I have a turbo 400 automatic with a 373 rear end and a 28 inc tall tire (P275 60 R15)in a 1970 chevelle dyno at 540 hp. I'm looking for the proper speedometer gear for the correct speedo speed.

I just put a new speedo gear in my 71 last night.
I have Th400 and 4.11 rear. Since I had switched from 2.73 open, I had to get the adapter 3932229 and went with 38teeth gear (lt Blue). I haven't tested its accuracy yet, but on the short drive I took last night, it seemed pretty close. Fred Aldrich gave me the specs. Recommend you contact him. You can find his email address in the Chevelle Engineer section. just search on Speedo gear.

There are links to 3 different web sites that may be of help here: http://links.chevelles.net/links/WorkShop/General_Info/ About 3/4 of the way down the page. May be something here also: http://links.chevelles.net/links/WorkShop/Estimators/index.html