How similar does 4x4 gearing need to be?

If you're swapping axles on a part time 4wd vehicle that will only be used in 4wd on loose surfaces how close does the axle gearing need to be? If your front axle has 3.73s but you can only get the rear you want to use as a 3.70 is that close enough or will it cause noticeable issues? 
 

I believe some off road racers actually use slightly taller gearing in the front axle to help with high speed stability so it seems like some variation may be acceptable. Anyone got any input or rules of thumb?

The difference in gearing in your example is perfectly fine.   OEMs will sometimes have variances close to that, due to different  differential assemblies that have ring and pinons come out close, but not exact.   I often will run different pressures f&R that will cause greater variances than that.

In fact, just driving in a circle will be more likely to cause drivetrain bind than that little of a difference in gearing.

I once drove a Chev pickup with 3.42s in one end and 3.08s in the other.

It was very interesting...

There was a Ford that had a slight difference from the factory 30 or so years ago.  Like, similar to your example. It will be fine on loose surfaces.  

In reply to Russian Warship, Go Berkeley Yourself :

Or having 1/4" less tread on your rears if the diameter of the tire is sufficient......

Are they actually different or are they both 10 tooth pinion and 37 tooth ring? Aren't 3.73 gears really 3.70 with a slight adjustment for the offset input?

You can get away with big differences if it's only used on loose surfaces, did that by accident once...then forgot to switch back to 2WD before going on the pavement and I thought that would be the end of the event right there. Drove straight onto some grass and the day was saved.

Interesting question- I'm wondering what gears are in modern Chevrolet Silverado models.  When I drive in 4WD/snow I'm always checking to be in snow and not dry pavement.  

Saying that a knucklehead at work borrowed our POS 250,000 mile crap Silverado and left it in 4 high for a week.  He thought it was AWD and didn't know there was a difference.  Nothing ever failed. 

I'm dusting off un-used brain cells here but I recall my 323GTX had a 3.9 at one end and 4.1 at the other. It had a button to lock the center diff and it was only to be done on loose surfaces. I knew the guy that had the other GTX in my city and he broke an axle turning into a driveway with the diff locked.

Datsun310Guy said:

Interesting question- I'm wondering what gears are in modern Chevrolet Silverado models.  When I drive in 4WD/snow I'm always checking to be in snow and not dry pavement.  

Saying that a knucklehead at work borrowed our POS 250,000 mile crap Silverado and left it in 4 high for a week.  He thought it was AWD and didn't know there was a difference.  Nothing ever failed. 

I stick my 2000 Silverado in 4hi some time in October and pull it out in April or so.  310,00 km...

matthewmcl said:

Are they actually different or are they both 10 tooth pinion and 37 tooth ring? Aren't 3.73 gears really 3.70 with a slight adjustment for the offset input?

Teeth is teeth, changing offsets won't change the gear ratio.

A 3.73 ia a 41/11 gear.  Probably closer to a 3.70 than running the same tire pressure front and rear in a nose heavy truck.

In reply to Pete. (l33t FS) :

Ah, thanks for that. For some reason I was only seeing people listing Ford 9" 37:10 as 3.73 last night. I was thinking some fuzzy things happened with hypoid offset, but it has been a long time since I have looked at gear design. Oddly, a sagging tire from heavy load or under inflation does not really change circumference much, even though the radius to the ground does. The full tread still goes around every rotation. You really only have the size change from stretching the belts, which is smaller. Thanks again for the 41:11. I have no idea why I couldn't find that, but I couldn't.

In reply to matthewmcl :

So, years ago, I had a 4 wheel drive Subaru.  The "transfer case" was a shift collar on the output to the rear driveshaft, you could shift into and out of 4wd at any speed.  I found that with the factory recommended tire pressures, it would bind a little in 4wd on the highway on dry roads and it popped/banged pulling it out of 4wd.

With some experimenting, I found that if I ran 44psi in the front tires and 35 in the rears, I could effortlessly slip out of and into 4wd, proving that it required those tire pressures to have equal rolling circumference.

Circumference *should* be the same, because the steel belts in the tread aren't very elastic, but I suspect that tread squirm has a say in the matter.

In reply to Pete. (l33t FS) :

There is definitely some stretch to steel, it is just not as dramatic as a flat looking tire makes it appear. I both want, and do not want, an old classic Subaru like that.

Thanks for all the responses, good to hear it shouldn't be a problem. Hadn't considered the difference tire pressures can make, interesting point.

In reply to matthewmcl :

It was 8-10 years old when I had it, was severely rusted, and had a top speed of 87mph in 4th gear. 5th slowed down.

But it also held a Mitsubishi 42" projection TV with the hatch closed.  Fit all my bike and other life stuff with ease.  Miss the car but also don't miss it, you know?

Bold to assume I have steel in my tires...

In reply to Russian Warship, Go Berkeley Yourself :

Bold to assume those are kept to tolerances close enough that you'd notice a small difference in gear ratio, too.

As to the rolling diameter not affecting rotational speed, how does a tire pressure system that runs off wheel speed work?

In the case of an underinflated tire, the radius decreases. That means it rotates more quickly than the other tires, because it does.  Circumference has nothing to do with anything, because you own a differential.  If you do not have a differential, you will tear E36 M3 up rapidly.

In reply to Pete. (l33t FS) :

Bold of you to think that there is only a small difference in rolling diameter at the shown 0.5 lb pressure.

Always soft?  I wouldn't be too concerned.  On solid ground I would need to know what xfer case.  Some of them like a Dana 18 will snap a U-joint before the transfer case gets a scratch, but I had an explorer AWD in the shop with a clunk noise.  The chain in the BW tcase had stretched so badly that it was getting tight on one side and skipping a tooth.  The cause?  One tire was a couple psi low.

In reply to Curtis73 (Forum Supporter) :

Dana 20, so no chain to stretch.

In reply to Racingsnake :

Full send.  Just don't drive it on solid stuff.