Corey

iTrader: (1)

https://www.yotatech.com/mpg/mpg.htm

Found some code on the 'Net.
I check my mileage each @ fillup.
Been keeping records since November of 1998 with the 4Runner.

I write down the miles each fillup, see how far I drove between fillups, then devide it by the number of gallons, then I get my result.

This calculator does the same.

Tacoma Dude

What would be REALLY impressive is if you could code the differences from OEM tire size to the new tire size and then get the REAL mileage

Actually this is still a good link to keep around, thanks!

4RUNN!N

Thats a nice way to keep track of mileage, but I use gasbuddy.com to log my mileage. You can also keep track of gas prices on that site.

midiwall

Still in progress...
http://midiwall.com/4Runner/mpg.html

Tofer

midi: according to that i will get better gas milage by going to 33s.. is that right?

midiwall

Wow... you have me re-thinking this Chris.


Ummm... The ledgend is mis-worded (I'll figure out something better). The point is that "stock" is what the speedo/odometer is calibrated for.

lesse... if you're calibrated for 30.6 (265/70R16), and you're running 31.6 (265/75R16), then the odometer is 3% _low_. When you do the normal mileage math, you need to multiply the result by 1.03.

So the point isn't that you'll get better mileage by going to a larger tire, it's that the odometer is farther off as you go up.

garrett1478

you might want to put in some if thens or something so if the modified field is left blank, it will only use the stock tire size field (1:1 ratio), instead of having people put in 30.6 as the "modified tire size"

other than that, very cool.

midiwall

Good one... done!

Tofer

cool.. it still saying i will be getting 1 mpg better by going to 33s... is that right? i thought you got worst mileage wiht a bigger tire

midiwall

I think you're looking at the tool a bit sideways..

This isn't predicting the mileage that you'll get with larger/smaller tires - it's correcting for the odometer not being calibrated to the tires that you're running.


Lemme try it from the mechanical side.

The odometer (and speedo) are tracking based on the number of turns the driveshaft makes (or depending on the vehicle, the wheel hub, or some object point in the transmission). It's calibrated at the factory to know what tire size you have, so it will show the right mileage/speed. The calibration is a ratio between the turns of the "thing" (driveshaft or otherwise) and the distance moved in one rotation of the tire. If you change the diameter of the tire, then you will affect this calibration.

Here's a random example...

Say your stock tire is 30.6" in diameter (265/70R16). That means that for each rotation of the tire, you will move 96.13". Since this is "stock" the odometer will be calibrated to that value.

Now, if you put on 31.6" tires (265/75R16), each rotation will move you 99.27", or appx 3" more. But, since the odometer still thinks you're moving 96", it'll be off by about 3%. So, it will read 100 miles when you've actually traveled 103 miles.

The amount of fuel required to rotate this new tire may or may not change - that depends on a lot of things like friction, which is a matter of tire compound, contact area, tread type, road conditions, etc.

So this calculator won't tell you about what mileage you'll get with a certain size tire, it will only correct for the mileage error induced by moving away from the stock tire size.

brandontrek

this is awsome...

midiwall

Mine? Corey's?

Either way, thank you to Corey for the base idea & code.

Anything else that you'd like to see in there? Is it worthwhile to allow metric tire sizes or are people comfortable with just jamming values in inches?

boogyman

wouldn't you have to divide the miles driven by the ratio as well? because the miles from the odometer are not going to be correct due to the larger tire size. if you did that, you'd actually be travelling less miles with the same amount of gas which would account for the loss in gas milage with larger tires.
Russell

edit: i think you're only supposed to divide the miles by the ratio and not multiply the result by it.

boogyman

yea.. that makes sense.

lets say you go 200 miles on 10 gallons of gas. on a oem tire size of 30.6". the unadjusted mpg is 20.

now you go to 32" tires. the ratio is 1.04

so now you divide the odometer reading (200 miles) by the ratio (1.04) which comes out to 192.3 miles.

the mpg now is 192.3/10 = 19.2 -- less than the original 20mpg you got with oem tires.
Russell

midiwall

Ummm, hmmm...

Okay, let's start with more succinct numbers. We're going to compare 30.6" tires and 32" tires, which will measure 96.132654" and 100.53088" in circumfrence respectively. (C = PI * D)

The ODO is calibrated for 30.6" tires, thus when the trip says 200 miles, it really is 200 miles.

Keep in mind that the ODO works by counting "ticks" at some point of the drivetrain, not by really measuring distance. We can figure out how many ticks are in a mile by looking at the 30.6" tire:
Call it 659... So every 659 ticks = 1 mile.

Now if you replace the 30.6" tires with 32" tires, and let the ODO continue to think 30.6", the "miles driven" will be incorrect.

If the ODO says "1 mile" then it thinks you've gone 63,360" (5280*12). After 659 ticks, the ODO will STILL say "1 mile". But, since the 32" tire is 100.53088" in circumference, you will have actually gone 1.04561001 miles:
...which is very close to the same number as:
So... to get the "actual miles driven", you need to multiply the miles shown by the ratio of the new tire to the old. If the ODO says 200 miles, then with 32" tires, you actually drove 208 miles. If you burned 10 gallons to do it, then you're looking at 20.8mpg, not "just" 20.

boogyman

i stand corrected.. i was working off the assumption that mpg was lowered with larger tires instead of assuming larger tires travel further than smaller tires on each revolution.

i think the mpg may be still be lowered with larger tires though simply from the fact that your engine has to work harder to turn the larger tires 1 revolution compared to the smaller tires, even though you do travel slightly further.

Russell

TechWrench

Of course we could get realy 'anal' with the tire diameter issue. As your tires wear down, the overall diameter decreases with the increase in tread wear. Therefore, if you have a deep tread model tire, you would see a 'decrease' in real MPG as the tires wear down.

midiwall

Definitely true, but the only purpose of this calculator was to adjust for the odometer error - nothing else.


Yeup, very true.