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Join Date: Mar 2006
Location: Kyle, Texas
Moontroll, like someone mentioned above, there are too many variables to guess why your gas mileage has gone down some. It is entirely possible that the new tires are it alone. If the new tires aren't identical to the old tires that were replaced, identical in circumference and weight, as well as tread design, they will have some effect on performance of your bike. If the front tire is slightly larger, it will make less revolutions at the same speed and distance, showing slightly slower on the speedometer, and less miles on the odometer. If it shows less on the odometer and you are used to going 120 miles on your trip meter before filling up, and it usually took 2.4 gallons, (roughly 50 mpg) and now after the new slightly larger front tire, when you fill up at 120 miles on the trip meter, (you've actually gone 130 miles) it takes 2.6 gallons (roughly 46 mpg when figured on 120 miles traveled) but you actually traveled farther than your trip meter says. It might not make that big a difference, just having a tiny bit larger tire, but the tire probably weighs a tiny bit more as well, and may have a different tread design and the rolling resistance may be greater. You get the point, I'm sure.
My 99 jeep wrangler had 31" tires on it since I bought it and put them on. It got right at 20 mpg in combined driving. 3 years later I put on a lift of 2.5" and put on new, very aggressive treaded mud grips, still in 31" tires. These new tires combined, I'm sure, with the 2.5" lift, dropped my gas mileage to 15-16.