904 or A518 help

[LeeM]

forgive me if this has already been addressed- new to the site and went through several pages but didn't see my question answered...
I am building a '74 Scamp with a '86 Police 318, a low shiny driver. I don't plan on drag racing at all but would like to increase my gas mileage with an extra gear. Of course I'm on a budget since I just had to replace my furnace... So has anyone swapped the stock 3 spd for a 4 spd? I am capable of the fabrication -just weighing the cost and overall benefit- oh and the mini tub is not complete and the rear end gear ratio has yet to be determined.

Thanks for your input

Lee

 

[70aarcuda]

you want a 46RH.....

that is what they are now called....

 

[fishy68]

I haven't done it but several guys on here have. About how much HP/torque do you think your engine will produce? It might be feasible to go with a A500 (also known as a 42RH) instead unless your running a lot of HP. The A500 is the overdrive version of a 904 so it's smaller and fits easier. Either will require some surgery to make it fit.

 

[rumblefish360]

Lee, pay off (In fuel savings/money in pocket) for this work will be a long time coming. But then again, we old car owners care little about this. If it an issue, then a cheaper way is re-gear, cam and tune it to work with a broom stick cam and 2.76 gears. Or less gear.

The bennifit of the extra get up and go with the lower first gear ratio is nice, but slight.
If you allready have the tranny, your ahead of the game. The rear mount from the trans to the car body is the only fabbing/thinking you'll have to do. Then, of course, shorten the driveshaft.

The O.D. ratio is .69. Multiply this by the rear gear ratio for the new final drive ratio and figure it out from there.

4.56 X .69 = 3.1464
4.30 X .69 = 2.967
4.10 X .69 = 2.829
3.91 X .60 = 2.6979
3.73 X .69 = 2.5737
3.55 X .69 = 2.4495