Simple Math or Minor Parts Can Keep You in the Know
How many "friends" do you know that have added 3 or 4 inches of lift to their CJ, bolted up a brand-new set of 33x12.50 mud tires, and headed out for the trails? Then along the way, the "Man" stopped this "friend" for a little chat. Although the "friend" argued valiantly that he was only driving a little more than 60 mph, the "man in blue" pointed at his radar gun, which flashed "71 ... 71 ... 71."
How can this be prevented? The first step is realizing that any change in tire size is going to throw off the speedometer. The bigger the change in tire size, the bigger the effect on the speedometer. Stepping up one or two tire sizes will affect the speedometer a little, but if you choose to go up more than a couple of sizes, change tire sizes by more than 2 inches (e.g., from 30s to 33s), or start getting into the really large tire sizes (35s and up), you should calibrate your Jeep's speedometer.
Larger tires have another negative effect on your Jeep. In addition to throwing off the speedometer, a tire change will also throw off the odometer, sometimes by as much as 5 to 10 percent with really large tires. Over the course of a couple of miles this difference can amount to a couple of tenths of a mile-a big deal when you're following a complicated route map.
Calibrating your Jeep's speedometer and odometer is not difficult. Eventually tiring of setting up a new worksheet every time a new set of tires was checked, the following system was formalized to simplify the process.
To get started, you'll need a facsimile of the accompanying Calibration Data Sheet to record the necessary information and a stopwatch (a digital watch will work, but a stopwatch makes things easier). It's usually a good idea to bring a helper along to record the data, too.
Next, choose a time period that has mild traffic, and head for the local interstate or any other stretch of highway with mileposts or mile markers that allow you to cruise for a distance at a constant speed. Once you are on the highway, drive at a constant speed, around 60 mph (it makes some of the calculations easier). Even if you have cruise control, a more exact speed can usually be maintained manually, because most cruise controls deviate somewhat over time.
When you pass the first milepost or marker, start timing, and record the odometer to a tenth of a mile on the worksheet and the speed in which you want to cruise. At periodic intervals, maybe every two or three miles, record the elapsed time, odometer reading to the tenth, and marker. The longer the run you make, the more accurate your calibration will be. Ten miles is usually adequate. Make sure you record the data at the end of your run.
Since you will more than likely be returning on this same highway for your next run, you might as well record another set of data to confirm the initial pass. Finally, don't try to do the math while on the road. If the results don't look right, you can always do it again.
To illustrate the process, we checked the speedometer on a '93 Wrangler that was recently upgraded from stock P205s to 33x12.50-15 Goodyear tires. Here's the statistics we came up with on a calibration run: We ran over a 10-mile course, holding the speedometer at 60 mph. The odometer registered 8.2 miles, and the elapsed time was 8 minutes and 54 seconds (8:54).
The speedometer error can be calculated with the following formula:
(test course miles x 3,600)
elapsed time of run in seconds
In the example test run, it took 8 minutes and 54 seconds to travel 10 miles; the actual speed turned out to be 67.4 mph, calculated as the following:(10 x 3,600)/8:54, or 36,000/534 = 67.4.
Dividing the actual speed by the speedometer's reading gives the speedometer error: 67.4/60=1.12, or 12 percent.
You can either do the math as shown above or use the accompanying Speedometer & Odometer Calibration Worksheet, which quickly and easily takes you through the same calculations-just drop in the numbers where shown, and perform the calculations indicated.