'69 Chevelle rear end gears

[Glock 18]

I have just gotten a 12 bolt rear end for my '69 Chevelle and I need help deciding what gear ratio to use. I have a 454 with around 475 - 500 HP. The rear tire is a Nitto 285/40ZR18 which is 26.97" tall. The car will be used for occasional road trips and Interstate driving. I want comfortable cruising RPM and plenty of low end power. Can anyone tell me what the RPM is on a 3:73 ratio gear at both 70MPH and 80 MPH? What is the RPM with 4:10 gears at both 70MPH and 80MPH? Thanks for any help or input! It is appreciated!!

[ssealeycpa]

I believe that the formula is RPM=MPH X Gear ratio X 336 divided by the tire diameter.
Using that formula, a 3.73 @ 70 mph will run 3,253 rpm (80 mph = 3,718).
A 4.10 @ 70 mph will run 3,576 rpm (80mph = 4,086).
IMO I would not run a numerically higher gear than a 3.73 unless you are running an overdrive trans if this will be a driver at interstate speeds.
Also factor in your first gear ratio on the trans that you are using (I run a TH400. it comes stock with a 2.48 first gear ratio but TCI and others makes a special planetary gear that will change this to a 2.75 first gear to give you a little more jump out of the hole which is what I run). I am happy with my 3.55 ratio although I have a 3,800 rpm stall torque converter which is a little wierd feeling when your cruising at idle speed and hit an incline and have the engine rev way up to start pulling again. Hope this helps.

[Glock 18]

Sorry I failed to add that I do have a 700R4 transmission in my Chevelle. Thanks for the help!

[66SuperSport]

This web site has an awesome gear/tire/rpm calculator with side by side charts and graphs. I have used this multiple times. Very cool!

http://www.fatboyraceworks.com/gears/

[alowerlevel]

http://users.erols.com/srweiss/calcrpm.htm

3.73
70mph=2300rpm
80mph=2600rpm

4.10
70mph=2500
80mph=2850


Try this one out too http://www.wallaceracing.com/gear-speed.php