My "new" '74 Duster- or why I need a project like a hole in the head

Dartsun said
I have the T-56 magnum for my 69 Dart project. It’s pretty far away from install but deciding rear gears for my 8.75. I’ll probably have a around a 26” tire. I do mostly highway driving and will want this car to be my daily driver. I was thinking 4:10s or 3:91s. The engine will be the blueprint 408 base with cam lift .238 intake .474 exhaust @50-110 degree lobe separation. What gears would you run? Thank you

Either of those would be fine I think. It also depends on what your 1st and 6th gear ratio's are in your T56, there are both 2.66 and 2.97 first's and .63 and .50 6th's. The spread between a 3.73 and a 4.30 rear gear is only like 400 rpm.

I went with the 4.30's just because I knew how my engine performed with my previous combo, even with a 3.09 1st and 3.55's I was slipping the clutch a little unless I was kinda hot rodding it. For a weekend car it wouldn't matter. But my car is my driver, I literally do not own a car newer than my Duster. So for commuting in traffic I didn't want to be doing that. And I was already running the freeway at like 3,300 rpm with the 3.55's and 833, so, even the 4.30's with the .63 6th was a massive improvement (like a 1k rpm drop). I also looked at what my rpm's would be in first and second at speeds that would lend themselves to autoX'ing, and the 4.30's would allow me to run faster autoX courses in second without dropping out of my power range.

Tremec has a really nice calculator you can play with, just plug in your transmission and tires and you can play with the RPM range to see what your speeds will be in every gear. I didn't see a reason to cruise below 2k rpm, my engine doesn't like it. And even with the 4.30's at 6k rpm the top end is still 180 mph, or in other words faster than I'll ever go even on a road course. On the street first gear goes by pretty fast, but it's still not the steepest first gear I've ever encountered.

https://www.tremec.com/calculadora.php