Gear & Tire size question

[Qwk88LX]

I have the gt-40x crate motor (other mods in sig.) with a C-4 w/ manual valve body. I currently have 3.73's but I plan on going with a lower gear. Either 4.10's or 4.30's. I'm also running 295/50/15 street tires. Since I don't have OD, I was planning on going with a taller tire: BFG 295/65/15 Drag Radials to somewhat compensate for the lower gears. I will also be upgrading to a 10" 3300-3500 stall converter. Does anyone know of a chart that gives the tire-gear final ratio? The car will be weekend driven, but will mostly be on the track.

side note: I also plan on swapping my heads for some Trick Flow TW heads & putting in a TFS Stage 1 cam, so I'll be running a Trick Flow HCI combo... I'm hoping for 300-310 at the rear wheels.

 

[jrichker]

WWW.corral.net has some calculators that will help.

 

[Qwk88LX]

Thanks, but everytime I click on "calculators" nothing happens, it just loads up the same page

 

[Michael Yount]

Quick tutorial - calculating the impact of tire diameter on effective gear ratio is just a matter of simple ratios. If you can multiply and divide (or can push buttons on a machine that can) you can figure it out.

Say you have 4.10 gears with a 28" tall tire. And you want to know what impact a 26" tall tire will have. 28/26= 1.076. 4.10X1.076=4.41. The 26" tall tire will make the 4.10's act like a 4.41 gear. Said another way - 4.10's w/26" tires is like a 4.41 w/28" tires. So, with 4.10's and 28" tires, you could either leave the tires and put 4.41 gears in, or you could simply put the shorter 26" tire on and leave the 4.10's in.

To calculate tire height - take the section width (295) and divide it by 25.4 to convert it to inches (it's in millimeters). 295mm=11.61" Now, multiply the section width in inches (11.61") by the aspect ratio (65 means 65%) -- 11.61"X.65= 7.55". This number is the sidewall height. To calculate overall tire height, take the wheel diameter (15) and add twice the sidewall height (2X7.55" - there's a sidewall at the top and one at the bottom). 15+(2x7.55) = 30.09". One tall tire. Do the same math - your current tires are 26.6" tall.

So, if you put the new tires on without changing the 3.73's, it will make your 3.73's like a set of 3.30's. (26.6"/30")X3.73= 3.30's. If you go with 4.10's and the taller tire, it'd be like switching to a 3.66 gear with your current tires. 4.30's and the taller tire would be like a set of 3.83's and your current.

Not sure what you're trying to accomplish - your overall gearing is gonna be pretty close to what it is now if you put the taller tire on with the bigger gears. So if the taller tire hooks better, or you like the look - make the swap. But if you're trying to impact engine speed, well you'll spend a lot of money on gears/tires to have it pretty much revving the same way it is now.

 

[Qwk88LX]

Quick tutorial - calculating the impact of tire diameter on effective gear ratio is just a matter of simple ratios. If you can multiply and divide (or can push buttons on a machine that can) you can figure it out.

Say you have 4.10 gears with a 28" tall tire. And you want to know what impact a 26" tall tire will have. 28/26= 1.076. 4.10X1.076=4.41. The 26" tall tire will make the 4.10's act like a 4.41 gear. Said another way - 4.10's w/26" tires is like a 4.41 w/28" tires. So, with 4.10's and 28" tires, you could either leave the tires and put 4.41 gears in, or you could simply put the shorter 26" tire on and leave the 4.10's in.

To calculate tire height - take the section width (295) and divide it by 25.4 to convert it to inches (it's in millimeters). 295mm=11.61" Now, multiply the section width in inches (11.61") by the aspect ratio (65 means 65%) -- 11.61"X.65= 7.55". This number is the sidewall height. To calculate overall tire height, take the wheel diameter (15) and add twice the sidewall height (2X7.55" - there's a sidewall at the top and one at the bottom). 15+(2x7.55) = 30.09". One tall tire. Do the same math - your current tires are 26.6" tall.

So, if you put the new tires on without changing the 3.73's, it will make your 3.73's like a set of 3.30's. (26.6"/30")X3.73= 3.30's. If you go with 4.10's and the taller tire, it'd be like switching to a 3.66 gear with your current tires. 4.30's and the taller tire would be like a set of 3.83's and your current.

Not sure what you're trying to accomplish - your overall gearing is gonna be pretty close to what it is now if you put the taller tire on with the bigger gears. So if the taller tire hooks better, or you like the look - make the swap. But if you're trying to impact engine speed, well you'll spend a lot of money on gears/tires to have it pretty much revving the same way it is now.

Thanks for clearing that up for me..I think I'll go with the 4.30's.. I like the look of the taller tires & I think they'll hook pretty nice. It's worth a try.