Under the Hood: New tire size changes odometer and speedometer readings

McClatchy-Tribune Information Services

January 23, 2018

Q: Three months ago I purchased 2010 Jeep Wrangler Sport. It came with P255/75R17 tires which are larger than the original tires which were P225/75R16. Is there an easy and inexpensive fix to re-calibrate my speedometer so everything is correct based on the present tire size? Thanks.

Fred S, Millville, N.J.

A: Your replacement tires cover about 8.5 percent more ground per revolution than the stock tires, making both the speedometer and odometer read low by this percentage, regardless of your speed. For example at an indicated 65, your actually doing 70.5 mph.

Unlike the old days when one could swap out the speedometer driven gear in the transmission or purchase a gear-driven cable adapter, this will require a software update to the powertrain control module or instrument panel module, as applicable. This could be something for the dealer to take a crack at. My concern is if the factory scan tool parameters would provide for such a significant change in tire size.

Jeeps and other off-road vehicles are often modified in this and other ways, and there are several aftermarket sources for DIY speedometer/odometer reprogramming. You could either purchase a device such as Rough Countryís Jeep QuickCal Speedometer Calibrator (applicable to 07-18 Wrangler JK models, about $140), or perhaps Hypertechís Speedometer Calibrator, which can also serve as an OBD-II code reader, for about $220. These plug into the under-dash OBD-II data link connector and make quick work of the task. Another option would be to check with nearby 4X4 specialty shops and see if they can use their tool on your vehicle for possibly a few less bucks.

Q: I was wondering whether the 2001 GMC Sierra HD 2500 Dual Cab truck has ever had an electronic recall. I have an electronic problem that I noticed on at least two other, different GMC trucks Iíve ridden in. Specifically, the ABS/Brake light is almost always on; the gasoline indicator works intermittently; and the radio/clock almost never works. All other gauges work. An Internet research brought up a possible problem: electronic cluster. Before I pay for an electronic diagnosis ($150), an electronic cluster (+$500), and installation fee, I wanted to get your input on this. Any ideas?

Rudy N.

A: I searched and was unable to find a service bulletin, OEM campaign, or recall that would tie any of these faults together, as well as any common physical connection. Certain Chevy and GMC trucks of your vintage were recalled for possible corrosion that might affect the ABS systemís front wheel speed sensors, resulting in undesired ABS intervention at low vehicle speeds.

I think you may have three different situations. Retrieving diagnostic trouble codes from the ABS system should shed light on the cause of the illuminated warning indicator. When the fuel gauge doesnít work, does it go to full or empty? This should help determine a possible cause. A scan tool check of fuel level (info from powertrain control module) versus the instrument gauge would guide a tech to perhaps the fuel tank sending unit as opposed to a possible instrument panel/gauge fault. Iíd start diagnosis of the radio/clock with power and ground checks at the radioís connector 1, terminals 5, 9 and 10.