http://www.gitout.com/forum/4wd-tech/bigger-tires-and-gas-mileage-how-to-determine-actual-gas-mileage Simple:Press Forum Version 4.0.1 http://www.gitout.com/forum/4wd-tech/bigger-tires-and-gas-mileage-how-to-determine-actual-gas-mileage/#p268 4WD Tech http://www.gitout.com/forum/4wd-tech/bigger-tires-and-gas-mileage-how-to-determine-actual-gas-mileage/#p268 Bigger tires are an excellent upgrade for off road driving but they also decrease gas mileage.  It's not uncommon to experience a 2-4 mpg reduction in gas mileage with your off road tires.  Why is this? 1. They are HEAVY.  Most weigh 20+ lbs more than stock tires. 2. Increased Rolling Resistance.  Aggressive tires increases traction off road and rolling resistance on road. 3. Incorrect odometer & speedometer.  Larger diameter tires will make your vehicle to travel further for each rotation. [caption id="attachment_592" align="aligncenter" width="560" caption="Tire Rotation Diagram - Distance Traveled per Rotation"][/caption] There's nothing that can be done about the tire weight or rolling resistance of your tires (beyond proper air pressure).  But you can do something about the odometer. There are 2 simple ways to learn how to calculate your gas mileage after increasing your tire size. 1.  With GPS:  Drive your vehicle at 50 mph (gps speed).  Read your odometer speed.  Take your odometer speed minus 50mph and multiply by 2.  This will tell you the % your odometer is off. 2.  Without GPS:  Measure the diameter of your old tires and new tires.  Divide your old tire diameter by your new tire diameter.  Subtract from 1.  This will be the % your odometer is off. [caption id="attachment_597" align="aligncenter" width="560" caption="Odometer Calculation Chart"][/caption] If your vehicle odometer registers 250 miles and it takes 16 gallons to fill... Multiply 250 miles by 10% (amount odometer is off calculated above) = 25 miles.  Add 25 miles to your odometer reading = 275 actual miles traveled.  Divide 275 miles by 16 gallons = 17.18 MPG Your non-adjusted gas mileage would have been 250 divided by 16 gallons = 15.62 MPG That's nearly 2 MPG better!  It's also good to know your "actual speed" on the highway and in school zones to avoid getting tickets.  We will talk about the other expensive alternative alternative (regearing) another time. Share your comments and questions! ]]> Thu, 15 Oct 2009 18:51:16 +0000