The F body T56 has a 2.66 1st gear. 2.66 x your 3.42 rear axle ratio = 9.1 final drive ratio in 1st gear. That's right in the sweet spot.
A 275/35/18 is a 25.6" tire which is a little on the short side.
At 80 mph with the F body 0.50 6th gear you will be turning only 1,800 RPM. At 60 you will be turning 1,347 RPM. This is geared kinda high (numerically low). This is what a lot of people have against the 0.50 6th gear in the F body trans.
With the GTO trans. 2.97 1st gear x your 3.42 rear = 10.2 final drive ratio. That's a little steep. Add in those short tires and it is likely to seem a bit steep.
At 80 mph the GTO 0.56 6th will have you turning about 2,000 RPM. At 60 you'll be turning 1,500 RPM. Better but 6th is pretty high on both transmissions.
Either setup will work with that 3.42 but you won't need or use 6th gear much. I usually shoot for 2500 RPM at 85 mph (you have to be able to do 85 or better here in Texas on the open road or you are getting run over).
Let's take a look at the T56 Magnum with your rear end and your proposed 275/35/18 tire....
The magnum (wide ratio) has the F body 2.66 1st gear and has a 0.63 6th. At 80 you'll be turning 2,300 RPM. At 60, you'll turn 1,700 RPM. That's a much more usable 6th.
My brother has the 2.66 1st gear behind his 6.0 LSX with a 3.42 rear in his 78 Camaro. When he takes off at a traffic light, he has to let off the gas as soon as the car is rolling because the car is zippier than the rest of the traffic. I don't think you want a steeper 1st gear than 2.66 with a 3.42. The GTO 2.97 first gear would be better suited to a 3.08 rear gear.
The 0.50 and 0.56 6th gears in the F body and GTO T56'S are for highway fuel economy. They are fairly unresponsive to throttle input unless if you are at high speeds (80+). They really need a 3.73 rear gear but that is getting a little low for the F body 2.66 and way too low for the GTO 2.97.
You can see how the wide ratio Magnum is the best of both worlds but the thing that sucks about it is it is so expensive.
This is not to discourage you. I'm trying to give you some insight on how to use transmission gear ratios, rear axle ratios and tire height to optimize your combination. Here are my favorite links for calculating tire heights and rpm's...
http://www.wallaceracing.com/tire-aspect-calc.php
http://www.csgnetwork.com/multirpmcalc.html
My suggestion is if you get a deal on a transmission, be it F body or GTO, go for it then optimize your setup with tire height and rear gear. Let me know if I can help.
A 275/35/18 is a 25.6" tire which is a little on the short side.
At 80 mph with the F body 0.50 6th gear you will be turning only 1,800 RPM. At 60 you will be turning 1,347 RPM. This is geared kinda high (numerically low). This is what a lot of people have against the 0.50 6th gear in the F body trans.
With the GTO trans. 2.97 1st gear x your 3.42 rear = 10.2 final drive ratio. That's a little steep. Add in those short tires and it is likely to seem a bit steep.
At 80 mph the GTO 0.56 6th will have you turning about 2,000 RPM. At 60 you'll be turning 1,500 RPM. Better but 6th is pretty high on both transmissions.
Either setup will work with that 3.42 but you won't need or use 6th gear much. I usually shoot for 2500 RPM at 85 mph (you have to be able to do 85 or better here in Texas on the open road or you are getting run over).
Let's take a look at the T56 Magnum with your rear end and your proposed 275/35/18 tire....
The magnum (wide ratio) has the F body 2.66 1st gear and has a 0.63 6th. At 80 you'll be turning 2,300 RPM. At 60, you'll turn 1,700 RPM. That's a much more usable 6th.
My brother has the 2.66 1st gear behind his 6.0 LSX with a 3.42 rear in his 78 Camaro. When he takes off at a traffic light, he has to let off the gas as soon as the car is rolling because the car is zippier than the rest of the traffic. I don't think you want a steeper 1st gear than 2.66 with a 3.42. The GTO 2.97 first gear would be better suited to a 3.08 rear gear.
The 0.50 and 0.56 6th gears in the F body and GTO T56'S are for highway fuel economy. They are fairly unresponsive to throttle input unless if you are at high speeds (80+). They really need a 3.73 rear gear but that is getting a little low for the F body 2.66 and way too low for the GTO 2.97.
You can see how the wide ratio Magnum is the best of both worlds but the thing that sucks about it is it is so expensive.
This is not to discourage you. I'm trying to give you some insight on how to use transmission gear ratios, rear axle ratios and tire height to optimize your combination. Here are my favorite links for calculating tire heights and rpm's...
http://www.wallaceracing.com/tire-aspect-calc.php
http://www.csgnetwork.com/multirpmcalc.html
My suggestion is if you get a deal on a transmission, be it F body or GTO, go for it then optimize your setup with tire height and rear gear. Let me know if I can help.
