I have been told that most all companies state the rear gear ratio based on a 28" tall tire. is this true?? I have a set of 4.11 and am running a 27/10.5/15 tire, so in that case, It would be closer to a 4.30 then?? Just wondering. I was going to move up into a 4.56 but I dont want it to become a 4.88 or something. I do drive the car on the street more than I used to so I may just stick with what it's got.

 

There is an easy way to tell. Simply divide the number of teeth on the ring gear by the number of teeth on the pinion gear. That will be your ratio, regardless of tire. BTW, this tread should be in the Drivetrain section not the Advanced section.

 

The gear ratio has nothing to do with the tire. It is the ratio between the number of teeth on the ring gear and the number of teeth on the pinion gear.
If you have 4.11's, 2.73's or 6.13's, you will always have that gear ratio regardless of what tire you use.


Having said that, the rear gears are not the last step before the pavement, and the EFFECTIVE ratio is affected by your tires. Taller tires make the engine "see" taller gears and shorter tires make the engine "see" shorter gears.
(2.73 = "tall", 4.11 = "short")

This can be useful in tire selection. Regardless of what gear you have in your differential, a tall tire will lower the engine RPM at a giver speed, which is useful for a street car. Conversely, you can use a short tire at the dragstrip to "lower" your gears.


If you want to know how much variance your tires create in relation to your stock tires, just take it out on the highway and make note of your odometer reading. Then drive for "X" number of miles, based on the milemarkers on the side of the road, NOT based on your odometer. The more miles the better, but you should drive a minimum of 10 miles.

All you have to do is divide the smaller number by the bigger number and you will know the percentage by which your new tires have changed your "effective ratio".

 

The majority of cars have a 26" tall tire not 28". As mentioned above, you're after an effective ratio. The mechanical ratio between the ring and pinion teeth never changes.

If you have 26" tall tires and 4.10 gears, going to 4.56 gears would be like putting on 24" tall tires. If you put 28" tall tires, the 4.10 gears would feel like 3.73's so to get the same performance with 28" tires, you put in 4.56 gears (not exact numbers but you get the idea). Same performance as in the engine still operates in the same rpm range at the same speeds.

On the dragstrip, gear ratio and tire size play a big factor. With a street engine topping out at around 5500 rpm, you don't need a lot of gear. If engine performance is increased and the engine now peaks at 6800 rpm, you need more gear to be able to use peak rpm through all the gears within a 1/4 mile. You need to stay in the gears longer (more rpm) but the distance to travel is the same so a deeper gear gets the engine through the rpm range quicker.

You never want to go with a shorter tire. A taller tire gives a longer footprint from front to back which will give better traction for straight line accelleration.

Since you want to drive one the street, don't be afraid of a high gear ratio like 4.10, 4.56, 4.88 etc. If you have a taller tire, you won't notice much of a difference. It's all going to depend on the operating range of the engine. The deeper gear also increases the torque multiplication so it can still spin that taller tire.

While a tubbed out race car may have 32" tall tires and 5.13 gears, A tubbed out pro street car may have 32" tall tires and only 4.10 or even 3.73 gears to make it much more driveable on the highway. It won't accellerate as quickly but the engine also won't be screaming at 4000 rpm cruising down the highway.

 

Got ya. I want to be able to drive the car on the street, but driveability is not my concern. I built my car for strip use, and have went from a 26 to a 27 tall tire. This should not make a big difference, but now I am thinking of changing the ratio from a 4:11 to a 4.56 or higher. I appreciate the info, I was just told that the manufacture states the ratio/ ring gear set at say 4.11 but they base it on a 28" tall tire, like stated above, if the car has a 32" tall tire, the ratio relevants out to say a 3.73 instead of the 4.11 stated. While it is a 4.11 gearing, how tall the tire is will determine how quickly the tires make a revolution. I understand. Thanks again.