MG Car Repair: MGB speedometer, speedometer drive, mgb roadster

Question
QUESTION: Hello Barrie.  I have recently bought a 1968 MGB roadster. It was fitted with O/D from new. The speedometer reads low: a reading of 30 is actually 35; 55 is 70. The needle is very stable and doesn't wave about (like the rev counter). I've had a number of helpful suggestions (wrong speedo fitted but I don't think it has been changed; different profile tyres but I don't think that's right either). I'm not v mechanical but it doesn't sound like a worn cable because the readings are high, not low. Any ideas or advice gratefully received.  Thanks.  Kevin

ANSWER: Hi Kevin.  The MGB up to 1974 had a speedometer calibrated at 1280 turns per mile.  If you look at the face of the speedo you should see this number printed on the dial in small letters.

From 1975 onwards the speedo was calibrated at 1000 turns per mile.  The difference is 28 percent.  My guess is your MGB has had a replacement overdrive fitted at some time, and they used a later one with the wrong speedometer drive gears.  

Unfortunately, it is a major job to remove the overdrive, strip it, replace the gears, and rebuild everything.  You can check by taking out the speedo drive pinion where the cable goes into the overdrive unit.  If the gear is white or blue nylon, it is 1280 TPM, and if it is red nylon then it is 1000 TPM.  The inner gear cannot be replaced without dismantling the overdrive.  

---------- FOLLOW-UP ----------

QUESTION: Thanks Barrie.  May I ask two additional questions?  If the difference is 28% would the error not be a consistent %age of the true speed? Does replacing the speedo, if it shows the wring number, have any effect?

Kevin

Answer
Hi Kevin.
The answer to both your questions is `YES'.

If the speedo has a number other than 1280 then it will read consistently wrong by the same percentage at all speeds.  If the overdrive has the wrong gears, that will also give the same symptoms.  There is a third possibility, but this entails removing the speedometer and taking the front off it.  In this third case, the speedo will read incorrectly by the same amount (eg 15 mph) at all speeds.

Before we go down that route, I recommend that you check the number on the speedo and the colour of the driven gear.