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Tire growth

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masracingtd1167:
Any way to determine how much a tire grows at speed ? Thinking about the 2585 Good Year in particular . When I went from a 4.56 to a 4.86 my car picked up . It crosses at around 9400 right now so I am thinking what's a couple of hundred more rpm gonna hurt ! So I am thinking of putting a 5.0 gear in it . My car is around 100 more lbs than a B/ND would be .

digster:
 Bill I don't think the percentage of tire growth number is  linear. Rim size and tire pressure will be big factors. Perhaps one of the experts will chime in with a formula. I'm interested too. I just switched to the 2585 and 12 in rims.

JrFuel Hayden:
Yes, slicks grow at speed, and yes it depends on wheel width, tire pressure, tire style, size, and tire brand. Looking at finish line photos it looks like around 2" with the Goodyear 31x12x15 on 12" rims.
One of our JrFuel racers, didn't like running the GY D2585 on 12" rims, because they didn't grow enough, so he ran 11" rims unless he needed more bite and then ran 12". He was using a shorter stroke 387 ci SBC, and his motor was out 50", different combo than ours.
My experience has been mostly with 31x12x15 GY on 12" rims because we seem to need the 12" traction. When I'm deciding on gears and tires, I use one of the online formulas. Because I use a RacePak I can see what RPM we get thru the lights, I have found the computer and the online formula is pretty correct, because of converter slipage it makes up for tire growth. I found I just need to add 1/2" to the 31" tire to get the numbers to add-up to the RacePak.
Just figure the tire size with no or very little growth to get your gear change RPM. If you don't have a data computer, try using a GoPro aimed at your Tach, or get a play-back tach. 

Jon, 800-624-3803

coupemerc:

Bill, A couple of years ago I got this from Todd Swaim...

You can expect the following increase in diameter in inches for the 31.0x12.0-15 tire.
100 MPH = 2”
150 MPH = 3.3”
200 MPH = 4.6”

Those numbers seem high when compared to my Racepak data. My data agrees with what Jon posted. The tire could very well grow like Todd says but because of converter slip, add about 1/2-3/4" to the 31" diameter and the speed/rpm equations work well.

masracingtd1167:
Thanks for the information Jon and Chuck . If I go by a half to three quater it comes out really close to where I am right now . 

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