*Note that on a non-overdrive like the 3 spd autos your diff ratio and overall top gear ratio would be the same because the highest gear in our 3 speeds is 1:1. 1:1= 1, times X, = X. The 4 speeds have an overdrive 4th gear with a ratio of .69:1.*

so if you have the common differential ratio (for the 604) of 3.6:1, and you multiply that by .69, you get the common 2.49 overall top gear ratio. so for every one revolution of your tire, the engine turns 2.49 times in 4th gear. you can also extrapolate the effective differential ratio by going backwards from the otgr with .69. i.e. you have a 2.49 otgr, 2.49 / .69 = ~3.6. I say effective differential ratio because it is actually the combined ratio of the transfer gears on the side of the transmission and the actual ring and pinion gearset in the differential. determining the split between those two is more complicated because the tooth counts/ratios of the transfer gearsets arent really documented here.

so if you are cruising down the road at 65mph and 2000 rpm in 4th, you can do 2000 / 2.49 = 803, so at 2000 rpm in 4th your wheels are spinning at 803 rpm or so. this will actually work for any gear if you know both the gear and differential ratios.

so if you have the common differential ratio (for the 604) of 3.6:1, and you multiply that by .69, you get the common 2.49 overall top gear ratio. so for every one revolution of your tire, the engine turns 2.49 times in 4th gear. you can also extrapolate the effective differential ratio by going backwards from the otgr with .69. i.e. you have a 2.49 otgr, 2.49 / .69 = ~3.6. I say effective differential ratio because it is actually the combined ratio of the transfer gears on the side of the transmission and the actual ring and pinion gearset in the differential. determining the split between those two is more complicated because the tooth counts/ratios of the transfer gearsets arent really documented here.

so if you are cruising down the road at 65mph and 2000 rpm in 4th, you can do 2000 / 2.49 = 803, so at 2000 rpm in 4th your wheels are spinning at 803 rpm or so. this will actually work for any gear if you know both the gear and differential ratios.

I've got a '93 Spirit ES 3.0 with a column shifter that reads (D)-3-L, and an '89 Spirit ES 3.0 with a floor shifter that reads (D)-D-L. Any difference, or just style points?

Also, I have a '93 LeBaron (AA) 3.0 with a column shifter that reads D-2-1. Does this have a lock-up torque converter, and does that make a difference to the formulas that Vigo presented?

Also, are any of the gears (differential or transmission) swapable in order to create custom ratios? I would love to get a 604 to run at about 1,500 at 65 mph...

Thanks,

Dave