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Running two different tire sizes

protechk5

protechk5

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I'm 99% sure ill be fine but please ensure me. Will I be alright in really soft sand with two 40" MT/r's in front and two 42" MT/r's in back? Ill air down the rear more than the front, so that should help out. Ill obviously not have it in 4wd on anything hard.

The only reason I am doing this, is bc I put a 4" cut in my sidewall. Ive been looking for 4 new 42's but could only get two for now. Goodyear MT/r's are on a national back order. Im going to the beach and would like to take my truck, so this is the only time I will run it like this.
 
I would keep them aired up to whatever they should be aired up to, other than that...shouldn't blow the truck up...in theory...

Don't run it in 4wd at all with the tires like that. Your front would be turning at a different ratio than the rear...not a good idea. Not for extended periods of time, at speed...
 
Years ago I knew someone with a 4x4 sand drag truck, it was an old Scout II with the front axle located farther forward. Anyways, he purposely used smaller tires in front to spin faster and help pull the truck straight down the run. This was with the same axle ratios front and rear. Said if he used the same size tires front/back the truck would not hold a straight line.

Personally, for how you plan to use it and the terrain, I think you could get away with it. But I don't have any personal experience with what you plan to do, I'm commenting on theory only.
 
I once ran a set of 3.73 gears in the front and 4.56 gears in the back. Truck handled a bit funny (dragging the tail around) but it did just fine in soft and muddy terrain
 
protechk5 said:
I'm 99% sure ill be fine but please ensure me. Will I be alright in really soft sand with two 40" MT/r's in front and two 42" MT/r's in back? Ill air down the rear more than the front, so that should help out. Ill obviously not have it in 4wd on anything hard.

The only reason I am doing this, is bc I put a 4" cut in my sidewall. Ive been looking for 4 new 42's but could only get two for now. Goodyear MT/r's are on a national back order. Im going to the beach and would like to take my truck, so this is the only time I will run it like this.
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It'll be fine as long as it's on soft sand, just avoid extended 4WD at speed.

And 496, if he used smaller tires in front with the same gear ratio, the front would actually be going slower with ground speed and the back would push. Was it the other way around?

My father is actually running 44s in the back and 38s up front, but he has gears that more than offset the difference, so actual ground speed is more in the front, and it does go very straight in the sand drags. So it can work, but your transfer case has extra stress on it.
 
folkenheath said:
And 496, if he used smaller tires in front with the same gear ratio, the front would actually be going slower with ground speed and the back would push. Was it the other way around?
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We're talkin like 1993 or 94 here. I was a tech at a local Firestone tire store and the guy with the sand drag Scout was the store manager. I remember asking why he had smaller tires on the front but may have forgotten every detail. At the time I was into cars and had not yet owned a 4x4 so I may not have payed too much attention to his explanation either. :dunno:
 
and dont think cause its in snow on blacktop in the winter it will be fine.

local guy last winter blew up the front of his ttb dana 50 f250 . :doah:

265/75/16 on 1 end and 245/75/16 on the other = :weapon9:

but yep in sand you be fine. and dont forget tire size on sidewall dosnt = act size when mounted on rim and aired up.
 
Early Jeeps, and some present day trucks ran a very slightly "faster" front ratio than the rear.
The idea was that the front would always be pulling ahead a little, and would keep the system from bucking back and forth.
Also so that the front tires would be pulling through the bad stuff instead of holding the rear back.

Like the rear might be 4.11 and the front would be 4.10.

But, a big difference between the two like different tire sizes would put a lot of strain on the system.

I would turn the hubs if you have some, leave it in 2wd until the rears started slipping.
Back off on the gas, drop it into 4wd and get back on the gas.
As soon as you got through that spot, hit the clutch or drop the auto into neutral and shift out.
Doing that will usually release the bind enough for you to shift out.
 
I would think if you aired down so tire diameter is the same front to back or very close you would be fine. I remember several years ago a truck with big tires in rear and small in front and two different gear ratios but it was figured out so with the tire diameter difference it was like having same ratios in axles.
 
It's not really the diameter that matters, but the circumference. The circumference doesn't change a lot with air pressure, but it's enough for those ABS-based low tire pressure monitoring systems to work. So I doubt you can make 42s match up with 40s, but you can probably get two different brands of the same size to match. In the sand it's probably fine.

As for ice, I find 4WD pretty twitchy when the tires aren't a matched set. 2WD seems more stable, IMO.
 
I did it on a suburban with a 203 tc and it worked finr in the SAND! Tried it in the snow and you could feel the xcase binding. The truck had a 4:56 front and a 3:73 back with 33" tires.


On edit: if you just air down tires 4wd probably won't even be needed.
 
70jimmy said:
I would think if you aired down so tire diameter is the same front to back or very close you would be fine. I remember several years ago a truck with big tires in rear and small in front and two different gear ratios but it was figured out so with the tire diameter difference it was like having same ratios in axles.
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Blue85 said:
It's not really the diameter that matters, but the circumference. The circumference doesn't change a lot with air pressure, but it's enough for those ABS-based low tire pressure monitoring systems to work. So I doubt you can make 42s match up with 40s, but you can probably get two different brands of the same size to match. In the sand it's probably fine.

As for ice, I find 4WD pretty twitchy when the tires aren't a matched set. 2WD seems more stable, IMO.
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It's not either of those, it's the radius. Measured from the center of the hub to the ground. I'm thinking a 42" tire aired down enough can equal the same radius as a 40" tire with more air pressure. That measurement should be the one that matters. With that measurement equal the vehicle should act as though it has the same size tires. Theoretically. Right? :dunno:
 
True it would be the radius from center to the ground. I agree If you have enough tire to make that distance equal or close no problem. Most of us air down anyway and that distance may change from time to time as we stuff a tire vs just rolling on the ground but doesn't seem to have ill effects on the the ring and pinion.
 
I tried to make sure it would work but ran a fully aired up 38" swamper (my spare so to speak) with my 42s aired down. I am fully locked up. Didn't do it for long.

I would probably only shift into 4wd as needed though just to be on the safe side
 
Sure, the flat tire has a short radius to the road, but that doesn't equal the new effective tire size. The entire tread surface still has to pass to make one revolution of the tire. But this distance has still been reduced because the tire pressure is low and the tread blocks are pushed together on the bottom and overall it just doesn't have as much circumference. So the aired down tire does act smaller, but not as small as the hub-to-ground distance suggests by itself.
 
Blue85 said:
Sure, the flat tire has a short radius to the road, but that doesn't equal the new effective tire size. The entire tread surface still has to pass to make one revolution of the tire. But this distance has still been reduced because the tire pressure is low and the tread blocks are pushed together on the bottom and overall it just doesn't have as much circumference. So the aired down tire does act smaller, but not as small as the hub-to-ground distance suggests by itself.
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496truck is correct in stating that the radius from the center of the wheel/axle to the ground, or more accurately the "rolling radius", is what determines all of this. So you can air down a 42" tires so it has the same rolling radius as a 40" tire and therefore get the same tire speed between them.

You are correct in saying the entire tread surface has to contact the road for each revolution, but you have more of this surface touching the road for each degree of rotation because of the increased contact patch. Same type of deal with bulldozer or tank tracks........just because the total length of the tracks is 30' doesn't mean that the vehicle moves 30' after each revolution of the road wheels.

Anyway, back to the OP. There should be no issue in running 4wd in the sand with the mismatched tires, especially if you air the back tires down more than the front. There will probably be less difference between the front and rear due to the mismatched tires than if the ratios and tires were the same but you did a turn in 4wd while in the sand.
 
Tank tracks are a different situation. Your speed is set by the speed of the drive wheels, whether the track is 10' long or 100' long.
 
496truck said:
It's not either of those, it's the radius. Measured from the center of the hub to the ground. I'm thinking a 42" tire aired down enough can equal the same radius as a 40" tire with more air pressure. That measurement should be the one that matters. With that measurement equal the vehicle should act as though it has the same size tires. Theoretically. Right? :dunno:
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And of course diameter (or radius) and circumference are linearly related, so it actually doesn't matter. 2-pi-r, pi-d, r, d, are all just fractions of each other.

As pointed out, the concern is that the road distance traveled is the same for front and rear per given revolution of the axles. (Unmolested NP203's actually have a center differential to account for this very phenomenon :) ) In 2WD it doesn't matter since the fronts rotate freely.

The road distance traveled is indeed the actual circumference of the tire surface at whatever tire pressure. It'll also depend on how soft or stiff the sidewalls are, how heavy the truck is, and so forth, as to whether you could for instance squish a 42" down to be close in size to a pumped-up 40".

[/nerd] :haha:

-- A
 
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Lamda Lamda Lamda and Omega Mu. :haha: Best part of the movie is the talent show, though ... as spoofed on Family Guy, no less.


-- A
 
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