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MPG effected by larger tire than original

Yahoo Message Number: 79484
How would I set up a math ratio to determine the increase in distance traveled with every revolution of my tire, since my tire has a greater circumference. My SilverLeaf indicates that my rolling MPG is 6.6 But I know the MPG should be better because I travel farther with each revolution than the original tire.
Or am I just trying to create a fantasy that my mileage is better than 6.6.

Stan

2005 Inspire
51406

Re: MPG effected by larger tire than original

Reply #1 –
Yahoo Message Number: 79489
Just multiply the 6.6 by Pi 3.1415926.
So now you are realy getting 20.73 MPG.
Don't let anyone dispute this.

Bob Wexler
SOB

Re: MPG effected by larger tire than original

Reply #2 –
Yahoo Message Number: 79494
I took the revs per mile of the smaller tires and divided it by the revs per mile of the larger tire and multplied the result times the measured mpg. When I went from 275 R 70 to 275 R 80 though the diameter increase was 5 or 6 % as I remember the fuel milage increased by 8 or 9% because the rpm of the engine dropped appreciably. TWI 98 Allure 30255 at the time. I could get almost 10 mpg driving ay 58 mph with my 36' Allure lightly loaded pulling a Saturn 300. 3100 lbs.
 

Re: MPG effected by larger tire than original

Reply #3 –
Yahoo Message Number: 79501
Stan the MPG in your engine ECM is calculated by the engine itself and Silverleaf uses that readout from the ECM. There is an input required , in the ECM, of the Revolutions per mile. The formula for the distance of one rotation is Pi X Diameter. Pi is 3.14 If you want it to read out properly, you will have to change the ECM. As someone else indicated you can do the formula they suggested manually. There are other issues with changing the tire size. The factory sets up shift points in the transmission and other data from this Rev per mile input. Since the engine is turning less at a given speed, you may actually hurt the fuel mileage and performance, by dropping below the best torque RPM, and lugging the engine, in certain conditions, like rolling hills terrain. It would be best to change it in the ECM for optimum performance.

Ron Baran 09 Magna7025