Do you need to replace all of your tires?? Read first before making a thread.

Is the rolling circumference the main issue or is it the rolling radius? Even with the recommended 32 psi front and 30 psi rear, the front tire bulge just above the contact patch is a bit more than the rear. The part of the tire that controls the revs per mile is the radius from the bottom of the tire to the centre of the drive axle. I have a leak from a nail in the left rear tire that is too close to the sidewall to patch and if I get a new tire, I should be able to run slightly lower pressure to match the rolling radius of the other tires.

The tires are Michelin Pilot Exalto 205/55-16 which are no longer made. The tread depth seems to be close to new tire specs, so I may be able to get away with one tire of a similar design by adjusting pressure. I have measured the height of the bottom of the wheel plastic centre piece from the ground and it seems to vary from 10.25 to 10.75 inches which would equate to a much larger variation than 0.25 inches of circumference. As expected, the lower values are at the front.

Radius doesn't count for anything. You want your axles all to rotate at the same rate to keep stress off the viscous coupling. Each revolution of the tire goes down the road a distance equal to the rolling circumference, not related to the radius which is distorted by the weight of the car pressing down on it (only at the bottom though ) Measure the circumference of the tire off of the vehicle with a tape or ribbon.

While unloaded radius is exactly related to the circumference, you will get more measurement error trying to measure the radius and then calculating the circumference, or matching radius tire to tire.

Is the rolling circumference the main issue or is it the rolling radius? Even with the recommended 32 psi front and 30 psi rear, the front tire bulge just above the contact patch is a bit more than the rear. The part of the tire that controls the revs per mile is the radius from the bottom of the tire to the centre of the drive axle. I have a leak from a nail in the left rear tire that is too close to the sidewall to patch and if I get a new tire, I should be able to run slightly lower pressure to match the rolling radius of the other tires.

 

The tires are Michelin Pilot Exalto 205/55-16 which are no longer made. The tread depth seems to be close to new tire specs, so I may be able to get away with one tire of a similar design by adjusting pressure. I have measured the height of the bottom of the wheel plastic centre piece from the ground and it seems to vary from 10.25 to 10.75 inches which would equate to a much larger variation than 0.25 inches of circumference. As expected, the lower values are at the front.

The tires are Michelin Pilot Exalto 205/55-16 which are no longer made. The tread depth seems to be close to new tire specs, so I may be able to get away with one tire of a similar design by adjusting pressure. I have measured the height of the bottom of the wheel plastic centre piece from the ground and it seems to vary from 10.25 to 10.75 inches which would equate to a much larger variation than 0.25 inches of circumference. As expected, the lower values are at the front.

Have you looked to see if any used tires are available near where you live? Some racer-friendly independent tire shops will shave a tire to match others. I had one Michelin Pilot All Season develop a bubble, found a used one to trade for 2 of my used tires locally that ended up being so close in spec that no shaving was necessary (they measured it). Also try ebay..

Good info, I'm looking at replacing all 4 tires....my concern, is the sensor/valve stem a replacement item at this time? Thanks ahead for any leads or response.

redonly

If one had two equal sized tires, one that was 1/8" smaller in circumference, and one that was one 1/8" bigger, and had a manual Suby with an open front diff and rear LSD, what would be the optimal tire placement? Feel free to ignore handling, and only consider centre diff health, unless you want to comment on handling too.

My guess: equal sized tires at the rear, and tires with a net 1/4" difference on the front, so that the front two averaged out to the size of the rears, the open diff didn't get chewed up by the difference, and the centre diff saw equal sizes at both ends. Downside would be having the smaller front tire wear faster than the rears, so things would get worse with time.

I realise one could just buy new tires, but there are some cases where new tires would be worth more than the rest of the car, in which case one might try to limp by.

Thoughts?

Thanks,

Andy

go to tirerack.com and see if the tires are on there, they can shave tires for you to match treads better.

1/4 inch tread circumference isn't bad and is still within the acceptable tolerances.

try to match treads in the back to save the lsd. but like the all knowledgeable internet might suggest you should be fine as long as the rear tires are within 1/4 in circumference or 2/32in depth difference, if you need to wear the newer tires on front to wear them closer to same depths.

I also work at a car dealership as a technician. I always replace mine in pairs unless all four are gone makes sense really

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