Calculating your ideal tire size for racing (rear wheel drive)
03-30-2007, 04:14 PM
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Calculating your ideal tire size for racing (rear wheel drive)
I have red quite a few discussions about people not sure what different tires sizes will do to their gearing, so Ive put together this post. (And to think, many of you thought that you would never use algebra in real life...i hope that you paid attention!)
Effective Gear Ratio
Gearing or Tire effect on trap rpm
To figure out what rpm's you will be hitting with a given tire size, you will need to know at least three of the following items, to solve for the 4th:
trap speed
trap RPM
current tire diameter
current gear ratio
I cannot do anything to help you if you dont know your trap speed or RPM, but I can help if you dont know your gear ratio or tire diameter.
Tire Diameter:
Gear ratio calculation
now that you have all of your variables, you can then calculate the effect of different things, such as gears or tire size.
rpm=(mph x final gear ratio x 336)/tire diameter
For my car, I know my tire size, gear ratio, and mph, so i can calculate my trap rpm.
rpm=(106x4.56x336)/26
rpm=162409/26
rpm=6247
It has been brought to my attention that many people dont have an understanding of basic algebra, so I will go ahead and include all of the different formations to solve for the different things. above, i explained how to calculate your trap rpm at given trap speed.
If you know your rpm, gear ratio, and tire diameter, and want to calculate your speed:
Speed=(Tire Diameter*rpm)/Gear Ratio*336)
example:
Speed=(26*6250)/(4.56*336)
Speed=162500/1532.16
Speed=105.73
You can also calculate your gearing based on your trap speed, rpm and tire diameter, but it will only be as accurate as you are with recording your rpm:
gear=(tire diameter*rpm)/(336*mph)
Example:
gear=(26*6250)/(336*106)
Gear=162500/35616
Gear=4.5626
If I have excluded anything about this topic, please let me know.
Effective Gear Ratio
To figure out your new tires effect on your gearing, simply take the old tire size, divide that by the new tire diameter, and multiply the outcome by the gear ratio.
Example:
I have 26" tires on my car with a 4.56 gear. if i go to a 28" tire, what will my new gears be?
(26/28)*4.56=4.24 this is my new effective gear ratio.
Example:
I have 26" tires on my car with a 4.56 gear. if i go to a 28" tire, what will my new gears be?
(26/28)*4.56=4.24 this is my new effective gear ratio.
To figure out what rpm's you will be hitting with a given tire size, you will need to know at least three of the following items, to solve for the 4th:
trap speed
trap RPM
current tire diameter
current gear ratio
I cannot do anything to help you if you dont know your trap speed or RPM, but I can help if you dont know your gear ratio or tire diameter.
Tire Diameter:
To figure tire diameter, you need to know what the numbers in your tire size mean.
lets take 245/45/17.
245 is the tread width, in millimeters. 25.4 mm per inch. so to convert 245 to inches, 245/25.4=9.65
45 is the aspect ratio. this number is the percentage of the tread width possessed by the sidewall, in other words, it is how tall the sidewall is. 45 is 45% of the tread, so its .45*9.65=4.3425 inches. that is the distance from the bead to the tread, since you have sidewall above and below the rim, you double it, so youll get 8.685.
17 is simple..its the size of the rim, and when you add that to the sidewall height, you get 25.685.
In short, to get the tread width, divide the first number by 25.4 to get inches. To get the height, multiply the first number by the second number, then times 2, plus the third number to get the assembled height.
245/45/17=X/Y/Z
tread width=X/25.4 we will call the result A.
assembled height=(A*Y*2)+Z
-or-
{[(X/25.4)*Y]*2}+Z
lets take 245/45/17.
245 is the tread width, in millimeters. 25.4 mm per inch. so to convert 245 to inches, 245/25.4=9.65
45 is the aspect ratio. this number is the percentage of the tread width possessed by the sidewall, in other words, it is how tall the sidewall is. 45 is 45% of the tread, so its .45*9.65=4.3425 inches. that is the distance from the bead to the tread, since you have sidewall above and below the rim, you double it, so youll get 8.685.
17 is simple..its the size of the rim, and when you add that to the sidewall height, you get 25.685.
In short, to get the tread width, divide the first number by 25.4 to get inches. To get the height, multiply the first number by the second number, then times 2, plus the third number to get the assembled height.
245/45/17=X/Y/Z
tread width=X/25.4 we will call the result A.
assembled height=(A*Y*2)+Z
-or-
{[(X/25.4)*Y]*2}+Z
While most people know what gears are in their car, some may buy a car that has been modified by someone and they may not know what gear is in the car.
Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion, but we know that counting the teeth isnt always an option.
To figure your gearing without taking anything apart, you will need to know if you have an open or limited slip differential. Once you know that, go through one of the following processes.
You will need to put the vehicle in neutral (automatic)or take the car out of gear (manual), and jack up the back end. You will also need a helper for this. Remember to always use jackstands for this, as you will be under the car. Take a piece of chalk or a crayon, anything that will show up on the driveshaft and the tire, and make an index point. You will need to count the times that these points rotate in relation to each other. Once you have made your distinct mark on the driveshaft and one of the tires/rims, have your helped hold the other tire in place.
Begin turning the unheld wheel, counting the rotations of the tire up to 25, and have your helper count the number of rotations of the driveshaft. For my car, 25 rotations of the tire causes 57 rotations of the driveshaft, if i had an open differential.
therefore, I take 57 (driveshaft rotations)divided by 25(tire rotations), and multiply the result by 2, or:
(57/25)*2=4.56
If you have a limited slip diff, both tires turn together and when you turn one, you turn both, so you simply take the number of driveshaft rotations and divde it by the number of tire rotations.
So for my car, with an LSD differential, it would be:
ratio=driveshaft rotations/tire rotations
4.56=114/25
Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion, but we know that counting the teeth isnt always an option.
To figure your gearing without taking anything apart, you will need to know if you have an open or limited slip differential. Once you know that, go through one of the following processes.
You will need to put the vehicle in neutral (automatic)or take the car out of gear (manual), and jack up the back end. You will also need a helper for this. Remember to always use jackstands for this, as you will be under the car. Take a piece of chalk or a crayon, anything that will show up on the driveshaft and the tire, and make an index point. You will need to count the times that these points rotate in relation to each other. Once you have made your distinct mark on the driveshaft and one of the tires/rims, have your helped hold the other tire in place.
Begin turning the unheld wheel, counting the rotations of the tire up to 25, and have your helper count the number of rotations of the driveshaft. For my car, 25 rotations of the tire causes 57 rotations of the driveshaft, if i had an open differential.
therefore, I take 57 (driveshaft rotations)divided by 25(tire rotations), and multiply the result by 2, or:
(57/25)*2=4.56
If you have a limited slip diff, both tires turn together and when you turn one, you turn both, so you simply take the number of driveshaft rotations and divde it by the number of tire rotations.
So for my car, with an LSD differential, it would be:
ratio=driveshaft rotations/tire rotations
4.56=114/25
now that you have all of your variables, you can then calculate the effect of different things, such as gears or tire size.
rpm=(mph x final gear ratio x 336)/tire diameter
For my car, I know my tire size, gear ratio, and mph, so i can calculate my trap rpm.
rpm=(106x4.56x336)/26
rpm=162409/26
rpm=6247
It has been brought to my attention that many people dont have an understanding of basic algebra, so I will go ahead and include all of the different formations to solve for the different things. above, i explained how to calculate your trap rpm at given trap speed.
If you know your rpm, gear ratio, and tire diameter, and want to calculate your speed:
Speed=(Tire Diameter*rpm)/Gear Ratio*336)
example:
Speed=(26*6250)/(4.56*336)
Speed=162500/1532.16
Speed=105.73
You can also calculate your gearing based on your trap speed, rpm and tire diameter, but it will only be as accurate as you are with recording your rpm:
gear=(tire diameter*rpm)/(336*mph)
Example:
gear=(26*6250)/(336*106)
Gear=162500/35616
Gear=4.5626
If I have excluded anything about this topic, please let me know.
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