I recently put 18" wheels and tires on my Terrain that originally came with 19" chrome wheels. The 18" wheels and tires came from another terrain (I purchased them on E-bay) and stiil had the original pressure monitors in the wheels. When I put the new wheels on the Terrain the vehicle would not recognize the sensors in the new wheels (the sensor I.D. numbers have to be programmed into the cars computer by the dealer). I did not bother to have the sensor numbers programmed since the Terrain will only store one set of numbers at a time and I do not want to have to program the sensors everytime I put the 19" wheels on the car in the spring. The downside was that the Terrain indicated on the dashboard display that the monitoring system was inoperable (error could be cleared by pressing the up or down arrow on the center stack) and a yellow warning light was indicated in the speeedometer. To my surprise after driving a few hundred miles the Terrain no longer indicated that the monitoring system is inoperable and the yellow warning light is also not activated (The tire pressures will still not indicate when the tire pressure menu is displayed). It appears that after driving for a number of miles the Terrain must deactivate the warning systems which in my circumstance is a good thing.