Change Rear End Gears?

[plwindish]

I'd like to have a little feedback regarding changing out the rear end gears in my '89 convertible. I bought the '89 last December fully rest-modded with a Stroker 383, TCI tuned and controlled 4L60 trans and a 9" Ford rear end with 4.86 gears. The car is down right scary with the amount of power it has at any speeds. Driving along, stomp on it and you are flying. I've taken the car on extended drives the last couple of weeks going around 200 miles round trip to Milwaukee and around 350 miles round trip to the Peoria area. MPG for the first trip was around 16.5 and 17.5 on the second trip, keeping the car at highway speeds between 70 - 75. Any thoughts as to what mileage might be with this power train if I swapped the 4.86 gear set for something in the 3.50 - 3.73 range? Engine rpm's are not too obnoxious because of the automatic overdrive, but I'm sure the mileage can be improved going to a lower (numerically) gear without sacrificing much power.

I don't think I will be putting a lot of miles on the car, probably less than 5000 annually. A rear gear swap should not be all that expensive compared to like swapping transmissions out. I may be driving the car to Dover and I don't want to have the national debt from the drive out and back. Any thoughts?

[mfg]

The key here is that you will not be driving this Avanti all that much, because of that, and the fact that you're basically happy with the way the car performs now,.......I say leave the rear end gearing alone.

[billy]

The great thing about the Ford rear ends is you don't have to set up the rear end gears, you just drop out your center section & bolt in another center section with 3.00 gears or what ever you want. It usually only takes an hour to drain oil, pull axles a few inches & change a center section. Check Craigslist for a used center section.

[Devildog]

I would just leave them alone for an occasional drive. If you are going to swap-out, I would go with the the 3:55. I have a 700R4 tranny (same gears ratios) as your 4L60 which I left the shop talk me into 3:70's It is OK, but I would like the 3:55 for 80 mph cruising (Texas has many 85 mph legal). the old rule of thumb was get a 10-12 total with 3.056 x 3.55 = 10.9 3.056 x 3.70 = 13.3 3.56 x 4.86 = 17.5 wow

With the 383 you have nice torque at 1800-2000 rpm I have an old low compression 400 cu in and get about 20 mph at 75+ mph.

Joe

[plwindish]

As much as I like the convertible in the summer time, I believe I'm going to keep the gearing as is and have it as a local cruiser. The car is crazy fast, but with the modified sport suspension, I feel every tar strip on the road. I have taken the car on a 350 mile round trip and my '76 Avanti II is a much better road cruiser than the '89 is. In June, I'll be driving the '76 from Chicago suburbs to Dover for the IM.

Joe, using your formula, my '76 with the 3.31 rear end would yield 10.1. I have gotten 22.5 on the road before with the rebuilt 400 motor and a rebuilt 200R4 swapped in for the THM400 trans. The original buckets are not as comfy as the '89's seats, but the ride is much softer and I have a ton more leg room in it.

Decision made. Thanks for the input!

Edited May 19, 2014 by plwindish