Thread: geering for ford 9 inch rear

Tweet

hi, new guy tyler here,recently purchased at auction 1967 chevelle with ford 9 inch rear super low geering, original 4 speed standard trans, 396 bb, rear tires are 255/60r15. can someone please tell me what to get for daily driver,ocational hiway gears? i like to order from summit racing, if you could recommend gears and part number that would be so great.thank you.tyler

A simple SWAG on my part. 3.25 or 3.50 or 3.73 / but there are many variables - even things like tire O.D. make a difference.

Those rear tires have an OD of just 27" and the stock 4 speed is a 1.0 final drive ratio. If you run 3.25 gears you'll be turning 2831rpm at 70mph. If you jump up to a 30" rear OD tire the cruise revs drops to only 2548rpm. If you drop down to 3.00 gears those numbers drop to 2613 (27's) and 2352 (30's) at 70 down the road. You've got to balance what you want for driveability, economy and performance.

thanks, if i want to keep tire size, what should i get for gears to run about 2300 rpm at 70 mpr?

Since this thread is not terribly old, just a few months then I thought I would add to it for
others that read it later.


A rear gear ratio of 2.7, trans final ratio of 1:1 and tires diameter of 27 inches would yield

a 2352 rpm at 70 mph.


If you have an OD trans with a final ratio of 0.7, then you could use a rear ratio of 3.9

and the engine would turn 2378 rpm.



TireSize_RPM-Calcs01-1.jpg


TireSize_RPM-Calcs01-2.jpg

What I do when I want low gears I put in the low pig I have a separate pig for hi way. Its is easier to pop the pig out than change gears

need to keep in mind low gears at high speed create a lot of heat. overdrive/short tires/low gears can lead to a burnt up rear end.

Originally Posted by cffisher

What I do when I want low gears I put in the low pig I have a separate pig for hi way. Its is easier to pop the pig out than change gears

OK, I will bite ...... what is a PIG ???
.

Originally Posted by shine

need to keep in mind low gears at high speed create a lot of heat. overdrive/short tires/low gears can lead to a burnt up rear end.

Good point, just because the engine is loafing along does NOT mean the rear gears are doing the same.
.

Originally Posted by Indy

OK, I will bite ...... what is a PIG ???
.

rear gear case with gears intact, aka pumpkin, third member.

I have a friend who still tells horror stories about when he got out of the Navy in '72 and drove back to S. Dakota from San Diego in July in a big block Chevelle with 4.11:1 gears, no air conditioning, a pregnant wife, and 2 dogs!!!! The right gearing is very important! PS, they're still married!

rear gear case with gears intact, aka pumpkin, third member.

Guess I should have figured that one out !
.

Originally Posted by Indy

Guess I should have figured that one out !
.

Don’t beat yourself up, it’s slang terms. Now you know and can stump someone else.

Originally Posted by Dave Severson

I have a friend who still tells horror stories about when he got out of the Navy in '72 and drove back to S. Dakota from San Diego in July in a big block Chevelle with 4.11:1 gears, no air conditioning, a pregnant wife, and 2 dogs!!!! The right gearing is very important! PS, they're still married!

I'm sure he realizes now, but if she survived that she's a keeper.