okay...heres a brain buster..

i have an 86 z
i swapped out the origional 85 mph speedo for a 140 speedo.
i also have of corse bigger tires. my problem is im not sure if the speedo is correct.

when i change the speedo, does the new speedo have to be calibrated or will it still work as if it were the origional?

also the cable that attaches to the speedo and trany is loose on the trany side... but it cant be tightened. the speedo needle does move so does the loose cable have any effect on the accuracy?

 

the speedo it self wont change in accuracy as for the tires what size did your car have and what do you have now? the loose cable could also effect it's accuracy.

 

Tire or rear ratio may change accuracy, you can get different gears for the trans side at the dealer. A loose fitting at the trans probably will not affect the accuracy. A dry cable will tend to make the needle bounce especially at low speeds.

 

but if i swapped mu 85 mph speedo for a 140 speedo and changed nothing else, is the speedo accurate, will it really read upto 140 mph?

 

Yes.
If the 85 speedo could swing around and had the markings, it would go to 140 as well. As stated, tires and gears will be what affects accuracy.