Rev counter

[350tasmin]

My mate at work is getting a cerby at the weekend and has the following question, any feed abck greatly appreciated..

The final drive gearing on the 4.2 should be 29.8mph
per 1000rpm. IE nearly 60mph at 2000rpm. I took the car for a test drive, the speedo read 50 mph at 2000rpm. Scratched head, thats 16.6% undergeared. Then realised that the 4.0litre straight six Cerbera was 24.8mph per 1000 rpm - 16% lower geared!. Checked the tyres - should be running 235/50 zr16, actually had 245/45 zr16's. Did the maths, only 2.6% different, so discounted that theory. The rev counter was also
dancing all over the place under hard acceleration making it difficult to read. Came to the conclusion the either it has a 4.0 litre diff/gearbox in it (unlikely), or the rev counter or speedo are screwed. Any issue relating to the rev or speedo that you can find out about will be appreciated ie is it a known
common fault, and what do I do about it!!!!!.

I understant the 100 amp fuse could show these symtoms from previous threads is this correct?

thanks

350tasmin

[johnmckenzie]

Rev counters are not the greatest point in TVR's favour. What year is the 4.2 you have? My early car went through 3 replacements until it was ok - replaced 1 for jerky responses, 1 for not going over 5000 rpm and then jumping to 6800 rpm and one for reading whatever it damned well felt like. Final one was fine. My 4.5 (March 1998) has been perfect since day 1 and it and the speedo are spot on at all revs and speeds (yes really! - even I'm amazed!). Are you sure about the 4.2 gearing. My 4.5 is geared at 28 point something per 1000 rpm, methinks?

Regards

John