Gear Ratio Terminology

[19_Chevelle_69]

The terms higher and lower can get a little confusing when reffering to rearend ratios. If someone changes to a higher gear ratio they are actually changing to a numerically lower ratio. If you were to swap a 2.93:1 gearset with a 3.42:1 gearset you would be switching to a lower gear even though the ratio of driveshaft turns to turns of the wheel is higher. If you swap a 3.42:1 with a 2.93:1 you are switching to a higher ratio. I don't how high ended up meaning low numerically or low ended up meaning high numerically, but they do.

A higher gear ratio would improve mileage and lessen wear on the engine somewhat but at the cost of less acceleration. A car is also capable of a higher top speed with a higher gear. Cars that are meant to do a lot of interstate cruising have higher gears, generally in the 2.48 - 3.08:1 range. Cars meant for acceleration have lower gears, usually something numerically above 3.08:1.

A numerically higher ratio will cause the same tire travel but at higher RPM. For example, a 3.42:1 ratio will cause the same tire travel but at higher RPM than a 2.93.

A lower gear (3.42:1) will require a lesser amount of torque than a higher gear (2.93:1) to accelerate at a given rate. A higher gear will require more torque to accelerate at a given rate.

An engine that puts out a small amount of torque will generally be better off with a lower gear ratio. An engine that puts out a lot of torque will accelerate quite well with a higher gear ratio. That's why the 4500lb 1970 Delta 88 with the police package could run 0-60 in something like 6.9 seconds despite having a 2.93:1 rearend ratio. It had the 455 with 500 ft/lb of torque and didn't need a very low gear to get up to speed quickly. (it also had an estimated top speed of 140). Those of us who just want insane acceleration put a low gear behind a big 'ol 455 in a light car and then spend the rest of their days trying to figure out how to eliminate tire spin!

[Every_Mn]

My dad has a hard time with this. Understandable, but it's sometimes hard to explain stuff like this.

[THEBOSS]

Gear ratios also multiply torque by the given ratio because of leverage.

[Mongoose]

If you don't know the gear ratio of the differential in your car, enlist the help of a friend and jack the rear end up on stands so both rear tires are off the ground and put the transmission in neutral. Use wheel chocks to insure it is safe to get under the car.

Put a chalk mark on the drive wheel or tire at the 6 o'clock position. Now put a chalk mark on the drive shaft on the bottom. Have your buddy slowly turn the tire exactly one revolution. While he/she (sorry Chero) turns the tire, count the number of times the chalk mark makes a revolution on the drive shaft. If the chalk mark goes around 4 times and a little bit, you have somewhere near a 4:10 or a 4:11 ratio. If it makes 3 and 3/4 revolutions, you probably have a 3:73 ratio. 3 and 1/3 times is a 3:30 and so on. It is not exact but you can get very close to your gear ratio numbers by using this method and you don't have to pull one bolt! :D

[74runner]

well...it works 4.5 turns for my 4.55 dana 60

[Daffy]

If you don't know the gear ratio of the differential in your car, enlist the help of a friend and jack the rear end up on stands so both rear tires are off the ground and put the transmission in neutral. Use wheel chocks to insure it is safe to get under the car.

Put a chalk mark on the drive wheel or tire at the 6 o'clock position. Now put a chalk mark on the drive shaft on the bottom. Have your buddy slowly turn the tire exactly one revolution. While he/she (sorry Chero) turns the tire, count the number of times the chalk mark makes a revolution. If the chalk mark goes around 4 times and a little bit, you have somewhere near a 4:10 or a 4:11 ratio. If it makes 3 and 3/4 revolutions, you probably have a 3:73 ratio. 3 and 1/3 times is a 3:30 and so on. It is not exact but you can get very close to your gear ratio numbers by using this method and you don't have to pull one bolt! :D
Unless to car has an open differential(no positraction) In that case you leave one wheel on the ground and rotate the other one two times and get the same reading from the driveshaft

[Mongoose]

Daffy is exactly right. I forgot about the open rear end problem. But it can still be done. Thanks Daffy.

But that is the ratio Every_Mn. For every revolution of the wheel, the drive shaft must turn X times. Try explaining it to him that way, it may be a simpler concept to understand.

[WINGNUT]

I would choose to count the number of teeth on the ring gear and the pinion gear. Then divide the number of teeth on the ring gear by the number of teeth on the pinion. A bit more work is required for this method, but it'll give the answer you need.

[Daffy]

You are absolutely right about that...But there's a lot to be said about saving a bunch of work and hassle...I owned a junkyard,we didn't want to tear down a lot of axle assemblies to find a certain gear ratio,,,So our system worked very well..

[WINGNUT]

You are absolutely right about that...But there's a lot to be said about saving a bunch of work and hassle...I owned a junkyard,we didn't want to tear down a lot of axle assemblies to find a certain gear ratio,,,So our system worked very well..

What happened to your junkyard.......did you retire?

[Daffy]

What happened to your junkyard.......did you retire?
Sold it....:D

[Letsrace]

Doesn't tire size also affect the ratio?
Say, 195/60R15 vs 255/60R15....wouldn't that affect the ratio? And if so, would it make it higher or lower?

[72Chero]

Speaking from the stand point of my Chero... I have .389 Richmond gears...and it was just a little low for highway driving with 235x60xR15's on the back so we switched it to 285x75xR15's and it brings the ratio to a higher gear we figure about .360's .... without actual measurements...we are unsure but it does make RED more streetable. I have friends with 4.11's and when they race they run smaller tires than what they drive around town on... hope that helps :eek: :rolleyes: :D

[Letsrace]

Speaking from the stand point of my Chero... I have .389 Richmond gears...and it was just a little low for highway driving with 235x60xR15's on the back so we switched it to 285x75xR15's and it brings the ratio to a higher gear we figure about .360's .... without actual measurements...we are unsure but it does make RED more streetable. I have friends with 4.11's and when they race they run smaller tires than what they drive around town on... hope that helps :eek: :rolleyes: :D
Okay, if I'm reading you correctly, the taller the tire is the taller the gear becomes vs a lower profile tire?
So if I'm running a 3:23 gear and I have 235/60r15 on the street and I want a lower gear for the strip, then I should run a lower profile tire, like a 205/60r15?
Anyone know how much of a change it would be?

[Daffy]

Okay, if I'm reading you correctly, the taller the tire is the taller the gear becomes vs a lower profile tire?
So if I'm running a 3:23 gear and I have 235/60r15 on the street and I want a lower gear for the strip, then I should run a lower profile tire, like a 205/60r15?
Anyone know how much of a change it would be?
Not much...You need a 50 series tire to make much difference. The 205 is just narrower not much shorter.

[72Chero]

Its easier to go to a higher gear than a lower gear...don't want to burst your bubble....

[Letsrace]

You're noy bursting my bubble. I'm just curious really, I've never paid that much attention to it until recently.
Daffy, I've always thought that the height of the tire was what made the difference. Isn't a 235/60 and a 235/50 the same height? The 50 is just wider than the 60? That's how it looks anyway.

[19_Chevelle_69]

The 50 in 235/50R15 is the aspect ratio. Meaning the sidewall is 50% of the tread width. (approx)

The P235/50-R15 wheel is 24.3" tall ... The P235/60-R15 wheel is 26.1" tall. Note both tire tread widths are rated at 235mm

(sizes are approximate, and will vary with air pressure and tire sidwall stiffness)

Have a look at this link...
http://www.miata.net/garage/tirecalc.html

[Letsrace]

But the width of the wheel can also affect the height of the tire. Well, at any rate, the higher the number such as 235 vs 205, the taller the tire, based on being mounted on the same size wheel.
The 50 or 60 is always width. A 50 series is always wider than a 60, and 60 always wider than a 70. How does width affect gear ratio, wouldn't it be the height that affects it?

[Daffy]

A 50 is always shorter than a 60 which is always shorter than a 70 in the same numerical size...They are also wider because less sidewall= more tread...

[PaPa Mike]

A 50 is always shorter than a 60 which is always shorter than a 70 in the same numerical size...They are also wider because less sidewall= more tread...

A 50 tire has 50% side wall and 50% tread, a 60 has 60% sidewall and 40% tread, and 70 tire 70% side and 30% tread. Therefor a 235/50, is shorter than a 235/60 and 235/70 because it has less sidewall. All three have 235 bead to bead width, just less/more sidewall/treadwidth based upon 50/60/70 series.
Hope this will help

[Letsrace]

But my question is, If I'm using a 60 series tire, how much does it affect the gear ratio if it is a 205 for instance versus a 235?

[PaPa Mike]

The only real way is to measure the height of each tire and use a ratio calculator to figure it out. There is a calculator on one of the forums to figure that but I can't remember where. If I find it 'll post it for you.

[PaPa Mike]

The only real way is to measure the height of each tire and use a ratio calculator to figure it out. There is a calculator on one of the forums to figure that but I can't remember where. If I find it 'll post it for you.

I found this one for top speed calculations, It has a tire size calculator, 235/50/15 is 24.2 inches tall, 235/60/15 is 26.2 inches, & 235/70/15 is 28 inches

http://www.markwilliams.com/calculators.aspx

[Letsrace]

I found this one for top speed calculations, It has a tire size calculator, 235/50/15 is 24.2 inches tall, 235/60/15 is 26.2 inches, & 235/70/15 is 28 inches

http://www.markwilliams.com/calculators.aspx
I'm just trying to figure out the difference in ratio between 205/60 and a 235/60, not the difference between 235/60 and a 235/50. The link to the calculator helped alot, thanks! :)

[72Chero]

Well basically nothing....205 is not as wide as a 235. The hieght of the tire is what changes the gear ratio not the width. 60 is a 60 is a 60....don't matter how wide it is. Its still a 60. If you drop the height to a 50 that will change your gearing slightly. Hope this helps....

[Letsrace]

Well basically nothing....205 is not as wide as a 235. The hieght of the tire is what changes the gear ratio not the width. 60 is a 60 is a 60....don't matter how wide it is. Its still a 60. If you drop the height to a 50 that will change your gearing slightly. Hope this helps....
See, that adds to my confusion. I understand what you are saying, but a 235/60 is taller than a 205/60. According to the calculator, the rollout in inches for a 205/60 is 77.3 inches, the rollout for a 235/60 is 82.3, that's a 5 inch difference. That surely affects the ratio, atleast alittle, but I think I have my answer now. Thanks everyone! :)

[PaPa Mike]

Glad we cleared that up :D :p :eek:

[Mongoose]

Changing tire size does not change the gear ratio. If you have a 3.83 gear, the drive shaft turns 3.83 times to make one revolution of the axle/tire.

Moving to a taller tire just moves you farther forward with each turn of the tire vs. a shorter tire. It does have the affect of changing the ratio, but your gears are the same no matter what tire you use.

To calculate the distance difference use the formula C=3.1416 x D Or this way : Diameter of the tire X(times) 3.1416 will equal the Circumference of the tire.

That measurement will be the distance your car moves forward with each revolution of the tire. You can calculate all kinds of good stuff with that. Like if high gear in the transmission is 1:1, 6,000 rpm on the engine would be 6000 divided by the axle ratio (3.83) that = tire rpm then X Circumference equals the number of inches your car moves in 1 minute. Have fun.

[Letsrace]

Changing tire size does not change the gear ratio. If you have a 3.83 gear, the drive shaft turns 3.83 times to make one revolution of the axle/tire.

Moving to a taller tire just moves you farther forward with each turn of the tire vs. a shorter tire. It does have the affect of changing the ratio, but your gears are the same no matter what tire you use.

To calculate the distance difference use the formula C=3.1416 x D Or this way : Diameter of the tire X(times) 3.1416 will equal the Circumference of the tire.

That measurement will be the distance your car moves forward with each revolution of the tire. You can calculate all kinds of good stuff with that. Like if high gear in the transmission is 1:1, 6,000 rpm on the engine would be 6000 divided by the axle ratio (3.83) that = tire rpm then X Circumference equals the number of inches your car moves in 1 minute. Have fun.
Cool, now I know how to calculate it. Thanks! It was more curiosity than anything. You and 72Chero confirmed that it isn't enough of a difference if any at all, to be concerned with. But that's good info! Thanks! :)

[kzo57]

The only other part of the equation is knowing that 88' per sec= 60MPH
this can all be setup in Excel if you have M S Office on your PC. from there you can print out a complete MPH chart for every type of rear ratio and tire hight!!