Making sense of the mpg guage with taller tires

Taller tire circumference throws off the gauge.

Here are the calculations for the new MPG after installing a taller tire:

• Old tire size: 265/70R17
• Old MPG: 19 mpg (Replace this with your actual mpg with stock/previous tire size)
• New tire size: 255/75R17

1. Calculate the overall diameter of the old tire and the new tire using an online tire size calculator or the formula: (tire width x aspect ratio x 2 / 25.4) + wheel diameter = overall tire diameter.

• Old tire diameter: (265 x 0.70 x 2 / 25.4) + 17 = 31.64 inches
• New tire diameter: (255 x 0.75 x 2 / 25.4) + 17 = 32.04 inches

2. Compute for the overall gear ratio. Divide the old tire diameter by the new tire diameter.

• Overall gear ratio: 31.64 / 32.04 = 0.988

3. Multiply the old MPG by the overall gear ratio to get the new MPG.

• New MPG = Old MPG x Overall gear ratio
• New MPG = 19 x 0.988
• New MPG = 18.772

 

I do agree many people just look at the mpg readout on the dash which without accounting for changes in tire size. I do sometimes wonder if the dash readout is on the conservative side with stock tires since most of us have found the speedometer about 3 mph higher than what a GPS shows at highway speeds, and 33” tires are potentially more accurate for the stock programming.

 

I have few questions (may be dumb questions) on this topic.

1) I think gear ratios cannot be calculated by the diameter difference right?

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2 ) Also diameter change drastically from tire to tire right? (just picked 2 tires)

VS
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3) This diameter changes and we not getting accurate distance might impact the reading right?


4) Wouldn't the wheel + tire weight impact the mpg?

(Stock Dunlop)

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Hankook DynaPro

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5) Wouldn't the friction by the area 265 vs 255 impact the mpg? PSI also impact the area then impact the mpg again?


6) Wouldn't the wind resistance impact the mpg? (I mean wind facing a 265 face vs a less area 255?)


7) Wouldn't the extra height gain impact the mpg? ( Eg - some trucks and suvs having wind deflector thing probably to keep the height low restrict the air going under may be save fuel?)

Pic of the Tundra active air dam / wind deflector thing:

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8) Also few other stuff like treat pattern impacting friction which again impact mpg? (may be?)


Not sure, just few things popped in to my mind.

 

That is what I noticed too with my 285/70R17 32.76" tires. My dash digital mph reading match with the road side radar speed reminder board (or what ever that LED speed showing thing called).


this thing ->

 

Maybe I can change the thread title to something more descriptive. What I wanted to say is that the mpg guage should read lower based on the computation I provided. If your actual experience shows worse mpg, then you have a starting point for guesstimating how much the other factors play into the decrease in mpg.

 

I get a little confused why a correction to the calculation would be necessary.

If 285/70-17s correct the speedometer to be dead but accurate, doesnt that mean it also correct the measurement of miles traveled? If that’s the case, the computer would calculate MPG perfectly. Wouldn’t it?

 

I’m actually curious about this, I wasnt being a smart aleck lol. Anybody know if a correction is actually necessary?

And also, does running 265/70-17s actually over-report miles on the odometer? Or is miles traveled on the odo somehow an independent measure?

I am looking for as many justifications for 285s lol. So anything that helps me convince myself that 285 are the “true” size

 

If your new tires are larger than stock, you would actually divide the calculated mileage by .988. So, if you're calculating 19 based on the odometer/gallons used, your actual mileage is 19.23×××

If you install larger tires than stock, you're actually traveling further than the odometer indicates. Of course, this measurement is actually slightly high with stock tires, but we don't need to get into that.

So, when people install bigger tires and their mileage drops, it's not actually as bad as they think.

 

Just get the 285's.



Seriously, though, My speed is dead on with the 285s installed. I'll have to pay closer attention to the actual miles driven versus the odometer to confirm the other part.

It's kind of funny; my 4Runner might actually be over 100,000 mi by now if I had stuck with stock size tires, lol! Instead, I'm at a measly 97,599.

 

Agree, this is what I thought too. Not sure how they calculate milage but I think it used to do with axle rotations and if so, larger tires take longer for full rotation and you cover more ground. I don't accurately track my mileage and just go by the computer but I have a sneaky suspicion that the 15.5-16.5 mpg I'm getting at the end of each tank is likely closer to 16-17. I'm not too concerned over it being off by 1/2 mpg.

I will say even though the speedo is correct now at lower speeds, it does seem to read 1-2 mph lower than gps speed around 80. When I had the 265's it was 1-2 high.

 

Mine seems pretty dead on with the 285's, and reads a tad low with the 295's. It seems like there's always a little variation in actual size from one tire manufacturer to another. It seems even worse with larger off-road tires.

 

I can seriously get lost in numbers, but doesnt the fact that 265s over-report miles traveled, actually results in less accurate mpg calculations than 285s

seems to me that because 285s result in accurate speed readings, it must also mean it’s measuring miles more accurately too. And also MPG. Im still confused. I might need someone to explain it to me slower, or louder haha

 

You're totally correct! This is exactly why you need 285's.

 

Very helpful!!! You’re right!!!! Yay 285s!!! Hahaha

 

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Seriously though, I don't think you'll be disappointed with performance or mileage, given that you have reasonable expectations. The little extra footprint is nice, and they look good on the 4Runner.