Anybody running different gears / tires front/back?

6BangBronk

6BangBronk

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Got a unique project in line and I think I could actually get away with not re-gearing one of the axles? It's a '60's F-100 with a factory flat bed. I'll be putting it on a 4x chassis but as it sits as a 2x, the rear rollers are slightly bigger than the front and to me it looks very good! Only talking 3.54 front / 3.73 rear so tires would be like 31's front 33's rear. I'd have to do the exact math and find the exact tires to run together but to me I think I could easily get away without regearing (Sorry Chase) and spend the money on tires instead? It'll be mostly street driven with maybe AT rear tires and street fronts.
But I could easily set it up this way and keep the nose up so it don't look all jacked up in the rear.

Any thoughts on this idea?
 
Depends on how comfortable you are with the t-case I guess. If its a 205, go with it. Anything less I wouldn't try it. And as long as you don't get out on pavement and thrash on it (in 4x4)
 
33/3.73=8.847
31/3.54=8.757
8.847/8.757=1.01

So the 33's with 3.73 gears will be turning 1% faster than the 31's with 3.54 gears. Measure your actual tire sizes and do the math. It's pretty simple.
 
Yea, I'd have to order 32"ish tires for the rear. But good point on the t-case though for it's getting a cummins/47RH/241 (Chain driven / aluminum shell) combo that just might not like it even if off by just a tad.

So I guess ECGS...
 
You would need to get those numbers very close, and pull them from actually measuring tires instead of the stated size. I would say that less than 1% probably wouldn't be noticed off-road.
 
4.11/4.09 is a 0.5% difference. 1% is technically double, but not that much more.
 
If my math is correct, for each revolution of your driveshafts your rear tires will travel 1/4" farther than your front. Doesn't sound like much but when your rear tires have gone a mile your front will have gone 50 ft less...

As long as you don't run it on pavement you'd probably be ok, but steering offroad may be an issue. Since your rear is traveling further, it will want to push your front tires.

Its actually fairly common in mud trucks to run a higher gear and/or taller tire in the front to make the front spin faster which helps steering. Same principle applies with RC cars when you overdrive the front axle or under drive the rear.
 
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Should be fine in low traction situations. Not recommended to be in 4x4 in high traction situations anyway. The math is (31/3.54)x3.73=32.66
 
My '97 4Runner has miss matched gears front to rear, bought it that way. 3.90:1 front 4.10 rear, seems the 3.90 gears are pretty damn expensive when they go, previous owner may not have even known that 4.10 had been installed.
I've used 4x a few times, pulls well but but I've always been in loose/wet conditions. Pulled real hard to the right when driving in snow, but that was probably more due to the steering rack being fubar at the time.
I've put in 4x once on hard surface, and it bound up pretty quick.

As for regearing the front, I have a 4.10 front diff, just been too lazy/cheap to swap it, never take it off road anyway. Meh
 
1% on a 33" tire is 0.33". Tire pressure could cause that much difference. If you really want, run the 31's a little higher on pressure and the 33's a little lower. As someone who knows about the science of metals, I'd run it regardless.
 
I've seen a few threads on Pirate about guys using different gears in each axle because of availability. I want to say it was rigs running 5.13s and 5.38s but may have been 4.10s and 4.30s. And they were running the same size tires front and rear. Not exactly and apples to apples comparison of your issue but may give you a direction to look for more info. Personally I think you would be ok as long as you keep it in the loose stuff.

Duane
 
As long as you got it pretty close you'll have no problems. Especially if you only use 4wd in low traction situations. Lots of old fords came factory with a little mismatch. My 78 trail rig had 3.54 front gears and 3.50 in the rear
 
Lots of vehicles still come with mismatch. 4.09 to 4.11, 4.86 to 4.88, 3.50-3.55, etc etc.

It's fine. Run it.
 
jeepinmatt said:
1% on a 33" tire is 0.33". Tire pressure could cause that much difference. If you really want, run the 31's a little higher on pressure and the 33's a little lower. As someone who knows about the science of metals, I'd run it regardless.
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Don't forget our friend Pi. Circumference = Pi x Diameter. So on a "true" 33 that 1% is about an inch. You can get into discussions about rolling radius when trying to figure this out. For street, less than 1% gearing mismatch should be okay. I would not do more than 2-3% offroad. One end or the other will always be fighting you. If you do anything you would want the front to be geared to roll slightly faster than the rear to pull it along, you don't want the drivetrain binding up "pushing" the axles together.

In the 1970s Chevrolet did some prototyping on an "automatic" part time 4wd system that used this principle with a one-way clutch on the front driveline. On the street the front axle would over-run the drive train until the rear axle would slip (slowing forward motion) then the drive train would "catch up" to the front axle and sent torque to it. I will have to look back and find that article, something I read a couple years ago.
 
OnlyOneDR said:
Don't forget our friend Pi. Circumference = Pi x Diameter. So on a "true" 33 that 1% is about an inch. You can get into discussions about rolling radius when trying to figure this out. For street, less than 1% gearing mismatch should be okay. I would not do more than 2-3% offroad. One end or the other will always be fighting you. If you do anything you would want the front to be geared to roll slightly faster than the rear to pull it along, you don't want the drivetrain binding up "pushing" the axles together.

In the 1970s Chevrolet did some prototyping on an "automatic" part time 4wd system that used this principle with a one-way clutch on the front driveline. On the street the front axle would over-run the drive train until the rear axle would slip (slowing forward motion) then the drive train would "catch up" to the front axle and sent torque to it. I will have to look back and find that article, something I read a couple years ago.
Click to expand...

Chevys with full time front axles and anything with 203 transfer case. 203 has a differential in it for this reason.
 
The 203 is not what I was referring to above. The 203 allows full time operation not because of different gear ratios but because when turning a front axle actually travels a slightly wider arc than the rear axle. The differential in the 203 allows this to happen without binding. Of course, it could also compensate for ratio mismatch if needed.
 
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