641 turns/mile on the speedo- not helpful. I have to change the rear end ratio (a project for later) or find plastic drive gears for the T-5 trans that will give a ratio closer to 3.46 of driven to driver when using a tire of 22.75" dia. I'm off to the local transmission rebuilder's to scrounge through his box of discarded plastic drive gears- unless anyone has any other suggestions of plastic speedo drive gears that will come closer to making a 1000t/mile speedo work accurately through a stock 3.9 rear end, using a tire that revs about 885 times/mile.
Maybe I'll have to swap the guts of a 1300t/mile speedo into a Jag speedo case somehow to get where I want to go until I change out the differential later.
Anthony Morgan

Anthony,

I just bought a new gear & housing at the local Chevy dealer. I had a handful of different ratios but the one I needed would not fit the converted angle drive which adapts it to the B speedo cable. There are two styles, short w/plastic shaft & gear, long w/metal shaft, plastic gear. The parts guy said the older short one was easier to find. He brought it right up & had it in stock. I'm sure the trans shop would have had a load too. Apparently there are a number of pinions available & a couple of output shaft gears.

Joe
Joe Collins

This thread was discussed between 03/11/2001 and 09/11/2001

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