Curious as to how much gear would be too much for a sbb with st300 trans? Goal is a street/strip car than can double as a driver. Eying the 3.90 gear sets, but seems the smart thing to do is go with 3.49s or close. Want to be able to go down the highway at 70 and not running 4,000 rpm.. but 3000rpm might be acceptable (even with the fuel prices).
Top gear is 1:1 so the rear ratio and tire height determines your RPM. 336/tire diameter X rear gear X MPH = RPM. Stock tire height is about 26.5" For 3.42 gears at 70 MPH, 336/26.5 X 3.42 X 70 = 3035 RPM discounting converter slip. Does that help?
Well another thing to consider are your choices PERIOD... With the 68-70 original rear you only have used gears to choose from OR a few 3.42 new gears left in the aftermarket still from Brad at Fabcraft in Texas. So please do consider this before getting all worked up on the ratio deal to deep.
I always thought by '69 the trans behind a 350 would be a T-350 3 speed not th ST-300. Whatever. That is where you can pick up some nice acceleration and not affect your cruise rpm. Change over from the ST-300 to a T-350. Your first gear in the 350 will be a 2.52 while the first gear in the 300 will be 1.76. A 1.76 first gear would need to have a 3.90 gear to have the same off the line pull as a 2.52 first and 2.73 rear. If you have 2.56 gears the 350 trans would feel like 3.66 gears. A good used T-350 with a shift kit would work great. Something to think about before changing gears. You could also split the difference and put in some 3.08-3.23 gears along with the T-350 and cruise rpm would still be under 3000 rpm at 70 and you'd really pick up off the line.
