Hello, I've been active here in years past, someday I'll reveal who I was. Some things I've posted are still stickies.
I am working on an LS376/480, with a 700R-4. Tires are 275/40R17 BFGs.
If you look at the GM Performance catalog, you'll find this engine is still making 400 ft-lbs at 6400 RPM, IIRC. If so, that's great!
This is not a MPG build, this is a city and highway driver.
So, here's how I have done this figuring for literally decades.
Get a sheet of college ruled notebook paper, and a pen. Down the left margin, number every other line. First line gets a 5, third line gets a 15, and so on. You should get to 155.
Across the top I noted the transmission ratios, and the axle ratio for that page, and the effective tire diameter. In this case, it's 3.059 1.625. 1.000. 0.696. 3.727. 24.9
The math formula is simple. Trans gear Timex axle gear times 336 times mph, divided by tire diameter.
That's 769 at 5 in first.
This gear would be great fun in town with a 2500 stall, but isn't what I'm looking for. For this engine, in this application, I'm going with 3.23:1.
You may be able to do z spreadsheet using tech, but this way does work.
When doing an automatic transmission, remember that most torque converters slip at least 400 RPM at WOT.
You really want your end result to be capable of 35-45 in first, and to cruise somewhere between 1500 and 2000 at 60. Wilder cams need more cruise RPM, as do smaller engines and higher static compression ratios.
​​​​​​ I'll be doing a 540 / T56 combo later this spring, I'll share that here.

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