A common 4x4 application is an upgrade to larger-than-stock tires. Once completed, this change immediately alters vehicle speed at a given rpm, rpm at a given speed, and effective gear ratio, which in turn affect both acceleration and fuel economy. Tire size, gear ratio, mph and rpm weave an intricate pattern of performance. Change one and all four are affected.; knowing any of the three, the fourth can be easily determined. The following four formulas illustrate the point:

If you are contemplating a tire size upgrade and know your rearend gear ratio, the table (at right/left/above/below) will offer a quick reference guide for figuring your engine rpm at 60 mph cruising speed. Likewise, if you measure your tire size and observe rpm and mph, you can calculate what gears are in your axles.

How To Calculate Actual Speed: With the change to taller tires, your speedometer will real "slower" than the actual vehicle speed. To determine the percentage of speedometer error, the formula is a simple relationship between old and new tire diameters.

Example: You’ve replaced your 28-inch OEM rubber with a new set of 35-inch all-terrains and you want to know your actual speed when the speedo reads 60 mph:

Ever wonder how far off your speedometer is with your new bigger tires? By using a simple ratio calculation, this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator, it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH

Another way of looking at this relationship would be to figure what the indicated speed would be if you were actually going 60 mph. In this case, the tire diameter relationship is flip-flopped to:

Using the previous example, your speedometer reading at an actual 60 mph is:

Gearing up: Using the above tire change as an example, lets say that your vehicle is currently running a 3.40:1 final-drive gear set. Now that you have changed to a taller tire, you want to determine the actual, or effective, final ratio. This can be figured by dividing the old tire diameter by the new, and multiplying by the current gear ratio (:1):

Dropping from a 3.40:1 to a 2.72:1 ratio will reduce off-the-line responsiveness and severely affect slow-speed trail capabilities. If your new 35-inch rubber is just what you want, but you now need to restore your vehicle’s low-end, the following formula will allow you to determine what gear set (equivalent) ratio should be installed to compensate:

Or, in this example:

By installing a gear set in the range of 4.25:1, you will not only restore your vehicle’s low-end responsiveness, you will likewise restore your speedometer’s accuracy.

Figuring gear ratio: Knowing what gears are in a given axle is a must when considering that axle for a swap. The actual ratio or reference code, will normally be found on either a tag attached to a bolt, or will be stamped into the axle housing. If it cannot be found, there is a simple method for manually (and mathematically) determining the ratio for any axle installed on a vehicle.

Raise both wheels of the axle, with the transmission in Neutral. (Make sure you support the vehicle with safety stands and block the front tires.) Make a reference mark on the driveshaft and on the differential housing. Next, without rotating them, make a mark on both tires and their respective fender wells. With a friend watching the driveshaft, carefully rotate both tires at the same time exactly one revolution. The number of turns the driveshaft makes will indicate the ratio. If the driveshaft rotates 4 ½ turns, for instance, the axle ratio is roughly 4.5:1.