Technical > Tech
Accurate Speedometers After Tire and Gear Changes
story and photos by Jim Allen
This is a driven gear for a Ford with a Borg Warner transfer case. A single bolt holds it in the t-case and a clip attaches it to the cable. Most cable driven speedometers are similar. Speedo drive and driven gears are usually available from dealerships, or in the case of older rigs, from places that specialize in NOS parts.
Tire and gearing changes are two common buildup items and changes in either area will throw your speedometer off by a little or a lot. Yes, you can calculate the difference and stick a post-it note on the dash, but that won’t get you a sympathetic ear when you forget and the local gendarme pulls you over. There are just a few standard ways to correct the speedo — some are mechanical, some are electronic. We’ll hit the high spots for you.
First, you need to know how far you are from right and in which direction. If your rig has an accurate tach, simply use the following formula;
| mph = | rpm x tire diameter |
| gear ratio x 336 |
To get the most accurate diameter, don’t go by the tire manufacturer’s listed diameter. With the tire installed and aired up to your normal street pressure, measure from the ground to the top of the tire. That’s your mounted diameter. Assuming your speedo was accurate before, you can calculate actual speed from indicated speed for a tire swap only, gear ratio change only, or for both combined with these three formulas.
| actual mph = | new tire diameter | x indicated mph |
| old tire diameter |
| actual mph = | old axle ratio | x indicated mph |
| new axle ratio |
| actual mph = | new tire diameter | x | old axle ratio | x indicated mph |
| old tire diameter | new axle ratio |
Aftermarket EFI tuners like Superchips’ MAX Micro Tuner have the capability to reset gearing and tire values to get your speedo reading correctly. The menus will list each area. In the case of this GM tuner, gearing can be adjusted from 3.08 to 4.10 and tire diameter can be set from 26-38 inches.
Getting to nuts and bolts, speedos come in two general categories, mechanical and electronic and each uses a different method of correction. The late model electronics are adjustable a few ways. If you can talk the dealership into it, their diagnostic equipment has the capability to alter some of the parameters. You can also do it via a feature in the aftermarket fuel injection tuning programmers, such as the Hypertech Power Programmer III, or the Superchips Micro Tuner shown in the nearby photo. You scroll through a menu and enter in the proper gearing and tire values to correct the speedo output. You can also install a programmable piggyback module, such as Superlift’s TruSpeed device, that only changes the speedo readings. The allowable electronic parameters varies somewhat from one OEM manufacturer to another.
With mechanically driven speedos it can be a lot more complicated and again, the methods differ slightly from manufacturer to manufacturer. There are two gears involved, the drive gear, which is on the output shaft of the transmission or transfer case, and the driven gear, which is on the speedo cable. Each drive gear can accommodate a small range of driven gears. If you need a ratio beyond that small range, you must also change the drive gear and that will mean partial tranny or t-case disassembly. In the case of having to change the drive gear, bear in mind that some speedo shops have correction devices that can be installed on the speedo cable.
There are several ways to calculate what drive and driven gears are needed, but we’ll distill it to just a couple. First, calculate the percentage your speedo/odometer is off, either using the formulas above or by measuring against mile markers on the highway. In the latter case, test over about 10 miles for consistency. Say your odo reads 9.1 miles over a 10 mile run. Divide by 10 to get a correction factor (9.1/10=.91). In the case of the formulas above, calculate the correction factor by dividing indicated speed by the actual speed (65 indicated /58 indicated = 1.12). The first example indicates a tire/gear ratio combo that is higher that original and the later indicates an overall ratio that is lower than original.
Pull the driven gear, determine the number of teeth and multiply it by the correction factor. Let’s say your gear has 18 teeth. Using both examples above; 18 x .91=16.38 teeth, and 18 x 1.12= 20.16 teeth. You round to the nearest tooth, which would be 16 teeth in the first case and 20 in the second. A simple formula is:
| new teeth needed = | current teeth x indicated speed |
| actual speed |
Remember that radical changes may require you to change the drive gear in the tranny/t-case. The dealerships were given charts of drive and driven gear combos and these will help get you set up. In some cases, you may find that particular combinations of gears and tires may be outside of correctable range, electronically or mechanically. That’s where the speed shop can help you with some custom correction parts.