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Author Topic:   Changing tire size
hobby39
Dirt Maniac

Total posts: 112
posted June 09, 2005 05:49 AM  
The track I run at changed their rules for this year and are allowing 60 series tires so last year we had to run 70 series. Were going from 23570R15 to 25560R15 I was told there should be about a 2 inch smaller roll out. Is there a way to determine the difference in RPM. My final out is right around 6.0.

dm3
Dirt Full Roller

Total posts: 40
posted June 09, 2005 07:14 AM  
Scott,
I did the math, and it came out that a 27560R15 would be the same roll out as a 23570R15. When I got the tires they were an inch taller and I had to go up to a 6.20:1 gear. I suggest you measure the tires before buying anything. I still haven't figured out why the "tire math" didn't work. Dennis.

29
Dirt Roller

Total posts: 21
posted June 09, 2005 07:37 AM  
The tire rollout difference for those two sizes will be a little less than 3" given that you are using the same brand tire. You could see an increase somewhere around 200-300 rpm's by the reduced rollout.

DangerZoneRacing
Dirt Maniac

Total posts: 176
posted June 09, 2005 08:46 AM  
I use to run a 235/70R15 on my right front...now im running the 255/60R15 and I got a 4" difference between the 2. The 255/60R15 being 84" and the 235/70R15 being 88". both measured at 30 psi. I was once told 1" =100RPM. Its realative close to follow by that, I've been playing with tire sizes on the rear to get my rpm's higher by a little. Its like just under 100rpm per inch of tire, close tho. JMO.

hobby39
Dirt Maniac

Total posts: 112
posted June 10, 2005 06:00 AM  
Dennis,
I'm not real worried about the roll out difference I run basically run zero stager and were getting 4 new tires so we should be fine with stagger. I could probably use 500 or 600 more RPM but I really was just curious what the RPM difference would be so I'd have an Idea and didn't have to worry about keeping that close of an eye on the tach. See you at the track tonight.

Thanks for the info that 1" = close to 100 RPM that basically the info I was looking for. I'm also switching brands for those of you wondering. There going to have to wait till next week as stated in my other post I'm also getting new wheels and were going to wait till I get the new wheels before mounting the new tires. Scott

    

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