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MG MGB GT V8 Factory Originals Technical - Speedometer Error

I have a 78 MGB with a 3.5 Rover and a Rover Transmission. The Rear end gears have been changed to the 3.07:1 (Whinning Gears) The stock speedometer reads aprox.50 MPH and I am doing over 70 MPH. I did ask but did not retain what gear can I get to get this closer. Speeding tickets are getting expensive !
Leroy Cook

Contact XKs Unlimited at They are official Smiths distibutors and should be able to set you upwith what you need.
Stephen Strange

Leroy, I have this setup, I just used a 1,000 tpm speedo and it's very, very close. I realize that different Rover trannies have slightly different speedo drive gears, but this should get you well on the way. I then asked a cop with a radar gun sitting next to a vacant road to time me a couple of times and see how far off I am. He was happy to help. Turns out my speedo reads about 2% fast. That's fine. (I have found that many modern cars deliberately calibrate their speedos 5% or so fast, just to make it a little less likely you will speed. My wife's BMW has a 160mph speedo but boy is it ever optimistic!)

There was some information given about the speedometer gear that is in the transmission. There were diferent colors and the number of teeth reflect the different set ups. Anyone know what I am talking about ? These were ROVER parts.
Leroy Cook

The different coloured gear sets should be in the archives, but if you have gotten more than one speeding ticket with a known calibration error, changing the gears will not solve the problem. Most people can stay at or near the speed limit, when they wish to, using nothing more than the tach.
George B.

Here they are again.
Orange gear = 20 teeth
Green 21
Red 22
Black 23
Blue 24
White 25
Unfortunately knowing this on its own will not help you calibrate the speedo. You need to measure how many times the speedo drive turns per wheel revolution and calculate how many turns this is per mile. (How to do this has been described several times over the last few months) Your speedo will have the number of turns per mile it needs on its face, for Fred this is 1000. If the number of turns you measure don't match your speedo, and it won't if it is as inaccurate as you say, you can do one of four things.
1. Get the speedo calibrated by a specialist - they change the gears so the miles recorded are correct and alter the magnetism so the speed is correct.
2. Change the speedo to match the actual number of turns per mile.
3. Change the gears in the gearbox so the turns per mile match your speedo.
4. Do nothing and keep paying the speeding tickets.
I'm not sure which of the above is the cheapest but in the long term it won't be number 4.

Geoff King

"Speedometer gear that is in the transmission. There were diferent colors and the number of teeth reflect the different set ups."
Does any one have these for a Camero T5?


When the local speedometer repair shop found my MGB to be too low for their calibration machine they told me to do the following…

Drive it down the road and watch the odometer. Mine read 1.5 miles per every mile I drove. Think of this as the driven speedometer gear is being turned 1.5 times more than it should be by the internal gear. It is slowed down by increasing the tooth count on the driven speedometer gear, so it doesn’t turn as far per turn on the internal gear.

Count the teeth on the removable speedometer gear and calculate the change. I can’t remember for sure, but I think mine had 12 teeth. Since 12 times 1.5 equals 18 I bought a gear that was mush closer, but still off. I took it back and traded it for one that had 20 teeth and checked again. Only a lack of ambition on a hot afternoon prevented me from going back for one with 22 teeth witch would probably have made it perfect. I justified the imperfection by thinking my passenger (and I won’t show her this) won’t complain as much when she looks at the speedometer.

I was surprised at how far off the math was, but it did get me close and should work both ways. In other words, if the odometer reads .8 miles per mile, you should be able to multiply the number of teeth by that number for example 20 teeth times .8 equals 16 teeth on the new gear.
George Champion

I have taken my blue speedometer gear to the local LAND ROVER dealer but they can not sell me what I hope is one that will read closer to my corect speed. They need part numbers. Anyone got the part numbers for the rover transmission that was originally used with the V8 with Rover transmission?

Leroy Cook

If all this fails or is too difficult to find a shop that will adjust the speedometer, buy an electronic speedo from VDO. Cheap, very reliable and you can calibrate the whole thing by yourself!
In addition you get good light into the instrument at night. Vision series works OK. No cutting into the dash.
All parts available at Summit Racing
Werner Van Clapdurp

Orange gear part number 219001 used on Rover V8 SD1 with 3.08:1 axle
Green 219002
Red 219003
Black 219004 used on 2300/2600 Rover SD1 with 3.45:1 axle
Blue 219005
White 219006
If LAND ROVER don't know these part numbers they are Rimmer Bros own, contact them at or or fax 44 1522 567600 or 'phone 44 1522 568000.
Using the gear that was originally in the Rover SD1 V8 transmission won't make your MGB speedo read correctly unless both the Rover and MG are looking for the same revs per mile.
Geoff King

This thread was discussed between 13/02/2001 and 07/03/2001

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