6l80e 6 speed what gear for my fab 9"

[robertjra]

ok a little background..... 1969 camaro l92 6.2 with a 6l80e i will be running a 28" - 29" tall tire looking for a good driver and road race im looking for a math formula on how to find out what I need

[Motorcitydak]

A quick search showed this as your ratio's for each forward gear 4.02 2.36 1.53 1.15 0.85 0.67. This does not take into account torque converter slip. Now, it will always be said with a 6 speed that you will get 1 gear that you will never be able to use. Your first gear is super deep which will get you moving like a stabbed rabbit but that gear will run out very quickly at a relatively low speed. I also saw that the max shift speed of that trans is 6500rpm.

Circumference of a circle is pi x diameter lets say you have a 28 inch tire. That tire will travel about 88 inches per rev.

Now lets say you have a 4.10 gear ratio. At 6500rpm in first gear will have an axle shaft rpm of 394.3 and you could shift into second at 32.8mph. That sounds pretty good and you would only need 1st and maybe 2nd gear for an autocross.

Once it shifts to 2nd gear if will sort of fall on its face tho only because of the huge gap between your first and second gear, your rev's are almost cut in half. There is nothing that you can do about that as far as the rear end ratio

Now for your cruising speed, we will say 75mph. With your 28 inch tall tire that is 900 axle shaft rpm, x the 4.10 ratio that is 3690 driveshaft rpm. That times your 6th gear ratio, your engine rpm would be 2472. That sounds pretty good to me. If you would like a little more, go for a 4.29 ratio which will increase your engine rpm to 2586 or 2749 engine rpm with a 4.56 ratio. Pretty much your choice from there

With the 4.56 your first gear will run out at 29.5mph, 31.4mph with the 4.29 ratio. You may want to lower your engines rev limit to 5500 or 6000 rpm which will change your shift speeds a little, but not too much.

I guess after looking at all those numbers, I would go for 4.56. You will have a lot of power going to the ground with that

Also, with a 4.56 gear, in 6th gear and the engine at 6500 rpm you will be doing 177 miles per hour. That fast enough for you?

[robertjra]

wow!!!! thanks for all the good info sounds like you know your stuff!!!! now what if I want that 200 mile mark? would the 4.10 be better? thank you so much for all your help!!!

[Motorcitydak]

Sorry it took so long for a reply, i was away for a while. How serious are you about that 200mph thing? If you really want to hit that, it will take a LOT of power to get a brick like a muscle car to do that. There is a car called the super cuda that hit 200 but it had a twin turbo'd Viper v10 that pushed out 1000hp and it was just able to do it with a proper suspension to allow it and many aerodynamically improving body modifications. I have some pictures of it if you would like to see or just google it, im sure it will come up

200mph would need 2400 tire/axle shaft rpm. Engine speed of 6500rpm divided by your 6th gear ratio of 0.67 gives you 9700 drive shaft rpm. Divide 9700 by 2400 and you get 4.04. So in theory if you had enough power to make it happen and a car with the proper aerodynamics to allow it, you would need something like a 3.92 gear depending on what is available for your rear axle