I double checked. The regular T-56 does have a .62 6th gear. The T-45 has .67 5th gear. So, with a 3.90 rear, at 2500 rpms, the speed for each tranny is as follows:
T-45, 5th gear-approx 71mph @ 2500rpm
T-56 5th gear-approx 60 mph @ 2500 rpm
T-56 6th gear-approx 77 mph @ 2500 rpm
Viper 5th gear-approx 65 mph @ 2500 rpm
Viper 6th gear-approx 95 mph @ 2500 rpm

Hope this helps.

 

Agreed with above posts.

Need to confirm your gear ratios and have LOTS of info on that very subject in my swap thread. Be sure to understand what your getting or you may not be happy with results.

Jazzer

 

U gotta love that viper tranny...

 

I hate the viper tranny!!! The gear ratios are a bit odd and dont do our cars justice unless we are built to the moon. And its kinda hard to pair rear gears with the viper spec. I like the Magnum T56. It holds 700 ft lbs and it has normal gear ratios from 1-4 and then has viper like gear ratios for 5th and 6th for cruising. So i think that would be the best option.

 

Actually, with 4.10s out back, the viper spec provides a very ideal final drive ratio in each gear for a bolt on car. You can easily play and compare ratios here..