Speedo running fast

Christopher

Christopher

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Just got my car back after having a bottom end engine rebuilt and balanced. Oddly the speedo is running about 8-10mph faster than it should be. It's not down to the engine delivering smoother power - i've tested it against my sat nav speedo, and even a roadside static speed indicator.

The gearbox, and speedo cable to it, were removed whiist the engine was out. Whilst it might just be co-incidental that the speedo itself has developed an internal problem at the same time, could anything else cause this to happen ? - i can't think of what, so any clues appreciated.
 
Christopher said:
Just got my car back after having a bottom end engine rebuilt and balanced. Oddly the speedo is running about 8-10mph faster than it should be.
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Are you sure they didn't install the wrong transmission? Perhaps confuse your's with that of another customer who has a numerically lower differential ratio? Ask the shop if another customer is complaining about his speedometer running too slow.

I suggest this half in jest, but like you, can't think of a better explanation.
 
Let's hope not ! - it wouldn't give me much faith in my new engine ... but it could be a possibility i guess
 
My simple minded theory is that a mismatch scenario (tranny, rims, tires, speed gears) would present a constant ratio between the actual speed and the measured speed. If it is more like a constant offset or a random offset but not a ratio then it is likely a fault in the cable or speedo itself.

To know if you have a constant ratio the error should be measured at several speeds and see if the error percentage is the same.
 
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Sometimes when you take this old stuff apart and put it back together things can change for whatever reason (Gremlins). Perhaps remove, clean, and regrease the connection of the cable and the tranny then go from there.
 
Do you have another speedometer you can swap out? The spring tension on the speedo needle has to be calibrated. As an example, I changed out the face on my speedometer and the needle rotated CC once and the speedometer ran about 10MPH slower. I know your speedometer was not touched but just a thought.
 
Be careful the speedo needle is brittle, but it can be nearly seamlessly repaired using Superglue. Don't ask, but I didn't learn this on my coupe.
 
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