Math deficiency: Talk to me about calculating mileage with non-stock tires

Hi all,

I recently replaced my tires on my 2019 Tacoma with 265/75-16's. The tire calculator chart says that the tire diameter is 3.4% bigger than my previous tires.

Given that my mpg readout is likely calibrated for the smaller stock tires and subsequently more revolutions per mile, would it be accurate to assume that my mileage is 3.4% greater than what my display tells me? If not, how would I go about accurately calculating my mpg other than using a third party gps device to determine mileage and begin from a full tank with subsequent refill?

 

no. you would add 3.4% to the miles driven per tank and then calculate the mpg. Miles driven/Gallon's used = MPG

 

use the vehicles trip meter...

fill it up, set it to zero, drive until gas light goes on, then fill it back up, take note of how many gallons you take on, and then divide

 

but if the tire size is different, will the speedo record the correct miles driven?

 

NONE of Ur electronic devices will match up.....................even if All components stock

the dash mpg info is close yet not as accurate as miles driven / gallons used

to get the very best / accurate info

have a loop U can drive under consistent conditions

cruise control is better than under foot

fill up.....zero the miles .........drive conservatively ( hi-way .....NOT commuter environs )

3 to 5 gallons used would be the min

divide miles driven by gallons at fill up

new tyres vs worn tyres WILL CHANGE ANY CALCULATIONS

Any TRUCK !!! even tho U are not using stock size

so being totally obsessive analytical retentive will drive U crazy

a good run will provide U reasonble info

 

1st thing to do is check your speedo against a good GPS and see how far off it is, or isn't.

Because of a typical 'read high' error in most OE speedos, *sometimes* a bit more tire actually corrects the speedo.

 

no. you take the trip meter and have to calculate the supposed vs actual numbers. If you know the speedo and odometer are off by a 3.4% variance it should be pretty easy to calculate.

 

This is correct. ([miles driven] x 1.034) / gallons

EDIT: Actually, you can switch that formula around like you originally thought and get the same answer. Whatever it tells you your MPG is, multiply by 1.034 to get actual MPG.

 

Understand this, but the commetn I quoted, the person said just use the trip meter. And I was asking if you have changed stock size tires, won't the trip meter be off.

 

Yes it will be off because the distance it's expecting to travel over one revolution of the wheel is less than what it actually is with the larger tires.

 

Hold up, so if I'm on larger tires am I getting better or worse MPGs than my truck computer is telling me?

 

Should be better because you're physically traveling further than it thinks.

 

Your assumption is, more or less, correct. The new tire has a larger circumference, so it takes longer to make a complete revolution, relative to the smaller tire. This is why the speedometer is off by that 3.4%. So for every 100 miles driven according to the dash, you've actually have gone 103.4. What is not accounted for is the extra weight of the new tire, adding to the unsprung weight. So it's not as easy as adding 3.4% to your number. Your mpg is better than what is shown, but just slightly.

 

Wouldn't it be easier to just fill up when needed and not worry about all that mpg stuff?

 

speedometre ......trp metre will be off from Normal tyre wear

so nit pickin tyre size is going down the rabbit hole

to the point if an accurate mpg with staock tyres................thats out the window now sizes have changed

driving & filling up is NOT goin to be determined by 3%

Ur gointo go & do no matter what

U only need a rule o thumb when trip planning.....................matters will rarely be as close as 3% !!

as i mentioned before new tyre vs same tyre wear are real world factors

 

This, but you need to know the difference in circumference, not diameter. You get that by multiplying stock and new diameters by pi and noting the percentage difference. You have metric sized tires on standard sized rims, and sidewall is a percentage of width, so there's even more math involved.

 

While this is all true. If you are trying to calculate your mileage off of an inaccurate number then your result's aren't accurate either.

 

if U had 3% more ‘ winky ‘ ........................Ur girlfriend would not notice !!

regardless of tyre size Ur going to drive to get to where UR going .............. destination achieved

no one in their right mind would stop 3% shorter

 

Agreed.

Using 100 and 103.4 miles and assuming 5 gallons to fill up, that equates to 20 and 20.68MPG.

That's not really enough to worry about, IMO.

 

This is the best method. Luckily changing tire size doesn't effect the fuel gauge.