which final drive ratio?

[southysuper]

I've been building a custom mid engine multi purpose rig for the past year. I've been on a rather tight budget, but so far I have gathered most of the major parts. I'm getting a little confused with the math and would like some help. Most of the calculators I have found online do not seem to take into account my "unique" setup.

Specs:
Propane Fueled SBC producing about 380 HP
Trans THM 425 Transaxle XXX Final drive
Tires 37" BFG on steel 15s
Rear Ford 8.8 (4.10) Gift from Friend
Front Dana 44 (4.10) Gift from Better Friend

As you may have guessed, I'm using the transaxle turned sideways (like a rock buggy) to give me AWD. I have not picked up my trans diff yet, but my options are limited to the following ratios:

3.07
3.42
3.55
3.66
3.70
4.10

Any ideas? I’m thinking 4.10 off the trans combined with 4.10 would give me like 16 to1 which is a little two high engine rpm, right? Should I go with the 3.07? See where I’m confused?

[MR.ROCKAPE]

SINCE IM NOT SURE HOW GM DOES THEIR RATIOS ILL THROW OUT A GENERAL # JUST TO MAKE A POINT....


SINCE YOU HAVE A TRANS AXLE THEY HAVE GEARS AS WELL AS THEIR OWN RING AND PINION.....


SO FOR THE SAKE OF ARGUEMENT...

SAY FIRST GEAR IS 2.5 TO 1 THEN YOU MULTIPLY THAT TO THE RING AND PINION RATION THATS IN THE TRANS (JUST SAYING A # ) SAY 4 TO 1...

SO YOU MUTLIPLY THE 2.5 X 4= 10 TO 1

NOW SAY YOU HAVE 4 TO 1 AXLE GEARS.....SO THE MATH GOES 2.5(TRANS FIRST GEAR) X 4 TO 1(TRANS RING AND PINION) X 4 TO 1 AXLE GEARS = 40:1

IM NOT EXACTLY SURE HOW TO FIGURE TIRE HEIGHT INTO THIS.....

BUT AS A RULE OF THUMB ABOUT 50 TO 1 WITH AN AUTO TRANS IS A GOOD START...

SO IF YOU CHANGED THE AXLE GEARS TO 5.13s YOU WOULD BE DEEP INTO THAT 50 :1 CRAWL RATIO.

I HAVE A BUGGY WITH A 4.3 THAT MIGHT MAKE 200 HP IT HAS A CRAWL RATIO OF 45 TO 1 AND I HAVE NO COMPLAINTS....OTHER THAN IT TRIES TO IDLE THROUGH THE BRAKES!!!:Big Grin:

HOPE THAT HELPS!!!

[southysuper]

Thanks,
That is what i needed to know. Your avatar is awesome!

[BryanD]

yeah, how ever you skin the cat you should be 50-60:1 final with an auto IMO.

the biggest issue as Rockape is dealing with; how to disconnect front & rear drivelines for manuvaring well. youl need some driveline disconnects on the output shafts, or as Rockape is going to try, on the axle side.