Wheel/Tire Diameter & Fuel Economy

I asked myself, how much do larger tires actually impact mpg? Then wondered, if everyone was using the dashboard fuel economy measurement, how much of this change in mpg could actually be explained by skewed measurement of 'a mile', to the vehicle anyways.

If my calculations are correct, for every 1" increase in tire diameter, your vehicles measurement of 1-mile will be off by ~3%.

That said, I'm curious how many are using the dashboard mpg readings to reach this conclusion?

And if you used a more hands-on, out of bands method to measure (like making the same trip every day and just know), did you reduce your speed by 3% per diameter-inch increase you added, to ensure you were keeping ~the same speed wind-resistance wise?

PS - I don't dispute that more material and rotational mass negatively impacts fuel economy. Just hoping to understand to what extent taking.

 

Miles divided by gallons.

 

lol

 

Yes size matters but weight of the wheels and tires also matter.

 

What is the lol? You asked how to calculate true mpg, I gave it to you. Have you even attempted that method?

 

I'm still shopping for tires. Keep reading about mpg - question is what method everyone else uses.

 

If you're concerned about maintaining your mpg's with a larger tire, get the lightest one you can find.

Firestone Destination P-rated AT's were the lightest 285 I could find at 42lbs each. They're also a fantastic tire. I'll be getting them again next time around.

BTW, your mph/mpg's are off with your stock 265 tires. When you put 285's on, they will be accurate.

 

From all I’ve seen when meeting up 4Runner folk that do increase tire size, I’m becoming an even larger fan of the 255/80 A/T choice. Increasing tire isn’t just an increase in radius, and not just an increase in mass (not just size, but load rating too), but also rolling width resistance and air resistance at higher speeds for those wider tires. I had 265/70s Toyo AT3s in S load (I think that’s regular car load rating), and now I have 285/70 Toyo AT3 in C load. I’ve seen about a 1 to 1.5 L / 100 kms less in long trips (1.5 - 2 mpg less) and at least 2 L / 100 kms less in short trips.

Additionally, since I’m little getting a bit up there in age, I’m sure having to change a tire while out on a trail wouldn’t be fun at all due to the weight. I can’t imagine trying to change 295/70 or bigger on my own, on a shelf road or in the rain.

 

If you want to maximize mileage, stick with the tires that it was delivered with.

 

Or something similar. Look up tire reviews that give you the "rolling resistance" of the tires. The best rolling resistance number equals the highest fuel mileage.

 

I would think most folks do factor in the diameter change relative to odometer reading. The catch is, was the odometer accurate to start? My change to 255/75 17’s was a +1.4% change to the miles / revolutions ratio, but my odometer was understating distance / speed by about 1.5%. So now my speedo / Odometer is within 0.1%. Correcting for math, my calculated milage is almost unchanged. (About 1/10 of a MPG deterioration.)

 

Delta odometer reading between fillups, divided by gallons put in

If running larger tires, correct the delta odometer reading by the ratio of the "revs per mile" figure published in the tire specs.

I've always wondered how the vehicle computes instaneous mpg, since there isn't a flowmeter in the fuel system.

 

Let be honest your foot will affect MPG more than tires.

Just get what you want and let something else bother you.

 

I think the biggest problem is the initial calibration. There seems to be a consensus that the stock 265/70r17 tires misreport MPH and therefore MPG when reported by the computer. Seems almost criminal and false advertising if true but many have experienced it.

The best method for calculating would be to use an accurate GPS to track actual miles (not computer reported miles) and divide by how many gallons it takes to refill. After changing tire size, repeat. Preferably, along the same exact routes. I'd compare those manual numbers to the computer provided ones (reset after every fillup - not a continuation). Only then will you have a true picture of what is real

 

So true - I'm going to have to use GPS to see what the stock calibration is. Not that I am infactuated with MPG, but it was a curiosity. I like the idea of narrower tires for some reason. Will have to read up on it