I have 4.56 gears right now in my 66 chevelle. I want to put a more streetable gear in it. I'm running a 26" tall tire. My setup includes a TH400 with a 10" TCI street Fighter converter. Which I believe is roughly a 3000 stall. I'm running a 355 with vortec heads. I'm using a Comp Cams XE268H camshaft with 1.6 rockers. Flattop pistons wich yields about 10:1. I have a edelbrock air gap rpm intake with a 3310 holley. Headers, 2 1/2" exhaust with dynomax ultra flows. Can I switch to a 3.55 gear or is that not low enough for this combo?

I'd go with 3.73's and a 26" tall tire..

I have the same engine combo but with 180 dart iron eagles in my Nova with 275/50/15's(25.9in tall). I have a super t-10 now but had a th-350 with a 2500 stall and 3.42 gears. The 3.55's will work better on the street than the 3.73's, but watch out for trans temp. With that high stall, you'll be riding the stall pretty hard. For street, I loved my combo. High stalls are not good on the street since you aren't going fast enough to not use the stall, especially with a 3000 stall. If you want better street manners, go with a 2500 stall and 3.73 gears. You should still put down 13's, but you won't scream down the street.

By the way, what were your times with the 4.56's???

Either 3.42's or 3.55's would be the best match for your motor combo. The 3000 stall is fine just run a big transcooler .

IMO, if you want better highway manners than I would go with a 3.55. If you are a little more "strip" oriented, I would go with the 3.73's.