Mounted some Mickey's and checked circumference. Mickey says 1/2" is max allowable. I have 98 7/8 and 99 3/4 or 7/8" difference. I had this issue many years ago but I can't remember the numbers. I called Mickey and was told too over inflate the small tire and let it sit in the sun and it seemed to work. At that time it was 105 in the shade, right now it's 50 here. I'm asking for real life experiences before I call MT.

I am like most everyone when it comes to matching tires, I want it exactly the same or within 1/4 inch at most. That being said, it is difficult to get a perfect match, not impossible, but difficult. Especially if you get them from a mail order store.

I have also been told that 1/2 inch difference is allowable. And yes, I have had some out by more than that. You definitely can put around 20 psi in the smaller tire and let it sit in the sun to "stretch" it out. It is a process that requires you to measure the tire every hour or so (depending on outside temperature) to get it the size you want. As with anything, it's not always an exact science. The tire will shrink back down some after it is set at the desired pressure and cools off, so you may have to do the process more than once.

Since it really isn't Spring or Summer, it would take a nice sunny day and more time to stretch the smaller tire. It might be better to call the business you bought them from and ask for a better matched set or send a single tire they have that matches either of yours.

It's good practice to always check the circumference of new tires once mounted and then to check them periodically through the life of the tires.

*You didn't say what size the tires are or what design. Are they small tires, radials or bias ?

They are 31.25 x 12.2 3073W bias.

If you have air in the tires now let all air out of the large one now. Try this first. Take the larger tire and try to shrink it. Put no more than 1# of air in it and place it in the sun for several hrs if possible. Flip it over 1x. You have a good chance of shrinking the large one to get it closer to the small one.

Living in WI (just snowed again today) I always try and change tires in June/ July so I am able to use the full sun if needed.

quote:

Originally posted by markemark:
If you have air in the tires now let all air out of the large one now. Try this first. Take the larger tire and try to shrink it. Put no more than 1# of air in it and place it in the sun for several hrs if possible. Flip it over 1x. You have a good chance of shrinking the large one to get it closer to the small one.

Living in WI (just snowed again today) I always try and change tires in June/ July so I am able to use the full sun if needed.

Agree with Mark 100%

I had a set a few years back that I mail ordered, mounted, and they came up 1" different in rollout. Called MT, they said I could try stretching/shrinking but I'd probably have to send them back. Since they were already mounted I figured nothing to lose by trying it, and I'd read here previously that people had more success shrinking them than stretching them but it didn't make sense to me.

I put .5-1# in the large one and 20# in the small one and left them out in the sun for 20 min. They got much closer together, with the large one shrinking more than the small one grew after airing them back up. Did the .5-1 and 20# for another 20 min, then they were the same. Then it was evening, the next morning they were 1/4" different. I stretched/shrunk one more time, then immediately put them on the car and made a run. They ended up being less than 1/8" different and maintained that for the life of the tires. The large one shrunk more than the small one grew by a lot. I wouldn't believe it if I didn't see it for myself. This was on a set of 3183s which are 33.5/16.5/16 so not sure if they are easier to change since there's more rubber.

I believe that the burnout/run "locks it in" as I had a set that were about 3/4" out and they didn't respond to this after they had been run. And I only noticed that driving through the pits, perfectly straight on a run, my guess is they grew the same.

^^^^ Agreed. Once you do the first burnout it’s over. What you see is what you’ll have afterwards. No changing.

When I put new tires on my car, I keep the sun off of them until I’ve been through the first BO. I’m fussy and want them 1/4” or less in difference if possible.

I was told by MT to shrink the large tire also and it worked for me

Thank you to all for sharing your experiences. IDK if it matters, but I've got race tubes in these tires. According to my weather app, 65 is about the highest temp for the next 10 days here. I've never heard of shrinking before, but very cool if I can get a result! I wonder if I fired up our wood burning stove in the living room would get some some movement! Mid 70's probably won't move them at all. I should've never bought tires from Jegs, but the deal was excellent and now I'm paying, lol.

This message has been edited. Last edited by: Ron Gusack,

^^^^^^Don’t feel bad about taking a good deal from Jegs/ Summit or others and getting tires that RO are different when inflated. I bought MT tires from Barker that were shipped direct from MT and it was a 3 week horror story to correct.

I had this idea when I was shrinking tires in the event I had to do it in winter. Make a cardboard box large enough to get the tire into. Take a section of flexible laundry dryer hose, attach it to dryer, then attach to box. Cut some slits on end of box to allow air to escape. Turn dryer on HI and monitor.

I like the dryer idea. Box or a small closet with a couple of heat lamps might do it to

My original idea was using a box and attaching the dryer hose to the exhaust pipe of my truck and think this could work as well while being outside.

Gree on shrinking the tire. I had A set I bought at a swap meet they where new and .750 difference. I let air out of the larger one let it sit then added some het with the blow heater. Rotating tire every few minutes. After two full rotations and tire is fairly warm. I let them sit and when inflated back to pressure it was 1/4" smaller than other one. It did stretch bak to about .375 larger.

Never had the stretching work as well but did it before finding out about shrinking tire.

I've never heard of shrinking slicks until this thread, thanks for the info guys!

As for dealing with RO issues, I've had none since switching to radials 10+ years ago. There's no plan for us to go back to bias slicks.

My newest Hoosier radials were advertised at 94" & measured dead-on. That reminds me, I haven't measured my new M/T radials yet; in the past they have been dead-on.

2BKING

Measured the new M/T radials & they're both at 94.4" at 20 psi.

2BKING

Yesterday I took the 98 7/8 tire and put 20 psi in. I took the 99 3/4 tire and put 2 psi in. About 20 hours later I put 14 in both and measured. They're both at 99 1/2. The tires sat in an unheated garage where the low was mid 30's and the high was mid 50's. Evidently they don't need heat to move, just pressure. Hopefully they'll stay the same, but this whole ordeal is unbelievable to me so tomorrow could be interesting.

Many years ago a Goodyear tire engineer told me that sunlight shrinks the sidewalls, and that I was correct to not make air pressure adjustments because of sunlight on one tire and not the other, which had always been my practice.
Every time I had tried that, the car always went (violently) to the side I let air out of.
He stated my belief was correct that during the burnout, everything equals out.

Just random notes.

In about 30 years of racing, I can’t say I’ve ever measured the circumference of a tire.

I live under a rock, and have never heard of shrinking the larger tire before. I sure would not have bet money that it would work better than stretching the smaller one.

quote:

Originally posted by Ron Gusack:
Yesterday I took the 98 7/8 tire and put 20 psi in. I took the 99 3/4 tire and put 2 psi in. About 20 hours later I put 14 in both and measured. They're both at 99 1/2. The tires sat in an unheated garage where the low was mid 30's and the high was mid 50's. Evidently they don't need heat to move, just pressure. Hopefully they'll stay the same, but this whole ordeal is unbelievable to me so tomorrow could be interesting.

This seems wild, that the tires still shrank in cold air. Crazy stuff for sure, and thanks for sharing!

quote:

Originally posted by Goob:
Many years ago a Goodyear tire engineer told me that sunlight shrinks the sidewalls, and that I was correct to not make air pressure adjustments because of sunlight on one tire and not the other, which had always been my practice.
Every time I had tried that, the car always went (violently) to the side I let air out of.
He stated my belief was correct that during the burnout, everything equals out.

Just random notes.

Never heard that one before, Dave. I don't carry a slick cover in the car, and when I sit in the lanes with sun beating on one tire I have always let some air out to even up the pressures just before a run. Might have to try out ditching that idea. Haven't had any noticeable extreme response from the car.

quote:

Originally posted by FootbrakeJim:
Never heard that one before, Dave. I don't carry a slick cover in the car, and when I sit in the lanes with sun beating on one tire I have always let some air out to even up the pressures just before a run. Might have to try out ditching that idea. Haven't had any noticeable extreme response from the car.

I do the same FBJ, might try not doing that this weekend.

2BKING