Work this formula backwards.
Mph=( rpm x TC) /(1056 x R1x R2)...........
Where; TC=tire circumference, R1=final drive(diff ratio), R2=trans top gear.
Say; your teener cruises nicely at 1800, and your 275s are 28 inchers(88TC), and your trans OD is .73.
65mph=1800 x 88 / 1056 x .73 x R1diff ratio; ...........R1 = 3.16; rounded up to 3.23;your new gears. Your srarter gear will be; 3.09 x 3.23 =9.98. This is fine for a stock cammed teener. Not so good for low compression teener with a bit of a cam.
For that application you will need about 20% more take-off torque multiplication. So 9.98 plus 20% equals; 9.98 x 1.20 = 11.98. now 11.98 / 3.09 = 3.88 rear gears. Which rounds up to 3.91s;easy-peasy.
On the other end of the scale, cruise will now also be 20% higher, so; 2160@65, still reasonable for fuel mileage!
Beware of going too far with a rear ratio as normal take-off will make the 1-2 shift ridiculously early .
If you go back to the 11.98 starter gear, and divide it by the standard 2.66 low gear, you will see that this is equivalent to running a 4.50diff! This is almost nuts in a DD(IMHO).A better answer is what Rusty said: 3.55s
3.55 x 3.09 =10.97. This is 10 % less than optimum for a LCteener, but still equivalent to running a 2.66 x 4.12; so pretty good.
But honestly the best answer is to bring up your Dcr(dynamic compression ratio) to where the teener pulls on the bottom-end, by itself. I know I would not be happy until the cylinder pressure was up around 165psi or better.
To that end; I respectfully recommend a compression test.
What you gave up in the A/T swap is the off-the-line torque multiplication of the converter.
That TC/A/T is what makes a low compression teener driveable with hi-way gears. Its kinda like a lower first gear in the automatic, kinda like the loc-up is another gear at the top.
So just to re-iterate; what Rusty and others said; 3.55s.
Hope that helps.
