Need help determining proper speedo and drive gear

[Nbf77400]

Hello all, I recently purchased a 1972 C20 that previously had a lot of backwoods engineering done to it. I'm trying to slowly bring the truck back to being functional and in proper working order. Long story short previous owners did a trans swap to a 700 R4 and never changed the speedo gear and I have no way to be sure they did the drive gear either. How do I determine the proper gear that needs to be swapped in so my speedo doesn't read 100 when I'm crushing at 70.

[clay68c10]

Do a google search for 700r4 speedometer gears, you should come up with links that will give you some charts to determine what drive and driven gears you need for a given tire diameter and axle gear ratio. There's just the gears in the trans tailshaft, nothing in the speedometer head. You may or may not have to pull the tailshaft to get the right combination.

[HO455]

If that fails go down to your local Chevy dealer and the parts counter should have the GM charts showing all the tire combos and the gear selections. Be warned not everyone who works at the counter will be interested in helping you figure this out just for a $10 dollar sale. They guys at Wentworth Chevytown here in PDX were kind enough to let me look through the book and make my own selection. Who knew there were so many combinations of tires and gears. There was like ten pages on 1972 trucks.