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Gear ratio vs Tire size ??? #538944 11/28/09 09:34 AM 11/28/09 09:34 AM
Joined: Oct 2007 Posts: 1,031 Raleigh N.C H hemiparts OP "Missile Pilot"
hemiparts OP "Missile Pilot" H Joined: Oct 2007 Posts: 1,031 Raleigh N.C	I know that if you use a taller tire that you effectively drop your gear ratio ( 5.13 gear with a 29 inch tire will act like a 5.00 gear ect...) How much change can be expected in changing to a shorter tire ??? ( 29.5 down to a 28) ????

Re: Gear ratio vs Tire size ??? [Re: hemiparts] #538945 11/28/09 09:41 AM 11/28/09 09:41 AM
Joined: Jun 2003 Posts: 52,972 Romeo MI MR_P_BODY Master
MR_P_BODY Master Joined: Jun 2003 Posts: 52,972 Romeo MI	Figure it out as a percentage change

Re: Gear ratio vs Tire size ??? [Re: hemiparts] #538946 11/28/09 09:52 AM 11/28/09 09:52 AM
Joined: Jul 2006 Posts: 4,595 On the south side of Nowhere S S/ST 3040 master
S/ST 3040 master S Joined: Jul 2006 Posts: 4,595 On the south side of Nowhere	5.13/28 = N/29 N = effective gear Multiply 5.13 x 29 then divide by 28 or using a 28" tire with 5.13 gear is the same as using a 5.31 with the original 29" tire. You can substitute the variable to find any of the combination. The text we have to use here doesn't illustrate fractions very well.

Re: Gear ratio vs Tire size ??? [Re: S/ST 3040] #538947 11/28/09 12:15 PM 11/28/09 12:15 PM
Joined: May 2004 Posts: 3,210 robin hood country deaks master
deaks master Joined: May 2004 Posts: 3,210 robin hood country	Why do some tyres have differences in circumference although they are the same diameter. Mick 69 Dart GTS 440 mopar .590 cam, Edelbrock heads, 3200# best et 6.45, 106.78, 10.14, 132.88 mph, 1.47 60ft best 60ft 1.36

Re: Gear ratio vs Tire size ??? [Re: deaks] #538948 11/28/09 01:10 PM 11/28/09 01:10 PM
Joined: Oct 2007 Posts: 4,213 New York polyspheric master
polyspheric master Joined: Oct 2007 Posts: 4,213 New York	Strictly speaking, they must have the same circumference (since it's C = Pi � D), but the expansion factor (how much the tire grows at speed) will differ from carcass construction (number of plies), pressure, rim width, etc. The circumference is probably a more reliable figure. Boffin Emeritus

Re: Gear ratio vs Tire size ??? [Re: hemiparts] #538949 11/28/09 03:13 PM 11/28/09 03:13 PM
Joined: Oct 2004 Posts: 1,946 Connecticut 5126brl No more top fuel
5126brl No more top fuel Joined: Oct 2004 Posts: 1,946 Connecticut	www.slowgt.com/calc.htm all kinds of calculators to do gear and tire stuff go to bottom of page and pick a page Last edited by 4406brl; 11/28/09 03:16 PM.

Re: Gear ratio vs Tire size ??? [Re: deaks] #538950 11/28/09 11:22 PM 11/28/09 11:22 PM
Joined: Aug 2003 Posts: 43,308 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,308 Bend,OR USA	Quote: Why do some tyres have differences in circumference although they are the same diameter. Mick The tire makers will post the sizes to make the public understand the sizes, then they will post the circumfeences for the racers looking at all the information, not just the size. I seem to remember some classes requireing a set size, some to the different brand tires sizes varied a little above the spec. and some varied a lot above the spec IE 9x30x15, maximum NHRA circumference allowed is 94 inches( 30 times Pi(3.14) = 94.2) I use to buy Firestone 9x30x15 that where 96.5 or larger regularly I would ask for the largest circumference sets they had, all of them where marked from the factory Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

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