Making sense of the mpg gauge with taller tires

Taller tire circumference throws off the gauge.

Here are the calculations for the new MPG after installing a taller tire. In my case, I went from 265/60R20 to 275/60R20, and I used 16 mpg as base.

1. Determine the old tire size and the new tire size.

For this example, the old tire size is 265/60R20, and the new tire size is 275/60R20.

2. Calculate the overall diameter of the old tire and the new tire.

You can use 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 0.60 x 2 / 25.4) + 20 = 32.519685 inches
New tire diameter: (275 x 0.60 x 2 / 25.4) + 20 = 32.992126 inches

3. Compute for the overall gear ratio.

Divide the old tire diameter by the new tire diameter.
Overall gear ratio: 32.519685 / 32.992126 = 0.98568019

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

New MPG = Old MPG x Overall gear ratio
New MPG = 16 x 0.98568019
New MPG = 15.7708831

 

If only it were that easy. It's a good start, but doesn't take into account increased weight, rolling friction, etc., just to name a few.

 

Use tire circumference to adjust actual distance travelled to indicated distance travelled, so if your odometer says 100 miles, you've actually traveled 101.5 miles, then divide by fuel used.

 

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 different factors play into the decrease in mpg.

 

Your math is a little off, you need to divide by the effective gear ratio. If you have larger tires you drive further for each revolution. Truck thinks you went 10 miles and you actually went 11 miles, so your mpg is higher. Likewise your odometer, speedometer, and range to empty all have the same effect where they will read lower than reality.

 

Sure, you're right...I tried this formula, and there is a bit of a difference. If you have a different equation, pls post it.


Using π ≈ 3.14159, the formulas for calculating the circumference of the old and new tire sizes become:
Circumference of old tire = (20 x 25.4 + 2 x 265 x 0.6) x 3.14159 / 100
Circumference of old tire = 86.99157 cm
Circumference of new tire = (20 x 25.4 + 2 x 275 x 0.6) x 3.14159 / 100
Circumference of new tire = 87.24157 cm
The difference in distance traveled per revolution between the old and new tire sizes is:
Difference = New circumference - Old circumference
Difference = 87.24157 - 86.99157
Difference = 0.25 cm
To calculate the new mpg reading, we can use the following formula:
New mpg = Old mpg x (Old circumference / New circumference)
New mpg = 16 x (86.99157 / 87.24157)
New mpg = 15.98

 

I feel like you're over complicating this. You don't need to use Pi for the equation because it drops out anyways.

Old tire = 34"
New tire = 35"
% change = 35/34 = 1.029 = 2.9% further distance driven

If your mpg is 16, then 16 * 1.029 = 16.47, if your speedo says 65 then 65 * 1.029 = 66.9mph, etc.

 

Not sure why mpg went up. Can you clarify for me?

 

In the above example, because the taller tire means you went further than the odometer indicated.

not something you can do with the economy computer, something you have to calculate at the pump. Miles traveled/fuel used.

just use the trucks computer as a pedal trainer. Try keeping over 15 as long a time as possible. Like get to the speedlimit quicker, so you can coast earlier.
Kick it out of OD under 55.

 

His is based strictly on the distance traveled.

If the computer says you went 16 miles per 1 gallon, you actually traveled 16.47 miles because of the difference in tire size that the computer doesn't know about.

 

It likely won’t in real life with larger tires. Take your real world gas used and apply the % of difference in distance traveled VS actual reading. The gauge is not the way. 400 miles on 28 gallons = 14.2mpg.
If your tires are 7% larger by circumference, then apply that to the mileage and recalculate mpg.
400 x 1.07 = 428 miles.
428 adjusted miles on 28 gallons = 15.2mpg.

 

I can save you a whole bunch of time... The equation goes something like this.

Gas divided by Tundra = Sucks

Old MPG = Sucks
New MPG = Sucks +/- a lil' bit.

 

Then to complicate matters the Toyota OEM speedo is usually a few percent too high compared to checking with a gps/etc. So ironically small increases in overall tire diameter often make the speedo accurate haha. This is part of why the left lane prius menace doing 57 in a 60 exists, person thinks they are going 60. This means the oem “miles” and mpg on the car are overstated in most cases I have seen (owned 4 Toyotas like this so far). The math above is still correct to adjust for the relative tire change diameter change.

 

FYI the odometer is incremented based on pulses of the wheel speed sensor. The speed signal is created from the same wheel speed sensors but the abs calculates it to a speed, broadcast in kmh to the cluster. The cluster reads in both signals. The milage increment is true to actual distance travelled. The speed coming into the cluster is true speed but the cluster puts a safety factor or in before displaying it.

I set my cluster to kmh, signal coming in was 115kmh (true speed matching gps) cluster readout showed 117 kmh with the safety factor.

 

My answer to this? Bourbon............ Carry on...........

 

Makes alot more sense now. Do you know if the speedo safety increase is a constant number added like 2kmh or 2mph type thing or more of a percentage of speed?

 

Pretty sure it’s a fixed percentage. The “safety factor” seems lower at low speeds.

I need to log it at several different speeds and look at the difference margin

 

Nice this is good info, percentage makes the most sense coding wise.