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.

How is this information used? This data shows that when the Wrangler's speedometer says 60 mph, we're actually driving more than 67 mph. That difference is big enough to cost you a speeding ticket. In addition, once you have obtained this number, you can either do the math in your head or make a chart to keep on your Jeep's dashboard. For example, in the sample cited, the difference is close to 10 percent, so 10 percent of the speedometer's reading was added to estimate the Jeep's speed.

The odometer error difference is a little easier to calculate: It's simply the actual distance divided by the distance recorded on the odometer. In this example, we covered 10 miles while the odometer recorded 8.2 miles; 10 divided by 8.2 equals 1.22 (10/8.2 = 1.22). This shows that for every mile recorded on the odometer, the car has actually traveled 1.22 miles, or that it has a positive 22 percent error in the odometer reading.

Similar to the speedometer correction, the math can either be done as shown above or the Speedometer & Odometer Calibration Worksheet can be used, which takes you through the odometer calculations. Simply drop in the numbers where shown, and perform the calculations. This information is important if you keep track of gas mileage.

Why don't the speedometer and odometer have the same amount of error? There are several reasons. Factory speedometers are rarely accurate, especially on older Jeeps and SUVs, and because of the manufacturing variation, you'll experience differences in the same make and model. Even odometers and speedometers on brand-new vehicles are sometimes out of sync because of manufacturing tolerances.

Up to this point, all we've discussed is tire changes and their effects on your speedometer and odometer. Axle gear changes will also affect your speedometer and odometer, and the same methodology used to check and compensate for tire changes applies to gear changes, too.

However, off-roaders usually install a lower gear (higher numerically), which has the opposite effect on the speedometer and odometer than a change to larger tires would have. For example, we changed gears in a 4x4 equipped with 33x12.50-15 tires from 3.55 to 3.90. In this case, when the speedometer says 70 mph, we are actually driving about 64 mph. In addition, for a large number of vehicles, if you go to a dealer's parts department and tell them what differential gear you are running, they can sell you an OEM speedometer cable gear that will get your speedometer/odometer close to normal. Obviously, a change to a higher gear ratio (lower numerically) (e.g., going from a 4.11 gear to a 3.55 gear) will have the same effect as going to a larger tire.

If you have a nice rig with larger-than-stock tires (or you've changed your diff gear ratios) and are not sure how fast you're really going, you should figure out your real speed-it might just save you a couple of bucks on a speeding ticket some day!

The Calibration Data Sheet is used to record the information that you'll need to calibrate your Jeep's speedometer on your "data run." Remember that the more data collected and the longer the test runs, the more accurate the results.

CALIBRATION DATA SHEET |
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Odometer | Milepost Time Notes | Speedometer__mph | |

0.0 | 0 | 00:00:00 | |

9.0 | 10 | 00:09:00 |

**Instructions:**

1. Collect the indicated data for the shaded areas and complete.

2. Calculate data elements as indicated in formulas.

**Application**

• Multiple correction factors by actual mileage and odometer readings to obtain vehicles actual speed and mileage.

**Notes:**

• If X is greater than W, reverse the entries.

• If not using a stopwatch, make sure to track time, and be sure to correctly calculate time difference.

Copyright 1998, M.C. Olmstead

Here's the Calibration Worksheet with the data from the example in the text. Drop your own data into the shaded areas, and follow the instructions to calculate your Jeep's speedometer and odometer corrections.

SPEEDOMETER AND ODOMETER CALIBRATION WORKSHEET |
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SPEEDOMETER |
ODOMETER |
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Stopwatch Readings: | # | min:sec | Odometer Calculation: | # | Miles |

Ending Stopwatch/Clock Reading: | A | 0:08:54 | Ending Odometer Reading: | T | 51,304.8 |

Beginning Stopwatch/ | |||||

Clock Reading: | B | 0:00:00 | Beginning Odometer Reading: | U | 51,296.6 |

Difference (A minus B): | C | 0:08:54 | Difference (T minus U): | V | 8.2 |

Total Seconds Calculation: | # | Seconds | Distance Calculation: | # | Miles |

Minutes (only) from C x 60: | D | 480 | Ending Milepost | W | 111.0 |

Seconds (only) from C: | E | 54 | Beginning Milepost: | X | 101.0 |

Total Seconds (D plus E): | F | 534 | Difference (W minus X): | Y | 10.0 |

Test Course Miles: | G | 10 | Odometer Error (Y divided by V): | Z | 122.0 percent |

Factor (G times 3,600): | H | 36,000 | Odometer Correction Factor: | 1.22 | |

Actual Speed (H divided by F): | I | 67.4 | |||

Speedometer Reading: | J | 60 | |||

Speedometer Error (I divided by J) | K | 112.4 percent | |||

Speedometer Correction Factor: | 1.12 |