The Stovebolt Site Forums The Shop Area The HiPo Shop 4x4 conversion. . .more questions

I bought a 1979 Ford Front axle (8-lug) with 4.09 gear ratio. My rear dually axle has 3.73 gear ratio. What I am wondering is which one to change to match the other.

I have a 5-speed tranny with overdrive 0.73 ratio. If I am running 35 inch tires would I want the 3.73 or the 4.09? I am looking for driveability with towing/hauling capacity on occasion.

Any advice or experience would be appreciated.

Thanks,
Craig

You would want the 4.09. With the larger tires, you'll need the deeper gear to compensate for the larger rollong diameter plus the extra weight. Most liley, the gear for the rearend will end up being a 4.10 or 4.11. Don't worrry about the slight mismatch, that is common on 4x4s.

If you know what engine RPM you desire at what road speed you can then calculate and decide what gears, tire size, and transmission ratio will get you there.
Try this calculator:
http://www.onlineconversion.com/bigger_tires.htm

I'm not so sure that 4.09 and 4.11 gears in the same vehicle (with same tire size) are a good idea?
Get a few more opinions on that before deciding.
Mud is one thing, but if you plan to use it where you have reasonable traction for both axles it could be a problem.

Grigg

I used to think the same thing before I went to auto mechanic school. In the transmission and driveline class, the teacher (who had been teaching mechanics since the early '50s) asked us the same thing. The entire class said they had to be the same. He pointed ut that with most 4x4s, the front and rear axles have different ring gear sizes, so the ratios will be slightly different than the advertised spec. Most manufacturers will round it off to the nearest common ratio. So inthe case of the truck that Craig got his axle from, the rear axle was most likely a 4.10.

you can also compensate different front/rear ratios with different size tires. if you don't mind the look

I don't know the answer to this question, but I would say that some four-wheel-drive vehicles act like they have different ratios at each end.

What I mean by that is once the transfer case is engaged suddenly they are hard to steer and feel heavier in the way they respond.

I won't name names, but GM did build every truck I've ever owned including the newest one, which is a '91. That truck is awful in four-wheel.

I'd suggest at least browsing some four-wheel-drive forums to see what they have to say.

Good plan.
I don't know enough to say for sure, but worth looking into.

I have heard that one hundredth difference in ratio is OK, and occasionaly used in original trucks. But I feel (don't know for sure) that identical ratios would be preferred, and if you did have different ratios the front should be the faster one.

If in your research you decide that matching ratios is actually preferred, then you're building the truck so may as well build it the best you can.

If you have a transfer case with an inner axle differential then it doesn't matter as much, but is that an option, and if so what transfer cases have it? I don't know.

Another thing, It's not "the size" of the ring and pinion that matters, it's the number of teeth, which when divided gives the ratio we're accustomed to seeing.
Curious if different manufacturers round differently and what we're really looking at sometimes is the exact same ratio if you actually do the math. For example 41 teeth on the ring gear to 10 on the pinion is 4.10:1 ratio (no rounding there though). What other combinations give a similar ratio?

Grigg

I just opened Excel and played a little.

45 and 11 yield 4.090909091.

This is interesting enough I may have to do some reading to better understand the issues behind the choices. I suspect it comes down to manufacturing simplicity.

you have to go up or down in equal or partial tooth amounts like; 82/20=4.10, or 20.5/5=4.10, bec of the .1, but obviously not practical

I am a little confused as to how this is all going to work. on the ford the frt pinion is offset to the drivers side and the gm is offset to the passenger side as is the t/c drop. you might have an oil pan in the way
ron

A 9:37 gearset gives a 4.1111111 ratio, which is the nearest thing to the 41:10 without being an exact match. I've seen that setup on some GM 4X4's, IIRC.
Jerry

Padresag,

The '91 Chevy tranny/transfer case has a driver side output shaft for the front. . .so the Ford front axle is what I needed with the drivers side pinion.

Grigg,
Thanks for the link to figure out ratios! If I stick with the 4.10 rear gear ratio, I SHOULD be cruising at 65 mph and only turning around 2000 RPM. Works for me.

What I don't know yet is how the weight of the truck will affect this. Won't it take more power and fuel to move a larger truck than a smaller one?

Craig

Yes it will. Whether the difference is discernable depends on other factors.

You don't appear to have said what the truck is, or what the motor is in this thread.

What motor?

How heavy will the finished truck be?

Sorry for the lack of info! The truck is a 1950 one ton. Engine is a 1965 292cid bored .030 over with .020 off the deck and .010 off the head. Stock other than that.

I might be putting a 2004 Dodge dually bed on it (narrowed)or a flatbed (depending on the looks of the Dodge bed!) The flatbed is quite heavy. Maybe the overall weight of the truck would be 4000 lbs?

Saturday I mocked up the front axle and suspension. . .Ends up being a little higher than anticipated/wanted. The 35" tires look right under it - quite tall!

Craig

I'm not an expert but I suspect with the 4x4 conversion you are under-estimating the final weight.

I'm a torque junkie, I'd put a bigger motor under the hood for a project like this one. At least a 454 and probably something like a 500 Cadillac.

I actually have a Buick 455 with turbo 400 transmission that I was thinking of using. I would then use an NP 205 divorced transfercase for the 4-wheel-drive. Now things are really looking heavy! Concerns were that the front driveshaft would be too long, rear driveshaft too short, steering clearance issues, fuel economy etc.

Plus I have to admit I've always loved the 292 ever since my first one! That engine never gave a lick of problems!

Craig

That Buick makes nearly 500 foot pounds of torque and isn't real heavy compared to the original motor.

Instead of using a divorced case why not eliminate the tailshaft and short driveshaft by marrying the transfer case to the TH400. That would solve the other concerns.

I've seen a nailhead in a 1-ton but don't at the moment ever recall seeing the newer big Buick in one of these trucks.

the 292 will do most things that you ask it to do
ron

" the 292 will do most things that you ask it to do"

I'm not looking for huge power. . .I know I'll get there, just not as fast as the next guy. I only occasionally pull trailer with skid-loader or construction tools. So most of the time I'll be cruising with only the weight of the truck and one occupant.