Miles per gallon

[Iron Clad Lou]

I have upsized my tires from 265/65/17 to 265/70/17...how do I adjust the onboard miles per gallon ticker? do I add a percentage? I know the speedometer reads 2 mph less, just wondering if I should add or subtract from the computer mpg...

 

[CJ3Flyr]

I have upsized my tires from 265/65/17 to 265/70/17...how do I adjust the onboard miles per gallon ticker? do I add a percentage? I know the speedometer reads 2 mph less, just wondering if I should add or subtract from the computer mpg...

I used two different GPS's and did the following:
1) I reset the tripmeter on the T4R and the warmed up, locked on GPS. I drove exactly one mile and noted the difference. Even with stock tires I discovered the tripmeter under-reported by .025%. So if the T4R tripmeter indicates 100.0 miles I have travelled 102.5mi. When I calculate mpg I just take my tripmeter miles X 1.025/gallons. The on-board mpg readout is famous for being optimistic. I read, here, that some of the people actually saw it get closer to correct with slightly oversized tires

2) I then compared indicated speed to GPS speed and discivered it was perfect. I mean I couldn't see the slightest difference between the two.





FYI- The Garmin and the Magellan showed no difference in speed indication or difference in distance travelled. Over time I have seen my 1.025 to be fairly accurate. When I road trip with the GPS at 100miles on the tripmeter the GPS shows 102.4-102.6. Local driving is 'off' because I am usually rolling by the time the GPS finds itself.

No GPS? Next time you're on the highway reset the tripmeter and compare it to the mile markers.

Hope that helps.

:cheers: