Axle ratio..3:08's? 3:36's? 3:55's?

[LarryH]

I finally found the axle ratio code stamping and found out it has 3:70 gears. I think i'll go with a little taller gear for highway cruising.

The car has a stock GM crate engine rated at around 260 hp, powerglide and 29x15.50 M/T Sportsmans. I don't really see going to a smaller tire (about 28 1/2" tall by 13 1/4" wide) anywhere in it's future. I'm undecided as to what ratio to go with. I had thought of going with 3:08's but i don't want to go to tall. I'm kind of thinking maybe 3:55's. Or would 3:36's be a better choice?

 

[der Spieler]

Here's a link to a land speed racers page with spreadsheets for calculating speed. The spreadsheets are designed for manual transmissions but the final ratio of 1:1 is common to most automatic and manual transmissions. Enter your rear end ratio and tire size then in the RPM column you can change the values to whatever you want. Most of the rpm values given are from 6000 to 9000 rpms but you can change them to lower values. The far right column will give the speed at the different rpms in the top gear. A 3.08 gear with 29" tires at 2000 rpms will give you about 56 mph. At 2500 rpms you are going 70 mph and at 3000 rpms you are traveling at 84 mph. A 3.36 ratio would be 51, 64 and 77 mph respecively.

The Link

 

[blownt]

I'd keep the 3.70 and try it first. I think you will like the 3.70 with the 29" tires. Definently try it out first. You can always change it later. Taller tires will give the same as a higher gear. You should have no problems on the highway with a 3.70 and 29" tires.

 

[LarryH]

The more i play with this link...

http://www.roadkillcustoms.com/hot-...lculator-for-Ring-Gear-and-Pinion-Changes.asp

...the more i'm liking the 3:36's.

 

[der Spieler]

Good, quick calculator. I'm bookmarking it.

 

[putz]

Mine came with wonderfull 2.56 figure those numbers with my 31 inch tires torbo 350 you have a hard time spinning tire it just goes..Can you say low R'S To change my gears I have to change the carrier also I guess. Have not done that much with gear changes.

 

[Rick]

Mine came with wonderfull 2.56 figure those numbers with my 31 inch tires torbo 350 you have a hard time spinning tire it just goes..Can you say low R'S To change my gears I have to change the carrier also I guess. Have not done that much with gear changes.

The carrier will stay just the gears change.Good choice keep the 256 thats what i have and yes i can pull the front end up but its only 2-3 inches.The gas milage is great and the sound is awsome.

 

[putz]

The carrier will stay just the gears change.Good choice keep the 256 thats what i have and yes i can pull the front end up but its only 2-3 inches.The gas milage is great and the sound is awsome.

Rick every where I have read says if I did change gears I need longer bolts and a shim kit. Probably not going to seeing I don't have a job now though.

 

[LarryH]

I decided to go with 3:36's. I ordered a rebuilt diff from these guys:
http://www.duntovmotors.com/differentials-63-79.php