Gears-Help

[ojimmyboy]

Hi to everone I'm new ,been working on a 72 GMC 3 year project,and just about done , lots of small stuff to finish, I don't have too much knowledge in differential, and gears.
My problem is I am moving to a latgrt tire size ,from 31''to 35x12.50x15'',I belive my now gears are factory 3.73,oh forgot ( AMT ) engine turn key Blue Print 383 stroker.
Need to change gears ,question is to what size? I have a formula a freind gave me ,but still not sure, (new tire 35'' ,times ratio, divided by old tire size 31'' = 4.2112903, sooo next size maybe 4.56, or 4.88 ? Help

 

[blazinzuk]

Grimmjeeper.com has a very good calculator.

Still the stock transmission?

With that motor I might just stick 4.10s in it but 4.56s would give you more oomph around town.

Mess around with the gear calculator till you get what you want your cruising speed at

Remember your tires will not be 35" tall more like 33.75 if you use static loaded radius, which will give the most accurate rpms

 

[chalet2506]

Th350 or a a sm465? Either way you 3.73's will be fine. Run them for a while and see how you like them. You can change later if you need to. 4.56 or 4.88 will give you better low end, but suffer on the highway. Add a tranny cooler if it is a th350.

 

[ojimmyboy]

Hey thanks, aand I've beed driving the blazer ,it now has about 1000 miles,yes I had the trens shop install a cooler.
lower end seams real mushey, slow,you know no jack rabbit take off,Ive done nothing to the engine,it was a turn key, dino tuned,so i left it as was( don't laugh )maybe timing is off.thank for your help