Final results on MPG vs Tire Pressure on G3

[Glider]

I wanted to tie some loose ends together on the controlled measurements I did of MPG vs tire pressure on the G3. Based on the number of time various files were opened, it seems that people have checked the earlier (incomplete) results much more than the final results in my other thread on this. Because of the confusion from the multiple files attached at different places, I am re-posting the final results here and adding a few comments.

First, you will absolutely get better mileage at higher pressure, but not as much as many people are reporting (at least, not on the OEM Avid S33 15" tires).

Second thing, there are three graphs listed at different speeds. I have no reason to believe there is a significant difference in MPG increase with PSI for the speeds I used, so use the TOTAL (last) graph which has all the data combined. That graph has a straight line fit and a curve fit. I recommend the curve, althought there is not much difference.

Finally, here's an example (use your own numbers) of how to use it. Suppose I have 30 PSI (average of front and back) in my car (set by the dealership for a comfy ride - like I got) and I am willing to go to 46 PSI. At 30 PSI, you read 61.2 MPG and at 46 you get 62.7 MPG. That's a difference of 1 1/2 MPG - and that is what you should get on average. If you were getting 58.5 MPG, you should go very close to 60 MPG on average.

Suppose your number is less, say 1 MPG improvement. Is it worth it? That's up to you. When you combine that with every other known technique (for example, slow down 2 mph and pick up another 2 MPG or whatever), raise the A/C a few degrees or turn it off when you don't really need it, etc. It all adds up to a major improvement in fuel efficiency beyond the great performance the car already has!!!

Happy motoring!!

[MikeDS]

Cool! So what PSI are you running now?

[Glider]

Originally Posted by MikeDS

Cool! So what PSI are you running now?

I'm at 46/44. I think I'll sit tight (no pun intended) for a month or so - make sure everything feels good. Then I'm looking to head to 56/54. That worked fine during all the test runs, but they were only for a few hours each.

Anybody who runs over sidewall max (sounds like a car accident in NYC) want to give us the benefit of your experience: handling, comfort and MPG?

[a64pilot]

I think your going to find there is a point of diminishing returns. I think it's going to be similar to this, 30 to 40 psi = 1.5 MPG, 40 to 50 psi = additional .5 MPG, 50 to 60 = no appreciable difference.
Don't quote those numbers, I just made them up, but I think the curve will be something similar.
In other words, I think you have achieved almost all of the additional mileage your going to get by tire pressure. I think really light weight wheels and light weight, low rolling resistance, narrow tires would be the next step. Probably take 1000 years to recoup the investment though. I appreciate your work, I knew there was an increase in mileage, I thank you for quantifying it.

[rrolff]

Great well thought out post!

Does anyone know the highest pressure allowed?

On the big plus side is tire wear - I bet at 50 you'll increase the life by 50%.....

[Glider]

Originally Posted by a64pilot

I think your going to find there is a point of diminishing returns. I think it's going to be similar to this, 30 to 40 psi = 1.5 MPG, 40 to 50 psi = additional .5 MPG, 50 to 60 = no appreciable difference.

Yes and thanks. Your idea about diminishing returns is right on, and so folks have the #s (the gains actually don't fall off quite so fast):

Original PSI.....New PSI...MPG Gain...Cum. MPG Gain
...30.................40...........1.00..........1 .00
...40.................50...........0.75..........1 .75
...50.................60...........0.45..........2 .20

Originally Posted by rrolff

...Does anyone know the highest pressure allowed?...On the big plus side is tire wear - I bet at 50 you'll increase the life by 50%.....

There is huge confusion about the "highest pressure allowed". The max sidewall pressure does not mean that exceeding it - within reason !! - places an uncompromised tire in danger of bursting. It means that you cannot safely carry a load higher than the max load by simply increasing the tire pressure beyond the max pressure. This is so misunderstood that Edmunds calls it one the TOP 10 AUTOMOTIVE MECHANICAL MYTHS (Why do they list 12??)

"10 Automotive Myths and Misconceptions " Edmunds Daily

(click where is says..... "click here" <-- yes, I overlooked that myself first time - ha ha )

Other people same the same thing. HOW MUCH OVER MAX?? You can't get anyone to quote this officially (liability issues), but unofficially I have heard that 25-50% over Pmax is fine (burst is around 300% over Pmax) for an uncompromised car tire. For a 44 PSI Pmax, that would be 55-66 PSI. But it is your call to make for your car. Don't do anything you are not comfortable with, right?

The word from longtime high-pressure folks is that tire life goes way up and there is no extra wear in the middle for SBR tires (unlike older bias tires)

Good luck!

[dogllama]

I finally looked into my tire pressure after getting consistent 48 mpg tanks while employing various hyper-miling techniques to maximize MPG.

My pressure coming from the dealer was around 33 psi for each wheel. I bumped it up to 46/44 and wow what a difference.

I am now getting a consistent 52 mpg and I actually like the stiffer feel.

I drive in LA with a lot of stop and go which makes me wonder if a higher PSI helps more in a stop and go scenario vrs long runs. I'm no expert, but maybe the force it takes to turn the wheel off the line is what was dragging my mpg down so much?

[Indyking]

Originally Posted by Glider

Happy motoring!!

Very good, thank you, but the hit on MPG was much less than I suspected. I'm amazed that the difference between 30 and 50 PSI is less than 2 MPG on average.

[windstrings]

Originally Posted by Indyking

Very good, thank you, but the hit on MPG was much less than I suspected. I'm amazed that the difference between 30 and 50 PSI is less than 2 MPG on average.

Based on what i"ve read, there is roughly 1mpg difference for every 10 extra lbs of air in the tire.

Realizing that tires are the very first level of suspension, I prefer about 40 - 44 psi.... any more than that the ride becomes stiff and less luxury feel as if I have a cheap suspension and can feel every rock and bump on the road... plus there is always the risk of rupture if you hit something of significance.

I hit the edge of the blacktop once where it had broken off pieces an it caused a blowout... that was running 60psi. At that point I realized how dangerous that was and stopped it for my families sake.

A cheaper and-or softer tire seems to tolerate higher pressures better, but the more expensive higher mileage tires that have harder rubber dont fare so well without ruining the ride.

Some tires just wont run the higher pressures without wandering all over the road... especially if they are new.. it feels like your running on tread instead of tires.

I agree with the curve of diminishing returns...... the risks, are not worth the benefit.

But the biggest thing that needs to be accounted for in a thread like this is whether they are running the 17" vs. 15" wheels or not.

Unless all the contestants are running the same size wheels within a few mm, its tough to rule out benefits of air pressure differences as its fairly common knowledge the 17" wheels deliver a near 3mpg hit vs the 15".

[Glider]

Originally Posted by dogllama

I finally looked into my tire pressure after getting consistent 48 mpg tanks while employing various hyper-miling techniques to maximize MPG. My pressure coming from the dealer was around 33 psi for each wheel. I bumped it up to 46/44 and wow what a difference. I am now getting a consistent 52 mpg and I actually like the stiffer feel….

It's great that you picked up 4 MPG. Probably some of that is coming from something other than tire pressure - maybe car break-in, better driving, etc.

Originally Posted by Indyking

Very good, thank you, but the hit on MPG was much less than I suspected. I'm amazed that the difference between 30 and 50 PSI is less than 2 MPG on average.

Yeh, I expected more too, but we have to take what we get. Stated more positively, "every little bit helps"

Originally Posted by windstrings

Based on what i"ve read, there is roughly 1mpg difference for every 10 extra lbs of air in the tire... Realizing that tires are the very first level of suspension, I prefer about 40 - 44 psi.... any more than that the ride becomes stiff and less luxury feel as if I have a cheap suspension and can feel every rock and bump on the road… I hit the edge of the blacktop once where it had broken off pieces an it caused a blowout... that was running 60psi. At that point I realized how dangerous that was and stopped it for my families sake…

I agree with the curve of diminishing returns...... the risks, are not worth the benefit.

Yep, 0.1 MPG/PSI is just about what I got, especially at the lower end of the curve. I've at 56/54 for 3 weeks and, to be honest, I'm not crazy about the transmission of bumps into the car. My FE has picked up by maybe 2 MPG.

Regarding the blowout, are you sure you wouldn't have gotten one at max sidewall pressure (which is supposed to carry no risks)?

Since you mentioned risks, and this is being hit pretty hard in another thread also, I have to say something about that. When I went (10 PSI) over max sidewall, I realized that my additional risks were minmal - maybe even negative - because I took another measure as well. I slowed down 5 MPH (from Speed Limit + 5, where I have driven for decades, to Speed Limit).

I now think that all the risks: decreased stopping distance, skidding in turns, blowouts from impacts, etc. are probably better than they were under my old driving habits at MFGR suggested tire pressure.

I will see how well I adjust to the harsher ride over the next few weeks. If not, I may come back down to 45 PSI or so (at the expense of 1/2 MPG)