Hi! I'm having the rear differential on my 68 vert redone at a local shop. I've looked at previous posts discussing gear ratios, etc. I plan on having a standard 400 build with just a little more punch, correct turbo 400 auto, with AC. I'll be doing highway driving and around town on 15" Rally II rims with 235 or 245 60's all around. I know tire height makes a difference. I'm thinking 3:36 or 3:42 gears with a posi carrier. I think 3:55 would be too high. Stock was the 2:56 open. Any huge difference between the 3:36 and 3:42? What manufacturer do you all most recommend? Auburn, Dana, Yukon, Eaton?? Appreciate the feedback!

Forgot to mention it's an 8.5 rear end from a 70's Nova, I believe. I pulled the code from the front of the right axle tube, just don't have it with me at the moment. Thanks!!

I had my tranny builder beefup my TH400 with better/stronger stock components and a shifter. You can you the stock shifter for the wholesome look or after market one. Either way your going to bury the guy with the 4sp standard. My 400HIO has A/C as well and thus came with 2:56 gears. The owner replaced the 2:56 with 3:91 and turned of the A/C. I bought the car and played with the 3:91 ripping up the pavement. Lot's of fun for a summer, but not practical for old farts. I have my 3:42 Posi available so I'm going to use it as I sold the 3.91.
I think it will be a good combo. Make sure you use the stock Qjet and your ready. IMO, get 3:42, I'd be pissed if the engine booged on 1st jear with 3:36

you wouldn’t even notice any difference with a gear spread of 3.36 to 3.55 at all.
maybe with 3.90 and lower you would but those ratios wouldn’t be much of a difference.

Here is a table I put together for comparing manual transmissions. The cruising RPMs are accurate for that tire size assuming 1:1 transmission top gear

For the driving you anticipate and the choices you're considering I'd say the 3.36. And I'd go with Eaton. Unless the turbo 400 has overdrive. I know nothing about automatics, if it does then I'd go for the 3.55.

3:55 is a great gear, but you need a 700R4 tranny that includes an 28% overdrive. Everything else is a compromise. Our FB with 3:55s and a 400 turbo was great around town but wanted to overheat and was very loud at cruise. Replaced the 400 turbo with a 700r4, kept the 3:55 and now have an essentially a 4:20 rear gear in 1st, and a 2:73 in fourth gear. Works great! There is a reason GM went to it, direct bolt in with the right parts.

If you're not up for dropping in an overdrive, 3.08s and a mild stall convertor.

x2 for 3.36 I have had 2.70, 3.36, 3,90, 4.11 and I like the 3.36 the best. Good for street and not so bad highway. Pulls about 3000 rpm at 60 mph.

^what he said.

My TH400 and 3:55 With 26” tell tires turned 3500 rpm at 70mph. It was not pleasant on the highway.
A 700r4 cured that.

I think a 3:23 would work well...

I would figure out what tire height I was going to use before deciding gear ratio. .25" isn't going to make too much difference but it all adds up. I run a stock M20 4sp with 3.23 gears. I like it. Things did change a bit when I changed tire size. It's not bad but I'm usually hunting for another gear when first getting on the freeway. I think a lot of it has to do with the noise it puts out. You don't get that in a daily driver so it sounds like it's running harder than it really is.