A 3.91 or 4.10 would be good and still deliver decent highway rpm with the A833. With a 27" tall tire and a 3.91 gear, you would be almost 2600rpm at 75mph; about 2700 with a 4.10 with the A833 overdrive. With my slightly smaller tire (26.85") and the slightly better .68 overdrive, I turn almost 2300rpm to do 75mph; engine sounds very happy there. It used to turn about 3300 for 75mph before. I drove it two hours to the Don Garlits Mopar show last weekend and got 30mpg holding 75mph the whole way. I am unaware of what the other gears in this transmission are, besides 4th of course. Just did some math to figure out what OD was and haven't gone any further. This transmission was in a 65 Mustang that kicked the driveshaft out at 150mph, but only cracked the output housing. Guy decided to upgrade to a TKX instead of fix the T5. He said the T5 had an upgraded gearset, was rated for 400hp, and cost him $3500 at the time he bought it.