changing tire size

Joe Capri85 · 03-20-2011, 10:22 AM

Hey guys I need new slicks and thought I would go a little bigger. Changing from a 28" tall to a 29.5 tall. good or bad choice and how much rpm do you think it will loose? Low hp car, 2910lbs track weight, dyno at 503 goes through at a 10.60 121 @ 6800rpm with a 4.11. I bracket and have been running a lot of 11.00 index, have been giving some thought to doing some 1/8th sst racing. The car is pretty consistant, but I figure a lil more tire wouldn't hurt. I also have a new 4.56 in the shop I could swap out but want to keep the rpm's around 6800. Thanks for any thought you all may have. Thanks Joe

Bill Baer · 03-21-2011, 11:26 AM

Well Joe 29.5" tire rolls 5.3% more per revolution than does the 28" so it should drop your RPM ~5.3% (6440 vs. 6800).The 4:56 is 10.9% more gear than the 4.11 which should have you spinning about 7140 with the 29" tire.
However if you can get something close to a 4:33 gear ( a 4:11 +5.3%) it should be right back @ ~6800

Joe Capri85 · 03-23-2011, 11:43 PM

Thanks Bill good Info.

03-20-2011, 10:22 AM	#1
Joe Capri85 Junior Member Join Date: Mar 2008 Location: Detroit area Posts: 14 Likes: 0 Liked 0 Times in 0 Posts	changing tire size Hey guys I need new slicks and thought I would go a little bigger. Changing from a 28" tall to a 29.5 tall. good or bad choice and how much rpm do you think it will loose? Low hp car, 2910lbs track weight, dyno at 503 goes through at a 10.60 121 @ 6800rpm with a 4.11. I bracket and have been running a lot of 11.00 index, have been giving some thought to doing some 1/8th sst racing. The car is pretty consistant, but I figure a lil more tire wouldn't hurt. I also have a new 4.56 in the shop I could swap out but want to keep the rpm's around 6800. Thanks for any thought you all may have. Thanks Joe

03-21-2011, 11:26 AM	#2
Bill Baer Senior Member Join Date: Jul 2007 Location: Nothern IL Posts: 596 Likes: 1 Liked 1 Time in 1 Post	Re: changing tire size Well Joe 29.5" tire rolls 5.3% more per revolution than does the 28" so it should drop your RPM ~5.3% (6440 vs. 6800).The 4:56 is 10.9% more gear than the 4.11 which should have you spinning about 7140 with the 29" tire. However if you can get something close to a 4:33 gear ( a 4:11 +5.3%) it should be right back @ ~6800 __________________ Bill Baer 3391 SC, 339B SC, QR Last edited by Bill Baer; 03-21-2011 at 11:39 AM.

03-23-2011, 11:43 PM	#3
Joe Capri85 Junior Member Join Date: Mar 2008 Location: Detroit area Posts: 14 Likes: 0 Liked 0 Times in 0 Posts	Re: changing tire size Thanks Bill good Info.