Gear ratio vs. tire size [Archive] - Chevy Nova Forum

: Gear ratio vs. tire size


63 Pro
14th-October-2007, 01:06 PM
Just curious if there is a formula to figure affected gear ratio vs. actual gear ratio with different tire sizes. For example, Say you have a 4.10, if you run 33" tall tires it will act differently than a 28" tall tire.
Even though the actual gear ratio stays the same would a 33" tall tire make a 4.10 act like a 3.90?

Mike Goble
14th-October-2007, 01:12 PM
Simple math....

gear ratio times old tire size divided by new tire size.

Your example, you have 4.10 gears and swap from 33" tall tires to 28" tall tires:

4.10 x 33/28 = 4.83

Your 4.10's with a 28" tire will act the same as your 33's with a 4.83 gear.

Let's say you swap from 28's to 33's

4.10 x 28/33 = 3.48

Your 4.10's with a 33" tire will act the same as 3.48's with a 28" tire.

63 Pro
14th-October-2007, 01:24 PM
Cool! Thanks Mike!:)

63 Pro
14th-October-2007, 01:54 PM
Well, now I'm confused, I did the simple math and got two different numbers, so, what am I doing wrong? Ok, let's say you have a 4:10 gear with an old tire size is 33" and switching to 29". 4.10 X 33/29 = 4.66. Now, lets say you had a 31" tall tire and were switching to a 29" tall tire 4.10 X 31/29 = 4.38. In both examples switching to a
29" tire but ratio is different? Should be the same, no?

Scooter
14th-October-2007, 03:38 PM
no you used a 31 inch and a 33 inch, yes they both switched to 29" but you started with two different sizes and therefor will have 2 different ratios.

agpd45
14th-October-2007, 04:29 PM
I think what he's asking above is how to find "true" gear ratio. For instance (correct me if I'm wrong) but aren't advertised gear ratios measured with a 28" tall tire? Anyway whatever they are measured with, I think he wants a way to measure effective gear ratio with a different tire size.

My GUESS is you take your gear ratio, divide it by 28 (which i'm using as the constant that the advertised gear ratio is measured by), then multiply by your new tire height.

NOTE* I'm assuming that they advertise gear ratios based off a 28" tall tire. That's something I think I heard but I'm not sure. If I'm wrong, just lemme know.

Mike Goble
14th-October-2007, 04:40 PM
There is only one 'true' gear ratio, and that is the number of teeth on the ring gear divided by the number of teeth on the pinion gear. This is irrespective of tire size, axle splines, brake size, etc. Changing tire sizes just alters the number of revs of the rear axles turn per unit of distance. The simple math described merely figures what gear ratios and tire sizes will generate roughly equal performance. For example: Your 4.10's with a 28" tire will act the same as your 33's with a 4.83 gear.

63 Pro
14th-October-2007, 04:55 PM
Ok, so if you start out with a 4.10 gear and a 29" tire it's a 4.10 ratio. If you start out with a 31" tall tire and the 4.10your still at a 4.10 ratio? Maybe I'm not asking the question properly or I'm just not communicating what I'm trying to find out very well.
Suppose you had no tire at all and you had a 4.10 rear gear.
How would you calculate a tire size that would give you an affected ratio of say 3.90 because a 3.90 isnt available for your application.

66 BADBOY
14th-October-2007, 04:56 PM
PM sent, I think my chart will help you out.;)

Mike Goble
14th-October-2007, 05:04 PM
Ok, so if you start out with a 4.10 gear and a 29" tire it's a 4.10 ratio. If you start out with a 31" tall tire and the 4.10your still at a 4.10 ratio? Maybe I'm not asking the question properly or I'm just not communicating what I'm trying to find out very well.
Suppose you had no tire at all and you had a 4.10 rear gear.
How would you calculate a tire size that would give you an affected ratio of say 3.90 because a 3.90 isnt available for your application.

If you can't get 3.90 gears, you'll never have a 3.90 ratio. It makes no difference what tire you use, the ratio is always the same. If you wanted to choose a tire that would make your 4.10's perform like a set of 3.90's, you would choose a tire that is 41/39ths as tall as your tire of choice with the 4.10's. If you had 28" tires with 4.10 gears and wanted the same performance as a set of 3.90's with the same tires, select a set of 29.43" tall tires.

66 BADBOY
14th-October-2007, 05:12 PM
The ratio will never change. Your driveshaft will still spin 4.10 times per one revolution of the tire. But since your tire size is now bigger (31 from 29), the car will travel FARTHER in those 4.10 revolutions of the engine.

So, with the 31 tire you will go farther per 4.10 engine revolutions, meaning your RPMs will be lower at any speed. I think what you are looking for is this:

If I have a 31 tall tire and 4.10s, what gear will act the same if I have a 29 inch tall tire. Is that what you are looking for?

63 Pro
14th-October-2007, 05:27 PM
If you can't get 3.90 gears, you'll never have a 3.90 ratio. It makes no difference what tire you use, the ratio is always the same. If you wanted to choose a tire that would make your 4.10's perform like a set of 3.90's, you would choose a tire that is 41/39ths as tall as your tire of choice with the 4.10's. If you had 28" tires with 4.10 gears and wanted the same performance as a set of 3.90's with the same tires, select a set of 29.43" tall tires.

Yes, I understand that the ratio will never change. Thank you for explaining it that way;) Yes, if you had a 4.10 ratio and you wanted to make it act like a 3.90 what type of formula or simple calculation would you do to get as close as possible to a tire size that is available. 29.43" obviously not but a 29.5 maybe a little better. How did you get the 29.43 for example?

66 BADBOY
14th-October-2007, 05:34 PM
take 4.10 and divide by 3.9 = 1.05

Multiply 1.05 x 29 and you get - 30.45

Mike used 28 inch tall tires in his calc, so that would be:

1.05 x 28 = 29.43

66 BADBOY
14th-October-2007, 05:42 PM
Its really an old math equation:

R1 x D1 = R2 x D2

Where:
R1 is the existing ratio
D1 is the existing diameter
R2 is the unknown (or known) ratio
D2 is the unknown (or known) diameter

The 2's are what you will have in the end, and like all math equations, you can only have one variable. So you MUST already know either R2 or D2. So if you know what tire size you want to use, enter that as D2 and this equation will give you the "effective" gear ratio that you would have.

In the case Mike did with the 28 tires;

R1 = 4.10
D1 = 28
R2 = 3.9
D2 = X

So:

R1 x D1 = R2 x D2
4.10 x 28 = 3.9 x X
To solve for X, you have to get it alone on one side of the equation. To divide both sides by 3.9:

4.10/3.9 x 28 = X

And thats the formula he showed you. X = 29.43. Its hard to show the steps correctly without being able to cross of stuff that cancels. But I hoped this helps.

The Big Al
14th-October-2007, 05:53 PM
Go to this site and play around!
Change your gear set and tire dia and see changes

http://www.hotrodworks.net/hotrodmath/hotrodmath.html

http://www.ecst.csuchico.edu/~traverso/hotrodmath.html

63 Pro
14th-October-2007, 06:04 PM
Barry thank you for the explanation!;) I got your PM, your too funny dude!:D:beer: Thanks Mike:). Al, thanks for the site! Thanks everyone for the input! I'm not confused anymore:D Amazing:rolleyes::D Hopefully this helps anyone else who may have wondered the same thing!