too much tire pressure causes poor MPG.

[fearghal]

hey all. not sure if this is common knowledge or not. but the dunlops on the car were worn so i took them off and replaced them with the nittos [265/70-17].

when i got the truck back it drove great but felt a little sluggish.. i checked the tire pressure and it was 40 [cold] on all 4 wheels, i figured i'd drive it like that and see what the mileage was like.

then i noticed the MPG readout was reporting 14.3mpg at 65mph.. worse in the city. i thought it was just the computer being confused until i saw a half a tank of gas guzzled in about 140 miles.

i lowered the pressure to 32 all round and noticed that the car was a little zippier at higher speeds and the computer was reporting 23mpg on the freeway.... bizarre. could it be the tread on these tires does something odd when they're heavily inflated?

anyway, note that too much tire pressure can negatively impact economy just as much as low pressure in some cases.

 

[sharp4runr]

There are a couple variables you're throwing at once. First you replaced your stock size tires with ones that have a taller profile so your calculations will be off (unless you've corrected for it). The Nittos will be heavier since they are a taller tire so your gas mileage will suffer slightly there. Then, it seems you're using the on board computer to tell you gas mileage when it's common knowledge here that the computer is off by 10% at times.

Just some things to think about...

 

[fearghal]

yup, i'd corrected for the size difference. also, now that i've changed the pressure - i'm getting the same BETTER mileage than i was was with the dunlops.

note - this is all based off the trip computer. i'll post my exact findings once i refill.

 

[TeCKis300]

When you over inflate, the overall diameter is slightly taller. Hence slightly taller gearing and more load on the motor. All the car sees is that the load is higher and it is not aware that it has traveled farther since it is calibrated for a stock sized tire. The is why the car feels slightly more sluggish.

 

[fearghal]

interesting. i'll buy that.

thanks.

 

[mole177]

is there a tool available for recalibrating the tires for the computer reading?

 

[Duc Hunter]

Recently I wanted to squeeze a few more MPG out of my truck so I bumped the tire pressure in my Revo's to MAX. Of course then I did not pay attention to how is changed because in every car I have ever owned more pressure meant better MPG cause the diameter change was so negligible it did not effect the gearing, but it did get the edged of the tread up and give better mpg cause of less rolling resistance.

Recently I drove 3 hours to my g/f's parents place, all freeway and noted the gas mileage. While there I had to take her mom for a ride and her mom was looking to buy a new 4Runner. I noticed my higher pressure made the truck ride rougher so I lowered the pressure to 34psi from the 42psi it had been to give her the best impression possible. On the trip home I did not pump the tires back up and guess what, I got noticeably better mileage.

I attribute this to the tire technology in newer tires, like my Revo's. They are designed to ride best when the entire tread id flat on the road (properly inflated for the load they carry). Too low or too high a pressure will change how the tread interacts with the road and affects the rolling resistance.

Best advice I have is, try it and see. Some tires will react different than others.

 

[fearghal]

i'm going to try this again with different pressures and do the math using miles travelled and fuel consumed. i'll report back.

from what the computer was reporting though, i was noticing a 9 - 10 MPG difference between nittos running ~42PSI and ~32PSI.

 

[MAC4Runner]

I've found the happy medium between MPG and comfort for the Nittos to be 34 PSI.

YMMV

 

[fearghal]

MAC4Runner, what pressure did you find gave you the best mileage?

 

[MAC4Runner]

37 PSI