Changing Differential Ratios and Tyre Sizes
This information is related to the Speedometer reading, the diff ratio, the transmission reading cog, and the wheel size, and is placed in this section due to it being interconnected to multiple electrical and mechanical engines
In order to ensure accurate speedometer readings without resorting to speedometer adjuster modules or finding and using a rare pursuit cluster, the following information has been gathered in order to make any calculations required to attempt changes in the Differential, transmission gear, and the wheeel size where the speedometer is still within the manufacturers reading percentages.
This information was gathered from FordForums and CarModder primarily, but has been reworked for manual calculation and slightly better accuracy.
Use the link below to view an interactive tool that can help with discerning changes in speedometer readings
JavaScript is required to use this calculator!Speed Reading Calculator
First you will need the change in diameter between the old tyre size and the new tyre size. Use the formula below for reference:
T = ([T2] - [T1]) / [T1] * 100
Where T1 is the original tyre diameter, and T2 is the diameter of the new tyres
For example, if you were to go from a 205/65R15 which has a 673mm diameter, to a 205/70R15 which has a 694mm diameter, which gives us approximately 3.12 percent change. Note this for later as T
The Tyre Size Calculator may be useful to get tyre diameters, rounded to the nearest mm
The next step is to calculate any difference in your diff ratio. Use the formula below for reference:
D = ([D1] - [D2]) / [D1] * 100
Where D1 is the original diff ratio, and T2 is the new diff ratio
For example, if you were to go from a 3.08 differential to a 3.45 differential, there is a calculated -12.01 percent approximate change. Note this for later as D
The last number required for the change amount is the speedometer gear, which is easily calculated using the number of teeth on each gear, the old and the new. Use the formula below for reference:
G = ([G2] - [G1]) / [G2] * 100
Where G1 is the original driven gear teeth, and G2 is the new driven gear teeth. The
G1andG2values are switched for this part of the formula as a increase in the teeth results in a decrease in the reading
For example, if you were to go from a 21 toothed gear to a 20 toothed gear, you would see a 5% change in speed reading. Using this formula, this gives a result of -5 percent change. Note this number for later, as G
Now that you have your Tyre change T, your Differential ratio change D and your Gear change G, you can calculate the total difference with ease by using the following formula:
X = (T + D + G) / 100 + 1
Using the examples in the other sections, the result calculated would be -13.85 / 100 + 1, or 0.8615. This is theoretically what reading you would have if you were traveling at 1km/h
After these calculations, any value under 1 will show a lower speed than you are actually traveling, and any value over 1 will show a higher speed than you are traveling. In order to avoid speeding fines, the best rule of thumb is to calculate the closest value to 1 as possible without going under