Calculating Mpg with oversize tires (1 Viewer)

[CharlieS]

oops, meant to post this in the 200 forum.

Math isn't my strong suit. Is there a simple way to calculate mpg when you run different size tires? Let's say the dash display says you are getting 12 mpg (I imagine that it assumes you have stock size tires to do this calculation), but instead you are running 34" tires instead of stock tires. It feels like the mpg should go down, but by how much?

 

[Czechmate]

Determine what percentage your larger tires are over stock 275/70r16. Tire size calculator will get you that info. A “34” diameter tire will be about 10% bigger than stock. Multiply your miles by 10% or .10 add that to you miles driven then divide by the gallons and you get you adjusted mpg.

 

[mudgudgeon]

Search for an online calculator

 

[Rusty Marlin]

Or buy and install one of these:

Landcruiser 70/80/100/105 & LX450/LX470 Speedometer Calibrator/Correction Device | Lutz Auto

The Lutz Auto 70/80/100/105 Series Landcruiser Speedometer Calibrator can calibrate your speedometer to display the correct speed, compensating for any modification that affects the speedometer calibration (ie. tire size and gearing changes). Correcting the speed with this calibrator will also...

www.lutzauto.com

I've had one of his Beta units for a couple of years (2021-ish).
Change tire size, make an adjustment.
Change gear ratio, make an adjustment.

It'll dial your speedometer output to within a 0.1MPH of your GPS speed with ease, and keeps the Odometer reading "real".

 

[Devils Paw 80]

oops, meant to post this in the 200 forum.

Math isn't my strong suit. Is there a simple way to calculate mpg when you run different size tires? Let's say the dash display says you are getting 12 mpg (I imagine that it assumes you have stock size tires to do this calculation), but instead you are running 34" tires instead of stock tires. It feels like the mpg should go down, but by how much?

Your mileage will be better than the theoretical display because the odometer will be shorting you on distance travelled. Bigger tires travel further on one rotation, right? Take your new tire size and divide by stock tire size. 34/31=1.097. Multiply that number by your display to get actual MPG. Answer is 13.16

 

[Chamba]

Easiest way is to use a GPS for actual ground speed.

Take your indicated speed from the speedometer, then devide it by the number on the GPS for the percentage you are off.

Then on your next full up, divide the miles travelled since your last full up by the number of gallons used and multiply that number by the percentage your speedometer is off.

So for example: your speedometer shows 70 mph, while the GPS shows actual speed of 82 mph. 82÷70=1.17, of 17% off. Thus your odometer will show 17% fewer miles than your truck travelled.

Fill your tank, reset your trip meter and then drive it like you normally do until needing another fill up; let's say the odometer shows 250 miles at your next fill up. You fill it with 20 gallons of fuel.

Now you will take the miles travelled, devide it by the gallons used to travel that distence: 250÷20=12.5mpg. This is your INDICATED fuel economy.

But now to correct for tyre size: 12.5×1.17 (the 17% error in readout from your GPS correction)=14.62 mpg CORRECTED fuel economy.

Remember that those computer fuel economy readouts are seldom correct on Toyotas (that's why they lost that class action lawsuit in the early 2000s for their wildly optimistic MPG numbers). The above will give you your actual MPG. Once you have a baseline, you'll get an idea how far off your computer is.

 

[ZackR]

For me the easiest way is to figure out how much bigger the tires are than stock, and multiply the mileage number shown on the trip meter by that whenever I fill up.

For example: If the tires are 10% larger and you are at the gas station with 200 indicated miles on the tank, take 200 * 1.1 = 220. Say it takes 15 gallons of fuel, then take 220/15 and you get 14.66mpg.

 

[oilbrnr]

If you really want to know your mileage, get a GPS corrected Scangauge and run a few tanks.

It is depressingly accurate.

 

[CharlieS]

Thanks all!

 

[Nad Aitch]

I've taken a cue from the boating world and shifted to gallons per hour. Compared to seafaring vessels of similar mass the ol gal's quite miserly with the petrol.