Another Gear Ratio Thread (67 Firebird LS Pro Touring)

[Rexenator]

I''m getting ready to order the rear axle for my 67 Firebird, so I need to lock in a gear ratio. I've scoured the forums, and it looks like most people are concerned mostly with track/drag performance. I'm a little more concerned with street drivability than track performance, and don't want to cook the tires at every start or listen to a screaming engine on the highway. I also don't want to go with a lower ratio and then decide that I want to change it. Here's my planned build:

Engine: GM LS376/480
Transmission: Tremec T56 Magnum with 2.66 1st gear and 0.63 6th gear (close ratio)
Rear Wheels: 18x11 with 315/30R18 Michelin Pilot Sport PS2 tires (25.5" tall)
Rear Suspension: Speedtech Torque arm w/Ford 9 inch rear end w/Eaton TrueTrac Posi

I've used some of the gear ratio calculators available, and at 75 mph I would be at 2305 RPM with 3.7 gears, or 2560 RPM with 4.11 gears. I'm not sure, but 2560 RPM on the highway seems a little high for a V8. I'm also concerned with traction off the line with 4.11s, but I don't have anything to compare against so I'm not sure what the performance would be with my engine/transmission/tire selection. Does anyone with more experience than me have any insight as to which ratio to pick?

[parsonsj]

3.25 or 3.34 is the way to go. I've built several cars with that transmission and similar tires and used on the street. Don't get hung up on some short gear ratio that will make first gear useful only for spinning tires.

[Z06vet]

I have run several different gear ratios with the T56 ratios. The first 4 gears are the same as the magnum close ratio, with 5th & 6th being slightly different. I can't imagine running a 3.2x gear. 4.10 is my go to gear. When I went from 3.73 to 4.10, I could not believe the difference. It really kept the motor in the powerband on every shift. I don't run a big camshaft- usually a stage 2. I would consider the 480hp motor more like a stage 1 cam. Now with the .63 sixth gear, I would say that 3.89 gear would be a good compromise. With a 335/30/18, .50 sixth gear, and 4.10 I would turn 2150rpm @ 80mph.

[icemanrd19]

I have the same setup as you and 4.10 gears. Absolute blast. 3rd gear is the best.

[Rexenator]

3.25 or 3.34 is the way to go. I've built several cars with that transmission and similar tires and used on the street. Don't get hung up on some short gear ratio that will make first gear useful only for spinning tires.

Man, that seems really low for this engine/tranny combo. At 70 mph, that would put me at under 2000 RPM in 6th gear, which I think would just bog the motor?

I have run several different gear ratios with the T56 ratios. The first 4 gears are the same as the magnum close ratio, with 5th & 6th being slightly different. I can't imagine running a 3.2x gear. 4.10 is my go to gear. When I went from 3.73 to 4.10, I could not believe the difference. It really kept the motor in the powerband on every shift. I don't run a big camshaft- usually a stage 2. I would consider the 480hp motor more like a stage 1 cam. Now with the .63 sixth gear, I would say that 3.89 gear would be a good compromise. With a 335/30/18, .50 sixth gear, and 4.10 I would turn 2150rpm @ 80mph.

Yeah, I had considered 3.89 just to split the difference. With 3.89s, that would put the car at 2400 RPM at 70 mph, and 1st gear would be 52 mph at redline. Even with 4.10s 1st gear would still get me to 49 mph, which I think would be fine for a street car. Probably a little short for a track/drag car though.

I have the same setup as you and 4.10 gears. Absolute blast. 3rd gear is the best.

How's the engine noise/drone at highway speeds?

[parsonsj]

Man, that seems really low for this engine/tranny combo. At 70 mph, that would put me at under 2000 RPM in 6th gear, which I think would just bog the motor?

I've used some of the gear ratio calculators available, and at 75 mph I would be at 2305 RPM with 3.7 gears, or 2560 RPM with 4.11 gears. I'm not sure, but 2560 RPM on the highway seems a little high for a V8.

Are we talking about 70 mph? Or 75 mph? Seriously, if the motor is bogging, downshift to 5th. 6th will be perfect for cruising. Autocrossing will be in 2nd gear, and you can run on road courses to 150 mph. If you're not convinced, split the difference and go with a 3.5x.

Let's keep all the differences straight too... a 335/30-18 tire is 1/2" taller than a 315, so that makes some difference as well.

[CarlC]

The Camaro has 3:73's but the 0.5 6th gear Magnum. For comparison to yours, the engine is 6.3L with a small cam similar to the LS3.

The ratios were chosen in order to be in 3rd and 4th for open track days, and in 6th for long open road cruise (70-80mph.) If the 3-4 open track gears were not in question, I would not hesitate to put a 3.3x:1 gear in it to make 1st more useable. The car as it is comfortably handles 60mph in 6th/0.5 OD.