Am I calculating my MPG correctly?

Trying to get a gauge on what kind of MPG hit I have taken since I switched from stock 265/70/16 to 265/75/16 tire size. Stock suspension/wheels.

Basically all I do is re-set the trip meter upon fill-up...drive till next fill up. Divide the miles driven by the amount of gallons I put into my tank. That's it.

Pre - 265/70/16 Rugged Trails - 19.5 mpg avg
Post - 265/75/16 Cooper AT3's P-Load - 17.9 mpg avg

Is there some calibration I need to do in addition to that to account for the large tire size or is that pretty much "true"?

 

I am running 285/75/16 and my speedo is off by 3 MPH. If I am going 60 according to the speedo, I am actually doing 63. That means the odometer is also off. I am sure there is a formula to figure it out but math was never my strong suit.

 

https://www.tacomaworld.com/tirecalc

correct your calc by the % difference in stock tire size (what your speedo thinks you had) and the current size.

 

Hmm ok, so for the sake of easy math...let's say I went 360 miles on one fill-up.

According to that chart:
60mph (old) = 62mph (new) on the odometer.

So I actually went 372 miles, and should use that number in my calculation?

Or am I doing that incorrectly?

Thanks

 

Your new tire is ~3.3% larger. You'll want to add 3.3%(new tire is covering more distance per revolution) to your mileage before you divide by the gallons used.

 

Got it, thanks!

 

Yes. ODO reading x 1.034 for the actual distance covered.

Couple questions.

Were the old tires stock size? (I have a little 5 lug, I don't know yours) Reason is the calculation should be based on OE size to the current size. That's because the speedo/odo are set for an OE size.

Also, speedos/odos are often off from the factory a bit. You might want some detailed checking with a tool like a scangaugeII and a GPS, just to see how things are in terms of calibration.

Or just drive it, and realize it's gonna use what it uses, and don't worry about it.

 

Yep, old tires were the stock tires that came on the truck originally (stock size). I am not THAT interested in getting to the minute details of this, was just curious if my approach was in the correct "ballpark".

Thanks again

 

If I'm reading this right, the BFG tire shows a max load of 2185 @ 35 pounds . The Cooper shows 2 tires in that size, one at 2470 lbs and the other at 3415 lbs. If the P-load is the heavier tire, then you've gone up a great deal in tire weight and load range, resulting in a tire that can carry more weight at a cost of fuel mileage. The difference between a 70 and a 75 ratio tire is 13.25mm or 33/64 of an inch diameter , so I don't think you're losing a lot there.

 

P is for passenger. Guessing the heavier one is E.

Using the tire calculator, the difference is 3.4%. Not earth shattering, but not insignificant either. Because of effective gear changing, at cruising speeds his corrected MPG should rise a tick or two above what it was.