Hi,

I just realized how much of a MPG difference tire pressure made. I have 8.5 inch A-SPEC wheels with 235-40-18s on them and I have been getting 16 MPG doing city driving. Then I checked the tire pressure and it was around 25psi on all 4 tires. Their max rating is 44psi so I inflated to 40. I also noticed in my garage, before when I took the car out of drive and put it in Neutral, (6 speed manual) it would sit there, now it will roll back. It is a really really tiny slope. My other cars would roll back so now I feel better about that. It is also way easier to push it forward with the engine off in the garage. If I come in and didn't leave enough room behind the car between the garage door, I sometimes like to just push it forward without starting it. Now it will roll much easier and I'm getting 18-19MPG in the city.

Has anyone else had an experience like this?

Jeff

 

I would not recommend running it at 40 though. remember that 44 MAX PSI is the MAXIMUM pressure you can run your tires when the car is FULLY loaded. (meaning 5 people & a full trunk)

I am not 100% sure but I think the recommended pressure from Acrua is somewhere in the 32 - 36 range. (check with you mechanic)

Running the tire pressure too high will cause uneven treadwear. (the center of the tires will wear out before the edges) It will also give you less traction & control on dry pavement as you are decreasing the contact patch.

The reason it rolls better is that when you run the tire pressure low you are increasing the surface are and this in turn means more friction / increased rolling resistance.

We have several mechanics on this forum who can give the recommended tire pressure if you let them know the make / model / size of tires.

 

Also its near impossible to quantify a 2 MPG difference because its impossible to duplicate the other conditions for the comparrison .. eg temp. Barometric pressure, humidity, traffic conditions etc etc . But its nice to know you "suspect" an increase.

 

Watch the TPMS and see how high the pressure goes while driving. If it exceeds 45, let some out.

 

Wow big gain..lower it to like 34-35 at most to be on the safe side because 40 is really high

 

Underinflation = W excess wear on edges, more road surface area
Overinflation = V excess wear on center, less road surface area
Exact inflation = U even wear across tire, ideal road surface area

 

A good rule of thumb is to start out at 80% of maximum and adjust up or down from there to your liking. 40 cold may be a little high. May want to back it down slightly.

 

That's the most creative use of the alphabet I've seen in quite some time.

 

You should inflate to the pressure found on the sticker on the driver's door frame (not door.. door frame). For an '04 manual, this is 35 pounds front and 32 pounds rear. For an '05 automatic, as I recall it's 32 on all four tires.

 

He's running Aspec rims. I can't find it now (there's a step when installing that show a new sticker for the Aspec rims). I think the PSI's are different? Anybody?

 

Can't speak for the 2004 8.5" rims, but the sticker that came with my 2005 A-spec rims called for 39PSI in the front and 36PSI in the back. So 40PSI is not outrageous. You want higher pressure in low profile tires.

 

The edges of my front tires are more worn than the center anyway, I'd like to even it out with a little extra pressure..


I suspect the 2 MPG because after running a few tanks through it, it was higher. I reset my MPG computer on every tank and make an assessment at the gas station and I do the same routine every week. Its the people who make assessments after driving 10 miles. They are the ones who didn't average out the variables. IE "Wow, drinking Mountian Dew increases my MPG because for 10 miles (coming down a hill) I got better MPG than when I wasn't drinking Mountain Dew (driving up a hill) Who Knew!"

 

If the outside of your tires are worn, you are under inflated. If the center is worn, you are over inflated. 25PSI is way too low even for the stock 17" wheels.

 

Hmm, you don't seem to really be "up" on this. Others have demonstrated a DIRECT relationship between tire pressure and fuel economy. (It's done ALL THE TIME for the hybrids). In breif, increasing tire DECREASES rolling resistance.