Differential Expert Advice

mrfoamy

Mrfoamy
The clowns who installed my Carli lift and Toyo tires can't follow their own work order...installing four Toyo AT2 35x12.5R18 F Load tires instead of five, to replace my LT275/70Rx18E BSW A/T's on the factory 18" rims. So in the event of a flat my spare will be traveling 5% less far than the other side. I originally planned to return for a new Toyo spare but started thinking...5% is not a lot, and by applying excess pressure in the spare the diameters should be even closer. I'm thinking with a damaged tire in Grand Staircase-Escalante National Monument it would be prudent to drive to Flagstaff, potentially 400 miles, to a trustworthy tire dealer. Flagstaff is close to my home base in Sedona and that return trip could be one day instead of two (with a stop in God knows where for God knows what replacement tire). Any opinions on just how much wear 5% represents on a differential??? How about a return trip from Montana...pushing it too much?
 

ttengineer

Adventurer
What type of rear differential do you currently have?

Open, LSD, Lockers, etc.

Also, if you paid for 5 tires I’d damn well make sure they give you five.


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dreadlocks

Well-known member
If you went and got your original fixed asap and kept speeds down I think it'd be fine.. if you have to cover some distance mebe replace the diff fluid sooner than later since you ran it at a high duty cycle.. I'm no expert but in a pinch it cant be that bad, factory donuts and are rarely full size, and even if they are they are almost never identical to the rest.. I think the worry is putting on thousands of miles with the diff never catching a break and cooking up its lube, dramatically increasing wear.
 

Buliwyf

Viking with a Hammer
Spider gears and cross pins have no bearing, just splash lube.




When your diff is diffing, those spider gears and side gears get nuked fairly quick. I've replaced plenty of wobbly gears and worn down cross pins in 1980's GM cars. The kings of one wheel peel.

Keep your speeds under 55. I'd get another wheel and tire. Keep the spare. So you can have two flats and still get home.
 

mrfoamy

Mrfoamy
What type of rear differential do you currently have?

Open, LSD, Lockers, etc.

Also, if you paid for 5 tires I’d damn well make sure they give you five.
I think the worry is putting on thousands of miles with the diff never catching a break and cooking up its lube, dramatically increasing wear.
Never thought of that, good point, maybe drive slower than normal.

Paid for four, electronic locker
 

mrfoamy

Mrfoamy
Hmmm, seems to me that there must be MILLIONS of cars with mis-matched tires driving all over the U.S. Or are they much less than 5% diff in diameter?
 

1stDeuce

Explorer
5% is no big deal. The relative speed of the spider gears at 55mph or even 70mph is not high enough to cause them to wear the pins excessively if your diff fluid is at the proper level. Look at it this way, many new cars and even some TRUCKS come with mini spares. They likely have a 5% or more difference, and cause no damage as they are frequently driven for thousands of miles by the financially strapped, lazy, or unknowing.

Second example is the front differential in most 4wd trucks, which uses an axle disconnect. In that setup, the diff case stops with the driveshaft, and the spider gears rip as one shaft turns with the tire and the other turns the opposite direction. That's at no load, but my truck has been doing that for 270k miles and still the front diff is fine.

If you are really concerned, or need to use the locker some to get back to civilization, you could always put the spare on the front and swap the good front tire to the back.
 

Regcabguy

Expedition Leader
Hmmm, seems to me that there must be MILLIONS of cars with mis-matched tires driving all over the U.S. Or are they much less than 5% diff in diameter?
Those are the people who wear two kinds of shoes. I always buy a set of four. At the very least match them if you're going to buy just two.
 

mrfoamy

Mrfoamy
5% is no big deal. The relative speed of the spider gears at 55mph or even 70mph is not high enough to cause them to wear the pins excessively if your diff fluid is at the proper level. Look at it this way, many new cars and even some TRUCKS come with mini spares. They likely have a 5% or more difference, and cause no damage as they are frequently driven for thousands of miles by the financially strapped, lazy, or unknowing.

Second example is the front differential in most 4wd trucks, which uses an axle disconnect. In that setup, the diff case stops with the driveshaft, and the spider gears rip as one shaft turns with the tire and the other turns the opposite direction. That's at no load, but my truck has been doing that for 270k miles and still the front diff is fine.

If you are really concerned, or need to use the locker some to get back to civilization, you could always put the spare on the front and swap the good front tire to the back.
GREAT COMMENTS, and thanks everyone.
 
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