1983 Grand Marquise w/ aod.
Overdrive gear is .67 give or take
1750 rpms or so ar 70 mph
25 inch stock factory tires, or whatevr ford was putting on 15 inch wheels.


Ok, figure this. Overdrive ratio .67, right? 33% right?
Add 33% to your engine rpms and you have your DRIVESHAFT speed, in this instance 1750 engine rpms x 30 (instead of 33 for simplicity)% OD = 2275..... This is how many rpms your crankshaft is doing.

take your 2800 rpms, you subtract 33% from that if you go with the OD transmission conversion kit. You would get 1875 or something in that ballpark as a new engine speed at 70 mph which means alot more throttle to play with. Think on that one for a little bit, soooooo (using a similar calk to yours)

Your NEW STATS would be: (Drum roll please)

At 2800 RPMS in OVERDRIVE you would effectively be lowering your 3.00 gears into 2.01 gears and instead of 70 or so be doing something very close to 100mph..... Or if you just want to cruise at 70 it woud be alot better for your engine wear and tear and nicer to the wallet at the as pump!

Hope this convinces you to go ahead withthe conversion, seems like an all around good idea no matter how you look at it. Might just need a more well built rear end to handle it is all.