What gears do I have?

[MoparMotel]

68 Charger 383 magnum/ 727 auto
15x8 Magnum 500's on 235-60-15 BFG TA's
60mph is about 2850rpm
65 is 3050rpm
70mph is about 3250rpm

Is there a formula to calculate what gears I have?

[1970Moparmann]

68 Charger 383 magnum/ 727 auto
15x8 Magnum 500's on 235-60-15 BFG TA's
60mph is about 2850rpm
65 is 3050rpm
70mph is about 3250rpm

Is there a formula to calculate what gears I have?

My guess is you have a 3.55 gear.   

I believe there is a speed conversion on Keislers website that you can put numbers in and see. 

[Ghoste]

That would be my guess also.  Have you ever tried counting wheel revolutions versus driveshaft revolutions with the rear off the ground?

[skip68]

3.55 

[Dino]

Sounds just as my car with the old gears which was 3:55.  I'd install them again if I had a 4th gear.

[MoparMotel]

Thanks for the help guys. I wanted to go to some 3.91's but only if I do my 4 speed swap so my rpms aren't too high.

[John_Kunkel]

The speed/rpm can only be used to estimate gear ratio if the speedo drive pinion is correct. Best to jack it up and determine the actual ratio.

[rebby]

Thanks for the help guys. I wanted to go to some 3.91's but only if I do my 4 speed swap so my rpms aren't too high.

Isn't the final drive on the 727 1:1 just like the 833? Or are you thinking of doing an 833-OD? I suppose that I could be wrong about the 727 too.

[MoparMotel]

I didn't really consider final drive... Just figured the 727 being a 3 speed and 833 being a 4 speed that extra hear would drop the rpms

[Homerr]

This may help:

http://vexer.com/automotive-tools/speed-rpm-calculator

[RallyeMike]

I didn't really consider final drive... Just figured the 727 being a 3 speed and 833 being a 4 speed that extra hear would drop the rpms

Final drive ration between 3 speed 727 and a 4 speed 833 are both 1:1 unless you are dropping in the 833 OD transmission.