Tire size change

[RodL]

I put a bit taller tire on my VX last week. On the trip home yesterday I had reset the GPS Odometer and my Tripmeter to zero. At the end of my 100+ miles I had 135 miles on my GPS odometer and the Trip meter shows 130. My MPH is right at 2 mph different. I show 70 on the speedometer and 72 on the GPS. Is there a math wiz that can help me with a calculation to verify me fuel mileage occasionally? My Ultra Gauge is currently showing 18 mpg it has been steadily climbing since I have gotten back to Iowa and am doing more than just city driving. In Florida I was happy to get 15mpg both with the gauge and calculation of Miles and gallons per tank.

THanks.

Rod

[JoFotoz]

I'm pretty sure if you divide the GPS mileage by the fuel consumed...

...you'll get an accurate MPG.

Jo

[Y33TREKker]

Since the different sized tires are going to throw off your dashboard trip meter, I'd trust the GPS mileage and simply plug in the gallons required to top off the tank when refueling, then compare that to your Ultra Gauge readings. Between those two results, you should get a fairly close idea of the mpg you're getting.

[RodL]

I'm pretty sure if you divide the GPS mileage by the fuel consumed...

...you'll get an accurate MPG.

Jo

That is correct, but I don't always have the gps reset. I guess I can use it for the occasional check as I questioned but I was looking for how to by just resetting the trip meter. I have a habit of always doing that. Don't have the habit yet for the GPS. May have to work harder on developing it. Thanks for the response. I do feel dumb after reading it though.

[Vendetta]

From my mobile, so forgive the lack of links.

Go to the tire calculator at 1010tires website (Google it). Put in the OEM tire size (again, Google it or someone here can share) and compare to your new tires' overall diameter. The result will give you the percentage difference. Use that to multiply your odometer reading to get true mileage, and divide that be how many gallons you took on. There's your mpg. Also, you can use that figure to get true speed. You're not a flat 2 mph off, it's a percentage.

For me, it's odometer mileage or gauge speed * 1.1043 to get the true figure.

-V

[Y33TREKker]

Please also keep in mind that tire sizes change with wear though, so the simplest method overall would still be to just get into the habit of resetting your GPS trip.

[dietz99vcross]

While the GPS would be the most accurate I would also say that it is going to be marginal in the comparison because your other tires also wore down as time went on. If your odometer said 130 and the gps said 135 then that should give you a multiplier (135/130) of 1.0384615. Since you are going farther than your odometer says you will be getting better mileage than indicated, roughly 3.85%. Instead of the 15 miles per gallon you were getting the error in odometer reading would only account for about 0.5 miles per gallon. Not sure about the additional improvement.

[VxSport]

On our 265/70 r16 we were 7.8 % off
On our 275/65 r18 we were 9.9 % off
Now we are on 275/70 R18 and are running 12.1 % off

These numbers were calculated using 10mile road tests, over several years and different tires. There are a few shops out there that can test and build a speedo adjuster (electronic)

[whiplash]

I think if you take whatever your trip meter says when you fuel up, times it by 135 and divide that by 130 you should end up dang close to your actual miles driven. You can then divide your gallons of fuel like normal to have your corrected mileage.

[tom4bren]

Rod,

Your odo is off by about 4% now. Just figure out your mpg as you always have and just add 4% to that number when you are done.

Tom