You might consider changing the size of your tires.
This is not a common problem. Many people have larger tires for aesthetic reasons. For various reasons, we’ve seen men reduce the size of their tires. Any changes in tire diameter can have serious consequences.
The final drive ratio affects acceleration and performance by affecting tire diameter. A Camaro with decent acceleration and a set 26-inch-tall tires and 3.55 rear axle gears can become a dog with 29 inch tall tires and the same gearing. It is important to compare the final drive ratios of different tire sizes before changing tire diameters. This will allow you to determine what axle gearing to use with the new tire diameter. It will also help you maintain or enhance your performance. To do this, you will need to know three numbers: the current rear axle gearing of your vehicle, the diameters of your tires, and the tire diameters that you wish to use.
Determining Current Tire Diameter
It is easiest to find out the tire diameter by calling your tire dealer. You can also use these formulas to figure it out yourself. These formulas can be used for light-truck and passenger tires that are metric-rated. You don’t need to calculate for Pro Street style tires, which are rated by their diameter.
As an example, we will use a P235/60R-15 tires: 235 is the section width and 60 the aspect ratio. 15 is the required wheel diameter. This is the basic formula.
Tire Diameter = 2x (section width/25.4), x (aspect/100), + rim diameter
The formula will look like this when you enter the values from the sample tire:
Tire Diameter = 2 x (235/25.4) x (60/100) + 15
Let’s now do the calculations.
Tire Diameter = 2 x (9.25 x .60) + 15Tire Diameter = 2 x 5.55 + 15
Tire Diameter = 26.9 inches (rounded down to 26).
Calculating Effective Drive Ratio
After determining the size of your tires and the potential replacements, it is possible to compare the effects of new tires on the final drive ratio. If you have 26-inch tires and a 3.55 gear, you can change to 28-inch tires. The effective drive ratio for the new tires can be calculated by multiplying the tire diameters and multiplying that number by the gear ratio.
26/28 = .92857142857
.92857142857 = 3.55 = 3.2964, or 3.30
As you can see, larger tires decrease your final drive ratio from 3.30 to 3.30. This is a significant difference. This can be used to assess the impact of smaller tires on your final driving ratio.
Calculating Equivalent Drive Ratio
You now know that tall tires of 28 inches reduce your final drive ratio. Now you can calculate how much gearing you need to match your original 26-inch tire/3.55 gear combination. Divide the new tire’s diameter by the original tire’s diameter. Next, multiply this figure by the original (3.55) ratio.
28/26 = 1.0769230769
1.0769230769×3.55 = 3.82
To match the performance of your vehicle with 26-inch tires and 3.55 ratios, you will need a 3.82 rear axle gear. To get the best result, a gear between 3.73 to 3.90 is recommended.