My 69 firebird has the Th-400 and I believe the 2.78 to 1 rear end. Hoping to switch to limited slip in the next couple weeks. If i am in 3rd gear, where the tranny is 1.00 ratio, then switching to 3.08 ring and pinion would raise my engine rpm by 10%...yes?


The car is pretty snappy even with the 2.78 gears, but I think would run a little "crisper" at a slightly higher rpm.

What gears have the rest of you used with a Th-400 tranny? How about 3.23 gears?

How will you be driving the car? A lot of highway? If not, you might think about jumping to a 3.55. That's what we have with our TH400 and real happy with the ratio.

What is your tire size? ie, 235/65/R15, I can give you engine rpms at specific speeds with different rear end ratios.

Thanks guys, I presently have 215R70-14. If I would like to know what rpm I would see with 3.08, 3.23, 3.55. Cold and frozen here in Vermont today, but the roads were dry, so I took the car for a ride this morning, covered in dust from sitting in my woodshop. It ran at about 1950-2,000 rpm at 55 mph in 3rd gear. At 147 mph it was revving at about 5,300 rpm..

Good Suggestion Colorado 68, and thanks for the help Dr. Drivability.



Just kidding I didn't really go 147 miles per hour. The rest of it's true though.

Hey, I just found the chart that you might be using to make this calculation. http://www.wallaceracing.com/gear-speed.php
Cool. It confirms for me that I most likely have 2.78 gears in the rear end based on the engine rpm I saw this morning.

I'll have to think about this a bit. Thanks again.

Let us know what you decide Turk and why. I'm going through the same internal debate.