View Full Version : Transmission question

02-13-2014, 10:08 AM
Don't know if this is an OK place to post this question, hopefully it's OK.

I recently read some stuff on gear ratios and started wondering. Can you change out the transmission gears from a close ratio to a make a wide ratio? If so does anyone have an idea how much a rebuild on a toploader should cost? I know I could sell my close ratio and buy a wide, but my close is a RUG-AJ which has the longer tail piece and the 1 3/8 input output shafts. Is the 1 3/8 really that big of a deal if my car only has 450ft lb torque?

The reason I ask is because right now I have a 2.32 1st gear and a 3.89 differential. 2.32 x 3.89 = 9.02 SLR - a 2.78 (wide ratio 1st gear) would give me a 10.9 SLR which would be much better out of the hole, especially when I get a posi-trac diff. I was thinking about going to a 4.3 diff, but that would only give me a 9.97 SLR with my close ratio trans and set my RPM's really high at the finish line.

I want the car to have some drivability on the street and interstate and anything taller than the 3.89 rearend would really make that tough. But I also want it to jump off the line. Right now I'm at 2600ish rpm to go 55 mph.

thx, Dave J

02-13-2014, 02:11 PM
Wow... First of all, transmissions are matched to the engine's torque curve. Most race engines operate at very high rpms because they don't produce torque at low rpms. That's why a matching transmission will be a close ratio (for that application).

For a cruiser or family car engine, a very wide ratio is more attractive. Those cars drive corner-to-corner all day long without fouling plugs or loping along.

If you change a gear in your trans, you need to change sets of them because of the cluster gears. Now, we're re-inventing the wheel which could prove to be very expensive. You're much better off using something already out there, like a trans with more gears.

I'm an 'automatic-type guy' who loves the AOD (yes, overdrive). My Mustang's RE gear is 4.30. I'm in second gear within ten feet at the light, usually burning straight through without noticing the gear change until the tires hook. By then I'm into third gear. Overdrive makes this setup perfect for intercity expressway driving but the car is way too stiff for long trips. The engine is a EFI 351W (aluminum/iron) w/roller cam.

There are great transmissions out there with six gears, if you want. You need to decide how far away from 'stock' you want to go. You can re-invent the FE setups or save a ton of money and buy proven, more modern and more available setups that are stronger and lighter. Or, you can enjoy the FE for what it is. - Dave

02-13-2014, 03:47 PM
Thx Dave,
This car is an ongoing project that will change gradually throughout the years. I wanted to get it running and then go step by step from there. Hence the 3.89 one wheel RE. Eventually I would like something like a 3.89 posi and a 5 speed (maybe a Tremec) that has a low 1st gear and of course the 5th gear for cruising. I'm not driving this car much and I love the 3 pedal cars - I just like that feel of rowing through the gears, and I want it to be fast in the quarter mile.

For the next couple of years I'll have to keep the toploader cuz a proper 5 speed and rearend will cost too much to do right now. Maybe I should just look to see if I can find someone to trade, or sell this one and buy a used wide ratio. But then I'd have to get a longer drive shaft and correct size clutch. Maybe for now I'll look for a 4.30 3rd member to get my hole shot better. That would be the cheapest until I have enough to go to a 5 speed. Hmmm, spending money for a temporary set up, until I can save enough for the right set up......this'll take some thought.

thx, Dave J