i recently put 3.80's in my 73' mustang. my speedometer is off by roughtly 18 to 20 mph. I remember back in the day when i had my tranny worked on, someone saying the gear was a "15" i think, this may be wrong...

It's a c-4 automatic*

either way... anybody have an idea roughly how many sizes i should go up that would adjust the mph by around 18 to 20?

any help is appreciated and thanks again!

 

Does not having the stock gears change the mph? If so then mine must not be stock either because im off by 10mph.

 

this calculator will help you figure out what size you need.

http://www.tciauto.com/tech_info/speedo_gears.htm