MPG difference with new 295/75/16

[BullElk]

With stock 275/70/16 on my '97 LX I would get 14 mpg around town, normal driving.

With recently upgraded 295/75/16 Hankook Dynapro mud tires I got 11.97 mpg. That is only the first tank with new tires but probably accurate.

I also noticed that between mile markers on the interstate my odometer measured .85 miles. 15% off

Sooooo.......would that make the actual "MPG" 11.97 or is there a calibration issue that needs to be accounted for to get a more accurate mpg with new tires? Adding 15% would get me back to 13.77 MPG.

Would I only loose .25 MPG with the larger tires with that calibration?

Thanks

 

[Otamiway]

Rough math... calculate the % difference(X) of distance traveled with old vs new tire (1 rotation). If your uncorrected speedo says you did 200 miles your actual miles traveled is 200 x 10X%. Use this new miles covered to calculate your actual mileage. HTH

 

[BullElk]

Rough math... calculate the % difference(X) of distance traveled with old vs new tire (1 rotation). If your uncorrected speedo says you did 200 miles your actual miles traveled is 200 x 10X%. Use this new miles covered to calculate your actual mileage. HTH

I think that is basically the same as what I did in my last calculation.

OLD tires.......305 miles on 21.7 gal 14 MPG
NEW tires.......260 miles on 21.7 gal 12 MPG

305 is 17% greater than 260....17% difference. Which is the same as:

.85 miles traveled with larger tires PER 1 mile of actual distance is 15% difference. (.85 is rough and hard to get accurate with rolling odometer)

Correct? or am I missing something.

Thanks

 

[e9999]

yes, of course. With bigger tires you have to correct the odometer reading. This is because the odometer does not measure distance, it measures number of revolutions.

 

[Paul_L]

Here's a tire size calculator that gives the % distance differences.

http://www.tacomaworld.com/forum/tirecalc.php?tires=275-70r16-295-75r16

You are probably looking at a 7-7.5% difference in distance traveled.
260 indicated mi. x 7.5%=19.5mi.
260+19.5=279.5 actual miles.
279.5/21.7=12.9 mpg.

Your new tires are heavier & have a higher rolling resistance than your old tires which can severely affect mpg.

 

[sdnative]

Going from 275/70R16 to 295/75R16 is about a 7.3% difference, so your actual mpg is (11.97 * 1.073) or about 12.9 as Paul said.

 

[BullElk]

I see that 7.3% is the difference in the size of the two tires but does that also equate to the increased difficulty/strain it puts on the engine to pull the vehicle...thus increasing fuel consumption and decreasing MPG by the same 7.3%?

 

[murf]

So....Would I only loose .25 MPG with the larger tires with that calibration?Thanks

According to your math which looks correct, you are right.

Not a bad trade off

 

[sdnative]

I see that 7.3% is the difference in the size of the two tires but does that also equate to the increased difficulty/strain it puts on the engine to pull the vehicle...thus increasing fuel consumption and decreasing MPG by the same 7.3%?

No. There are so many factors involved, it's not a linear relationship. Only way to really find out how changing tire size affects MPG is experiment. You only change one variable (tire size) and keep everything else the same, collect the data and analyze the results. In the real world, you can't keep everything else the same (the way you drive and outside influences change day to day), but you do the best you can and you'll get close.

The 7.3% comes into the MPG calculation above because your speedometer is off with the larger tires. Since MPG is miles divided by gallons, if your miles are off your MPG is off. Need the 7.3% factor to get correct miles traveled.

 

[rjinga]

I'm running 285/75R16s, which according to the Tacomaworld website is a revolutions per mile difference of -5.1% verses the stock tire size. Does that mean I can take my current MPG of say, 15, and multiply it by 1.051 to get the actual MPG of 15.765?

 

[CruiseOrlando]

it took me a little while to get to my "exact" MPG after getting 305's.

How'd I do it? I stopped caring about MPG

 

[eurorom]

I think your vehicle looks nicer with the new tires, you should get better performance offroad, so the trade-off in gas is a mute point! Enjoy the ride!

 

[murf]

How'd I do it? I stopped caring about MPG

The Brotha's got it figured out!

 

[86tuning]

At 10~15% difference your math is close enough. For larger differences you'll have to consider that if your speedo says 0.90 miles per 1 mile of actual travel, you're at 11% because 0.90/1 is 1.11

If you feel that the 1% difference in the math is not significant, you're correct. It's the difference between 12.0 and 12.12 mpg.

But if your tires were 25% bigger, the correction factor is more significant.

/10 cents/

fwiw, I don't calculate mpg anymore. It's $/month that I keep track of. And then at the end of the year, I look at miles/year and figure that I spend thousands of dollars on fuel more than if I drove an old civic as a daily driver.