Rear tire size

[joedoozer]

Going to a new track this week. I want to mount a harder set of tires for the rears, in case I need them. What kind of RPM change can I expect going from a 92RR/88LR to a 89 or 90RR/85 or 86LR?

[4bangerhotrod]

a couple hundred rpm, but dont go down to that small of a tire if you dont have to. theres a reason they have a max tire size.

[joedoozer]

It's just in case. I have never been to the track and I want to make sure I have the right compound if I need it. I really don't think I will.

[sj valley dave]

Are you talking about the stamped size or real measured size? We have to run D-55's and have gotten as big as 95" and as small as 85 1/4" out of a stamped 92 and 88..LOL

[joedoozer]

I don't know exactly was size they are, I haven't mounted them yet. They are stamped 90 and 88. The groove does move down the track as the night goes on, I just didn't want to be way off with smaller tires. Roughly how much RPM per inch is it? And what is "too small" to run on the rear? I have other tires in my pile, these were just the best 2.

[drtrkr244]

Going to a new track this week. I want to mount a harder set of tires for the rears, in case I need them. What kind of RPM change can I expect going from a 92RR/88LR to a 89 or 90RR/85 or 86LR?

That caculates to about 2%. For example, if you are turning your motor 8,000 rpms, you now will be turning approx. 8160.

So you should drop down 15-20 points on your rear gear.

[AmickRacing]

Are you talking about the stamped size or real measured size? We have to run D-55's and have gotten as big as 95" and as small as 85 1/4" out of a stamped 92 and 88..LOL

I'm not sure what's going on with the tires this year (WRS-2 55's). The 90's are turning out about average in size, but the 90's I've mounted and seen mounted are in the mid 89's when measured. Dang tough to get stagger with a tiny RR!

[JJ128]

Take the bigger tire size divided by the smaller tire size, you will get a number greater than 1. Take that number times hom many RPMs you turned with the bigger tire and that will be how many RPMs you would turn with the smaller tire, assuming you are running the same speed.

Or, take the smaller tire size dived by the bigger tire size, you will get a number less than 1. Take that number times your current gear ratio, and that will tell you what gear you should run to turn the same RPM with the smaller tires. Again, assuming you are running the same speed.

With your example, the difference will be what drttrkr244 said.