Iqraam

I just put on brand new Bridgestone Alenza's to replace the cupped Alenza's I had and the tire pressure is set between 32-34 psi but I am getting crappy mileage. I cant do better then 20mpg. However, on my old set of tires I had my psi set at 40 and was doing atleast 22-23mpg. I am not sure what to do to get better mpg.

1koolfella

iTrader: (1)

I live in the North East, was on my way to Strauss Auto for my yearly vehicle inspection and come to find out that Strauss Auto Center is no longer in business. They closed all of their locations.

So I decided to go to PepBoys to have the inspection done, Since then my gas millage really sucked. I checked my tire pressure and it was 45 psi. Brought all 4 tires down to 32 psi and now my gas millage is much better. Not sure if I should inform Pep Boys or just find another place to have my vehicle inspected next year. There was no reason to put more air in the tires.

Adamjeeps

New tires have more diameter than old worn out tires, so your speedometer is fooled into thinking you are going a little bit slower, therefore your calculated mileage will be less because your odometer has recorded slightly less actual miles than you have driven.

Not to mention the new tires also weigh a bit more than the old ones.

1koolfella

iTrader: (1)

Is there a rule of thumb for the front set of tires as well as the rear set?

If you're carrying more people or cargo in the back, should there be more or less psi in the rear tires?

Adamjeeps

Go by the door sticker. Believe it says 30 front and rear. Load does not matter in this application.

spacecraft

I don't think that the diameter difference you're talking about can be big enough to make any meaningful difference (could be calculated, anyway). But one thing I'm sure of is that the effect is exactly the other way round: with less diameter, you're actually covering less distance than the speedo calculates, therefore the calculated mileage will be more, not less.

Adamjeeps

The odometer turns based on the speedometer, not actual miles driven. The speedometer can not adjust itself for tire size differences.

Lets say you have 2 exactly the same cars going exactly 60 mph. One has 31 inch tires and one has 30 inch tires.

The one with 31 inch tires will be showing a lower mph on the speedometer because the tires are rotating more slowly. In turn, the odometer will also be registering slightly less miles traveled than the car with 30 inch tires.

So, the 2 vehicles are getting in reality getting the same mpg, but the calculated mpg is lower on the vehicle with 31 inch tires because the odometer is not rolling as quickly as the 30 inch tire vehicle. you would be calculating mpg based on say 350 miles driven versus say 320 miles driven, even though in actuality the miles driven is the same.

Any math wizards want to jump in as to the difference in diameter of a new vs. worn out tire?

spacecraft

Exactly. In fact, as I said, with the lower diameter wheel, the calculated mileage is higher.
In your previous post, you said that with the worn out tires (=lower diameter, or 30" in your latest example) "your calculated mileage will be less because your odometer has recorded slightly less actual miles than you have driven".
Now you're saying that it's with the 31" that the reported mph is lower, which is correct.
Re. calculating the difference between a new and a worn out tire, I can calculate that if you're interested.
Just give me the tire specs you wish to consider (is 235/55R18 ok?), and the surface wear to assume.

spacecraft

Incidentally, it's not necessarily true that the real mpg is the same for two identical cars running at the same speed but with different diameter tires, because the two engines will spin at a slightly different rpm.
And it's impossible to say on paper if and how much that can affect the actual fuel burn.
At a guess, the 31" tire car should get a slightly better mpg because its engine spins at a lower rpm, but in some conditions it could be the other way rounf.

Adamjeeps

So ,in conclusion, can we say the calculated mpg will automatically go down slightly with new tires due to them being slightly larger than old worn tires?

takeshi74

You're talking a difference of 10-11/32 versus 2/32. It's insignificant. There's a rule of thumb for avoiding speedo issues (an issue if you get tires with a significantly different sidewall height) and the margin for that is much larger.

Proper inflation helps with fuel economy but most will see a bigger difference in altering driving style.

jfelbab

It's all explained quite well in this link: http://www.tirerack.com/tires/tirete...jsp?techid=177

spacecraft

Aaargh, not again, iit's the other way round!
You got it right with your 31" vs. 30" example, when you said that "the calculated mpg is lower on the vehicle with 31 inch tires".
That is correct, and obviously applies also to the new vs. worn tires.
And in your analogy, the 31" is the new and the 30" is the worn, of course.
Though as also Takeshi74 said, in the case of new vs. worn the difference is negligible.
I'll do the math using 10/32 vs. 2/32 later, I must rush out now.

KonaRX

Also may want to consider that new tires have a higher coeffiecient of friction due to the increased amount of rubber/sips etc.
Either way, probably increasing the PSI a few may help with the mpg. I have read that 'low rolling resistance' tires (like they put on Priuses etc.) and many new cars (to pump up MPG) will do slightly better with mpg. My only experience with tires like that are they transmit more road noise and do less well when cornering.

back to the original question. I run 35 and it does help ever so slightly with mpg.
Have fun !