Adjust MPG calculations with tire size?
 

Howdy,

I've added larger than stock tires to my truck. Using GPS and highway mile markers, I know that I'm at a differential of 1.073. How can I add this to fuelly to adjust the MPG calculations derived from my odometer readings at fill up?

In other words, for every mile my truck thinks I drive, I've really gone 1.073. This really adds up over a tank of gas, and I'd like for fuelly to reflect that. How can I change it?

Not sure if you can do it in an automatic setting in the system but you can keep a calculator handy and multiply the odometer number before putting in the distance traveled.

I could do that, if I wanted to use a "trip" odometer reading for each fill up. However, I'd really prefer to keep using the truck's actual odometer and only extrapolate the MPG numbers.

One easy way to do it is to make the adjustment to the gallons pumped. This way, the odometer mileage stays consistent without need for complicated tracking, and the miles per gallons come out right.

In other words, if you are traveling 1.073 miles for every mile the odometer records as 1.000 miles, this means you are actually traveling 7.3% more miles than the odometer says you are. So, multiply the gallons pumped by 0.9320, and divide that into the odometer-reported miles traveled to get your adjusted miles per gallon.

Example: the odometer says you drove 400 miles and used 20 gallons, multiply the 20 gallons by 0.9320 to get 18.64 gallons, to give you a miles-per-gallon of 21.46. If your odometer had the correct mileage, the miles driven would have been 429 miles (400 x 1.073), which when divided by the 20 gallons pumped would give your the 21.46 miles per gallon.

You already answered your question, multiply the odometer reading by 1.073 for the actual miles travelled.

Another way is to divide the diameter of the old tires versus the new ones. That number should be very close to your already calculated 1.073 number.

Look at it this way. if you go from a 24 inch to a 25 inch diameter tire, you will go 25 miles instead of 24 miles on the old tire. You will indicate 96 miles while you travel 100. The number is 100/96, slightly greater than 4% more.

Or just have this app coded with whatever voodoo is needed to be able to enter your tire size differential and then it does it for you. If I wanted to do my own calculations I wouldn't be using this app.