71 Trans Am looking for a taller gear
 

I have a 71 T/A 4 spd with 3.73 gear. Thinking of going to a 2.90 or 3.23 something in that range.for better highway driving. What is the easiest way to do that and also have a speedo calibrated. I know it it not like a 9 inch where you can just swap out center section but I do want to keep it stock as possible.

I'm very happy with the 3.23 I went with. Nice balance of acceleration, highway cruise and traction. Since my original carrier was toast (restoration shop didn't check if for lube before driving), I replaced it with a Yukon posi and 31 spline axles. I went with a RIchmond gear set.

1970 Esprit
4.25" stroked 350, 413 cu in
Comp XE276HR, 9.3 cr, 6X-4 Heads
Turbo 350
442 Lb Ft@4300

Maybe work the problem backwards. What RPM do you want to be at what MPH?

2:90 gear at 2800 should be about 75mph which is about where I would like to be.

Venom - Assuming you have the original 8.5” 10-bolt rear, you can get new 3.08 gears from several sources. Also, assuming you have 26” tall tires that allows 776 revs per mile x 3.08 = 2391.5 rpm at 60 mph. Divide by 6 to get rev’s/10 mph increments x 7.5 (for 75 mph) = 2989.4 rpm. If you can source 2.73 gears (I don’t think there are any choices in between) they would drop the rpm at 75 mph to 2650 rpm. Taller tires would drop the rpm a little bit.

Dennis

Anything taller than 3.23 you’re going to be slipping the clutch starting out in first gear a little more than you’d probably like.

Had a 1969 330 hp 400 4-speed Firebird all original drivetrain with the wide ratio Muncie and 3.23 rear end and it was acceptable on clutch slipping in low gear start off but not great.

The 1976 T/A 455 cars with the anemic 200 hp 7.6:1 engines all came with wide ratio 4-speeds and 3.23 gears.

3.08 would be the max I’d go with a 455 HO 4-speed.

Since your TA is a 71 I assume it's a M22 transmission. My brother's 72 TA has a M22 with 3.23 rear gears and it is less than desirable in racing situations with the 2.20 first gear. However, if you want to go 60 mph in first gear before shifting its great :pound:

Maybe consider a TKX transmission swap to have 5th gear?

Unless you have the need for a strict restoration, that's the solution for these woes. Low 1st, tall 5th. That's the ticket for driving pleasure..

I’d have to agree with b-man on this. I put 3.08’s in my wife’s car. 455 with 2.64 first gear and 27 inch tall tires.
It’s good on the highway , about 2700@70 mph. I’d like a little more gear reduction in first. More clutch slippage than I’d like from a stop.
Overdrive trans would be the best solution, if you’ve got the money. Unfortunately I don’t, so I’m thinking of trying a set of 3.23’s in that car.

Good luck!
Murf

I am not opposed to that at all and is an easier solution. just worried about it being more difficult to calibrate the speedometer.

Borg Warner made a ST-10 that had a 3.20 first gear, which I think would be a drop-in replacement for your M22. That would work pretty well with a rear end as low as a 2.73. These were used in the 1982 Trans Am and Z-28s with factory 4-spds. I had a 1982 T/A 4-spd I bought new.

Keep in mind that as the first gear ratio goes numerically higher the strength of the gear set drops accordingly.

So there’s always a trade off.

The 1982 F-body cars had anemic 305 V8s so those weaker first gears were fine, for backing up a 455 HO with twice the power not so much.

another idea:

If you want to get 3.08 gears for your differential;
Maybe you might consider swapping out an M20 or BW-ST10 for the original M22.

If you get an (1971+) M20 or a BW-ST10, they would swap directly in place of your original M22, and offer you better first gear to accommodate a taller gear set.

If you stick close to something original, you can just get the same combination of speedo drive and driven gears to accommodate for correct speedometer reading - and remove any question to what you need.

Truman Fields (a Pontiac racer) personally suggested the BW-ST10 with the 2.64:1 first gear ratio.

It should be worth noting that an M40+A/C equipped 1971 Trans Am would have had 3.08:1 gearing.

If you want to go the other way, and you're open to an aftermarket transmission (and headache involved with an aftermarket swap) why not leave your gearing in the differential be as-is, and go to a transmission with an overdriven top gear;
A Richmond five speed, with a 0.77:1 top gear would make your 3.73: effectively a 2.87:1 .

I have a '70 Lemans with a SD Performance 461 (4.25" stroke 400 + .030") with an M20 (2.52 1st) and 3.08 rear gears (7.7616 multiplication ratio) and it doesn't require excessive slipping of the clutch to get moving. It does have 9.5 to 1 compression and a Bullet RAIV copy cam with 1.50 rockers, but still makes tons of torque down low. With 27" tall tires it runs at 2900 rpm at 75 mph.

My 455HO GT-37 has its original 3.31 12-bolt rear gear, but I swapped out the original M-13 3-speed for a BW-ST10 with the 2.64 1st gear (8.7384 multiplication ratio). This HO has 8.6 to 1 compression and doesn't require any more clutch slippage than normal. Feels about the same as the Lemans with higher compression despite the 0.9768 ratio advantage of the HO. With 27" tall tires it runs at 3100 rpm at 75 mph.

More compression seems to compensate for higher 1st gear ratios.

Dennis