Speedo reading - very inaccurate - reasons?

[Troogly]

I seem to have a very under-reading speedo. When at 70 the indicated speedo reading is 50 mph. When traveling at 105 the speedo is circa 80 mph!

I had the gearbox changed recently and wondered if that could have caused the problem. Any ideas on what the cause might be and how easy it might be to fix it?

Thanks all

[spend]

If you have the pre-cat with the drive from the speedo, it sounds like they didn't swap over the speedo pickup. Three are 3/4 versions, if your old box is around probably easiest solution would be to swap the pickup over (still a bit fiddly). If a garage did it go and screw them

[Troogly]

Thanks very much for your reply - I think you are spot on and I will be picking this up with the garage on Monday!