CMARNOLD78
Ready to race!
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- DC
No, I didn't really expect either. Had seen other reports of it and hadn't really thought about tires & fuel economy prior. The stock Potenza's fit into the 'Grand Tourer' tire category which are also designed to help fuel economy. There was also another post in here from someone who went from 'eco' tires in their Jetta and got a significant drop. The Pilot Sport are also have a chunkier tread and are actually noticeably literally sticky after driving (like race wheels). Also after doing a quick bit of research when you have new tires the circumference of the tire is greater leading to milage calculation errors in the trip computer*.
I think all of the above led to the actual AND calculated drop in MPG
*From Tire Rack "New, Full-Treaded Tires Travel Farther per Tire Revolution Than Shallow-Treaded, Worn Tires
Vehicles are programmed with their Original Equipment (O.E.) tire's revolutions-per-mile to allow their odometers to calculate the distances traveled. Unfortunately vehicle odometers aren't always 100% accurate and the tire revolutions per mile will change as its tread wears.
The diameter and circumference of a new, full-treaded tire is greater than that of an old, worn-out tire. Considering that many passenger car tires are molded with beginning tread depths of 10/32" to 12/32", the tires will be approximately 1/2" to 6/10" shorter in diameter when they've worn to the minimum tread depth of 2/32".
In order to determine how much odometer error this dimensional difference might cause, the Tire Rack team drove one of our 2008 BMW 328Ci test cars on a set of new, full-treaded 205/55R16-sized tires, as well as another set of the same tires shaved to worn-out (2/32") tread depth. We used a Global Positioning System (GPS) to measure the 100-mile test distance on a dry Indiana Toll Road and maintained an average speed of 70 mph. We then compared the number of miles traveled indicated by the vehicle's odometer to the GPS receiver and highway mile markers.
When the GPS indicated exactly 100 miles had been traveled while the vehicle was equipped with new 12/32" deep tires, the vehicle's odometer registered 99.4 miles. When the vehicle was then equipped with another set of the same tires shaved to 2/32" of remaining tread depth, the odometer indicated 101.0 miles. While the 100-mile test distance didn't change, essentially the vehicle's odometer overstated the distance traveled by about 1.5% when equipped with the worn out tires.
Since drivers traditionally monitor their vehicle's fuel economy by dividing the number of miles traveled as registered on the odometer by the number of gallons used to fill the tank (or by simply letting the vehicle's trip computer handle the task), the accuracy of the vehicle's fuel economy calculation is dependant on the number of miles indicated by the odometer.
This means that the vehicle's fuel economy (whether calculated by the driver after filling up or the trip computer) would instantaneously appear to drop by about 1.5% when fitted with new tires simply because the vehicle would actually have traveled 1.5% farther than it did when equipped with its recently removed worn-out tires."
