I was wondering for emergency use only, if you had to use the 33” spare you could air down the 35’s to 20 psi and air up the 33 to 50psi and disable the limited slip function? Therefore, decreasing the circumference difference and get you home without damaging the drivetrain? You know if you’re driving a big circle the drivetrain compensates for the different speeds of the outer tires.
33” Spare with 35" Tires
- Thread starter Lextech
- Start date
That is a good question that I had for my Explorer ST as I have the same situation. Running larger tires than the spare. The computer will generate a 4WD error and disengage the system until corrected.
The answer seems to be yes, you can deflate the larger tires and make sure the the smaller spare is fully inflated so that it doesn't have a big bulge at the bottom to compensate for the size difference. This should get you out of a pinch without damaging the drivetrain. I imagine Lexus has similar driveline safety protocols as Ford does.
Below is something that I pulled from another conversation on a different forum.
The usual concepts of diameter, circumference, and radius we apply to a solid objects don't work very well for tires. That's because tires are flexible and don't roll at any diameter/circumference that can be measured directly.
For example: If one takes a freestanding tire - one jacked off the ground - and measures its circumference, then lowers the vehicle to the ground, then measures how far one revolution moves the vehicle, that second measurement will be about 97% of the first measurement.
For this reason tires generally use revs per mile as an indicator.
HOWEVER, rolling diameter does change with inflation pressure. That's because the steel belts are 2 layers parallel wires one angled the opposite direction to the other. X-ray the tire and the belts form a diamond pattern.
Continuing that analogy, if one were to pull on opposing corners of a diamond (which is a form of a parallelogram), the diamond gets longer, but narrower. The opposite happens when the belt goes through the footprint - it gets shorter and wider! Not a lot, but you can measure it (with difficulty!) That is called pantographing.
One can actually measure the difference in rolling circumference due to inflation pressure. I did it once and published the results in some thread on some web site, but I can't seem to find it now.
It's that phenomenon that is used by some vehicle's ABS system to sense changes in inflation pressure, instead of directly measuring the inflation pressure.
And a comment on rolling diameter vs brand of tires: There is no law that says a tire of a given size MUST have a certain diameter (rolling or otherwise!) - and that means that it is possible to ruin an AWD unit by mixing makes/models of tires on one of those! That's why tire manufacturers do NOT recommend mixing makes/models on AWD and 4x4's. (Please note: It is also possible for different makes/models to have similar rolling circumferences (revs per mile) - and there would not be a problem, but unless you know for certain, it's not a good idea.)
No, you can't do that without causing problems. Airing a tire down does not change its rolling diameter, the length of the tread is still exactly the same. An aired down 35" tire will roll the same distance as a fully inflated 35" tire with one full rotation, and both will be further than a 33" tire of any inflation level.
perfect explanation.
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