Bigger tires are an excellent upgrade for off road driving but they decrease gas mileage on road.  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 from aggressive tread.
3. Incorrect odometer & speedometer.  Larger tires travel further per rotation.

Tire Rotation Diagram - Distance Traveled per Rotation

Tire Rotation Diagram - Distance Traveled per Rotation

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.

Odometer Calculation Chart

Odometer Calculation Chart

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!

6 Comments on Bigger Tires and Gas Mileage:
How to determine actual gas mileage?

  1. david says:

    thanks for the advice. I have no gps, and I don’t trust my measurements or my odometer because of what I am about to explain.

    I have see that when I drive by a roadside speed trap, the reading on the sign might tell me I am going 21 mph, when I am watching the speedometer which tells me I am going 25 mph.

    May I calculate my percentage of wrong mileage based on this?

    25-21 = 4 and 4 * 4 = 16?

    Would that mean that my miles are counting 16% more than my actual distance?

  2. Ah Yeah says:

    It sounds like your tires are smaller than stock or the gearing is lower than the stock tire size. On most off road vehicles with larger tires the speedometer reads slow so you may think you’re doing 60mph when in reality you’re doing 65 or 70 mph. In your case your speedometer might read 60mph but you may only be traveling at 50mph. I’d recommend testing your speed at 60mph as low speeds can often be less accurate. Many smart phones (including the iPhone) have “speedometer” apps available for free. — I’d recommend testing at 50+ mph for more accurate readings.

  3. Jason says:

    David, your math is wrong…. First of all you need to subtract the speedo from the sign reading not the other way around “21sign-25speedo=-4″ then multiply by 2 not 4. So -4×2=-8. So in the example you would take 250x(-.08)=-20. Then add that number to the 250 from the odometer (or just subtract in this case) but it would be 250+(-20)=230
    The fill up was 16 gallons so you would have to divide 230/16= about 14.4 MPG if it took 16 gallons to drive 250 miles calculated by your odometer (really 230 miles after the correction was made)

  4. Jason says:

    Wait a minute after looking at this again…….. it seems like the wording used in the chart is different than the wording used in the example. Depending on the method you use the chart “GPS-Speedo” or the wording “Speedo-GPS” you get different results. The chart says 17.18 corrected MPG but the wording above it seems to say 14.04 corrected MPG.

  5. timmy says:

    why dont people keep saying odometer when referencing the speedometer…dumb

  6. Ah Yeah says:

    Your speedometer and odometer will be equally slow or fast percentage wise. But you are correct when referring to speed it doesn’t make sense to reference odometer.