I switched out the rear ends in my car and now my speedometer reads 5 fast. What do I have to do the correct this?

Also if I switch out my 85mph speedometer for a 115mph speedometer what would I have to do to make it read right?

Thanks for your time.

 

The rule of thumb is a decrease of 1 tooth on the driven gear the speedo will read 2mph higher. Look here for more info:
http://www.bgsoflex.com/speedo1.html

 

Look up the rear end code stamped on the housing. Example- Mine is a 4HY. 4HY in the code book= 1987 3.23 POSI rear end Non-BorgWarner.
Next, go to the dealer and tell them you need speedo gears for a 3.23 rear end with either auto/manual trans which ever you have. It will probably help if the rearend you put in is the same year as the car, but if not, they should still be able to figure it out for you. You should also check if the speedo is electronic or cable driven.
This will only work for factory produced gear ratios. They won't be able to help you this way for aftermarket gears like 4.11's

 

I put in a posi rear with 3.27 gears(borg warner) out of an '86 T/A. My speedo is cable driven. Would I be able to just take the gear out the the t-5 that was in the T/A also? or are they incompatible? Thanks for all the help.