How much of an increase in mpg can I expect with overinflated tires?
For the first time ever I overinflated all four tires on my VX today. I put 45 psi in the front tires and 40 psi in the rear tires. I have always run 30 - 35 psi never going over 35.
Do the other overinflaters in this org have pre and post mpg data that shows let's say a 2 - 5 % increase? For my car that would translate to 1 to 2 extra mpg. I'm just asking because I will probably need to check tire pressure weekly to maintain the "high" pressure. I don't check nearly that often now. I didn't notice any handling difference on dry pavement today at 60 mph. Cornering felt a little weird but it didn't feel unsafe. My goal is to achieve 50 mpg average through a variety of techniques - this one being the first. |
*bump* anyone?
I was wondering too...
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It really depends on the type of tire,size and weight of the vehicle,type of surface among other items . I can assure you the gains can be/are quite good ,but only up to a certain point. I've been told that point is at the outskirts of Stoopidville,USA and happens to be where I reside.
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I think that perhaps the best way to answer that question is to break it down for what it is. It's a method of reducing rolling resistance. You can isolate it and measure that with a slow roll-down test. The big question, is how well you are able to take advantage of lower rolling resistance. At speed it won't make as much difference because of aero drag.
On a porker like mine, it really extends the coast and makes continual coasting on the slightest of grades possible - good thing too because it sucks more than enough gas going up these very slight grades. :eek: |
I run 50 psi in my tires that are rates for 35 psi and it drives beautifully. I cannot give you a hard figure on FE increase on that one because I did that together with installing LRR tires.
Last weekend however I drove my wife's car, a 2003 Buick LeSabre (yes, a big *** full size car), all the way up to Michigan and back, with my wife and kid in there as well and luggage in the trunk. My wife and her stepdad who the car originally belonged to, never got past 29 mpg on the highway which I thought was already good for a big car like that. Before we left I increased the pressure from the 33 psi that was in there to around 52 (sidewall said 44 psi max). We then left for our trip. It has a computer in there that lets you see live mpg and average trip mpg. I drove close to 65, say 63/64 mph and got an average on a 200 mile trip of 37 mpg, with a Buick LeSabre! It even surprised me! What can I attribute it to? Well, I drive pretty much with FE in mind but apart from not going fast i did not use any 'tricks' other than increase the tire pressure. So without even calculating it you can see what a difference the tire pressure can make. |
With my 2007 Toyota Yaris, the increase in MPG is 5% to 7% when I take my PSI to 50. The tires are rated for max of 44.
Now if your tires are very low (under specified psi) this can cause a loss of 10% or more off your MPG...this is really bad! |
I put my tires at 55 psi. At 55 psi, I "feel" I am probably getting 5%-10%. I felt that their was a significant increase in my EOC rolling distance, when I changed it to 55.
I had run 45 for a short period of time, but I didn't feel their was a significant increase in rolling distance, over 35 psi. After I moved it to 55, the increase was significant enough that I decided to quit fiddling with it and just leave it at 55 psi. I figure with it that high, even if it loses a few pounds, it is still probably not going to be real significant, until I think to check them again. I have probably put about 15,000 miles on my car, since I went to 55 psi and I haven't had any issues with blowouts, bad handling, or all of the other issues people are uncomfortable about. FWIW. |
Here are some easy rules about rolling resistance you can use to estimate the effects of pressure and other tire properties--
1. Rolling resistance is inversely proportional to tire pressure-- double the pressure, cut the rolling resistance in half. I WOULD NEVER RUN MY TIRE PRESSURE HIGHER THAN THE NUMBER ON THE SIDEWALL. The tire pressure that counts is the pressure while you are driving. Tire pressure goes up about one psi for each 10 degrees F in air temperature. Well-pressurized tires only run one or two degrees warmer than what they touch, but road surface can get very hot. I run my 44 psi tires at 40 psi and that works well for me. 2. Rolling resistance is inversely proportional to tire diameter. That number is published in tire catalogs. An additional benefit not related to rolling resistance is that the engine becomes more efficient at lower RPM, which you also get when you increase tire diameter. Watch out: when you put on larger-diameter tires, your guages will read differently and you can be fooled into thinking that your mileage is going down when it actually went up. And, careful about getting tickets with a lower-reading speedometer. 3. Low-profile tires will always get worse mileage than the high-profile types, that read /55 or higher. Sorry, no formula rule here. Tire width normally has no effect on mileage. 4. Rubber formulation is very important. Tires differ a lot in efficiency. Do your homework and buy the best tires. Check your mileage change with the new tires, and return them within 30 days for a refund if mileage goes down. (Be sure to ask about the 30 day guarantee when you buy, and you may want to keep your old tires just in case.) Now, drive carefully-- improves car mileage, and yours too. Ernie Rogers |
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https://www.officer.com/article/artic...on=19&id=27281 Of course, always go with what's good for you ;) |
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Can anyone comment on whether it's better, on a front-wheel drive car for example, to run the front tires a couple PSI lower than the rears? For me, it's a cheap way (along with removing the spare/jack/carpet from the trunk) to give the car an aggressive stance. However, I wonder if making them all the same pressure, or running the fronts higher than the rears, would be best.
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>>ernie: skeptical about dbl psi/half rr claim.
Inversely proportional makes sense, but not linearly and not with a 1x1 slope, if nothing more than because drive train resistance is not affected by tire pressure. Max mpg gain is probably when you start really slipping in the corners, and max mpg loss is probably when intitial resistance is "much" larger than actual rolling resistance. In either case, your tires may not last very long :-) |
I'm still stumped as to what I should set mine as, both the manual and tires say a max of 35...maybe I'll try 40 next time I fill up?
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I have 50 psi in 35 psi tires and they work great. It's up to you of course, but just to show you what's possible. And you will be able to tell the difference in your mpg...
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Alrite, thanks. I just wasn't sure how much of a max before say, the tires gave out or soemthing.
Yeah, I'm one helluva newb, but I want to get bac into the 40's again...got htere once (370/8.8mpg) but all the others have been in the 36 range...what's weird is that its the same trips, over and over again...oh well. |
Just a $0.02 worth...make sure your tires are in good condition, and if you really pump up the tires, consider getting them balanced after inflating.
One of our cars, the tires need to be balanced and it got MUCH worse after over-inflating...and the other car, nearly lost the tread due to separation...not sure if it was due to the inflation, or junky Michelins?? |
Very good point about tire condition. All four of my tires are new (or practically new). And - to my surprise - the VX Enkei wheels are holding the pressure rather well. I just added a few psi to them today exactly 1 week after the initial overfill. Just as the bad guy Emile in the classic film Robocop says when he fires the mini cannon for the first time - I LIKE IT!!!!
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Hmm...just went from 35 to 43, maybe will go higher, but I think I feel it taking longer to slow down... but the rides a little harsher, but its all good =]
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I'm a member of the don't-exceed-sidewall-spec group. The owners manual is a suggestion based on estimated load in the car. There's leeway built in for people that load their car full of other people and misc. heavy cargo. That's why it's safe to screw around with that number.
The sidewall is the max pressure that the specific tire manufacturer determined is safe. Basically the point where they shed liability if the tire blows up while you're inflating it up to 55psi. I personally think it's insane to go 5, 10, 20 psi over the sidewall. Chances are you'll be fine 99% of the time, but the 1% chance of an Interstate or worse, inflation-time blowout isn't worth the extra 1mpg for me. All that being said, obviously countless here have exceeded sidewall with no ill effects... this is just my personal feeling on the subject. OM = 34psi; SW = 44psi; Mine=40psi |
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LOL! Good comments
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Okay, we can disagree on exceeding side wall "max pressure." I prefer to believe that the engineers that set "max pressure" really meant it. Quote:
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Ernie Rogers |
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Ernie Rogers |
I use 40 psi
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I have to say, for my own car I've settled at a very similar practice to your decision. My oem is 27 with optional increase to 32 for better FE, my sidewall 44, my tires are at 40ish. Please don't guess that loading up your car/truck will stress highly pressurized tires. It's the other way around. Increasing tire pressure raises a tire's load capacity, up to the tire's sidewall rating. So, at least according to the published information, running at less than max sidewall gets you less than rated capacity and increasing beyond that point gets you no additional gain. You can find a lot of this at tirerack.com if you dig for it. I'm putting in another plug for the reference quoted earlier in this thread: https://www.officer.com/article/artic...on=19&id=27281 Long story short, they put Crown Vic police cruiser tires (44 psi rated) + wheels on a Ford Ranger p/u and pumped them up to 100 psi. Then gave it to a race driver to do some stunts with. Success. The tires hold up far better at 100 psi than they do at 44. There is quite a lot of additional information in the article on the topics of safety, handling and reliability of tires at different pressures. Draw your own conclusions, but please do read the article. I'm not recommending running 44 psi tires at 100 psi for street/highway use. However I'd certainly consider the max sidewall to be allowable, and 10-20% more to be doable. In actual practice, 40 psi feels pretty hard in my car so I'm not inclined to go much higher. But that's my car and my tush. As usual, ymmv and I suppose ytmv also. |
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So I did a quick search... and found a patent for bias ply tires... It makes a reference that says the steel cord safety factor should range between 4 and 11, 7 being a target. Now that's not directly related to inflation pressure - but should be related to hoop stress (P*D/2).... Again, this isn't really an equivalent - and I don't know if the mentioned factor of safety applies to traditional radially belted tires... Really, I doubt I'll find the actual number anywhere as it's probably trade secret. ---- Quote:
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I am also thinking of a case of a tire with wheel run out (out of round). So we basically have a tire in the shape of an ellipse (a very minute one though). The areas with the shorter sidewall will have more stiffness and translate that into the the vehicle suspension - as compared to the higher sidewall. So that, combined with the assumption that the normal force is constant leads me to the difference in taller versus shorter sidewall is the k. Thus the deflection for the stiffer (shorter sidewall) should be less than the taller sidewall... So that paragraph above would mean that sidewall stiffness AND inflation pressure are variables that determine contact patch size (something tells me stiffness from the tire is much less than inflation stiffness) Of course, this could be *** backwards - and x is constant and the force changes (although, intuitively - that seems wrong or I am missing a key point). In any case, it doesn't matter for me either way. I have no intention to get a shorter sidewall :p I just like this sort of discussion :p |
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1) max sidewall is spec'd for cold pressure, meaning cool morning temps before driving the car. and 2) max sidewall and also the mfg oe recommedations allow for increases in outside air temp during the day as well as increase in tire temperature from driving. So, check tires when cold. Drive with confidence after that. |
I wouldn't run your tires way too much over what the side wall states.
Over inflating your tires will wear out the center. If you think about it, it will also give you less contact patch to the ground, that is one reason why you are getting lower rolling resistance and getting better mileage. But you pay for it at the end with premature tire wear. Something to keep in mind. You get short term benefits of better gas mileage at the expense of your tires. Gary |
Overinflating is not really a cause of premature wear, low pressure is the major cause of that.
It may affect wet handling adversely, so practice. Dry handling effects are good, the sidewalls are nice and stiff and do not flex as much in the corners (them guys that drive on two wheels use like 100 psi). Them steel belts keep things lined up pretty well so there's no appreciable bulging from overinflating. |
Low Profile Tires on a larger diameter rim for the same or bigger tire diameter won't hurt mileage ( I know you meant on the same sized rim but it could cause confusion).
There is a point where rolling resistance will increase with tire pressure...if the tire is set so high it stops conforming with the road surface, it can be detrimental. Tires do grow convex and/or convex with different psi's. This will happen in the middle of a tire and causes different running temps along the inside, middle and outside edges of a tire...you really want pretty even tire temps for best results. No reason to get uneven wear on tires and then all the money you saved on fuel goes to new tires. Agreed that low psi's are much worse, so err on the side of high psi's. |
I've read that on modern steel belted tires additional pressure doesn't cause the center bulge as on non-belted tires.
The proof seems to be in the anectdotal reports of drivers who have run their tires at considerably more than max sidewall pressure for tens of thousands of miles with no uneven wear detected. In fact the additional pressure reduces rubber flexing. Flexing creates the heat that softens rubber and destroys tires, and those runnning higher pressure long term report tire life at least as long as expected if not longer. |
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An easy test would be to roll your tire onto flexible measuring tape, then pump up and measure the diameter change :p |
One could also paint a stripe across the tread and drive around "normally" for 25 miles or so with them pumped up and see if they wear evenly or not.
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I would have to disagree.. Both overinflating and underinflating tires will cause premature wear. It's not just underinflating it.
I've seen pictures of over inflate tires and it's wear after 20k miles from tire companies. The middle budges out. Yes, the steel belts are there and the one in the picture was steel belted. Gary Quote:
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The difference is not readily discernable to the eye...use a pyrometer and take temps across the tread...you WILL see a difference. The higher temps in the middle show an overinflated tire and uneven wear will result. You just have to weight the cost savings verses the wear factor...you have to assume that the vehicle is properly aligned
Higher pressures are used in Auto-X to keep tire squibb to a minimum...you dont' have time to build up heat on a run Lower pressures are used in Road Racing...heat builds and psi increases For the street you are pretty safe unless you run underinflated...BOOM!!! |
sigh, we can go through this all again, or we can re-read the previous debates:
https://www.gassavers.org/showthread.php?t=1727#6 People here are reporting even tire wear with lots of pressure and 10s of thousands of miles. Many of the overinflated tires here are pretty skinny to begin with and being steel belted radials they seem to hold their shape better with extra pressure. You have steel belts going in every direction, if it were to bulge out in just one part of the middle tread it would have to pull in from another part of the middle tread, but that part of the tread also has extra pressure forcing it into the shape defined by the steel belts. I know it seems counter-intuitive and unconventional, maybe the center bulge theorists are a hold over from before steel belted radials? I don't have enough miles to consider myself a first-hand expert, just trying to think it through in the mean time and make sense out of the reports of "overinflating works". |
Rubber stiffness doesn't support any of the load
Yes, "inflation stiffness" is greater.
When you push down on an uninflated wheel and tire, it won't even support your body weight-- you can neglect the stiffness of the rubber in determining the area of the contact patch, so-- F (load on the tire) = P (tire pressure) x A (contact patch area) Let's try some numbers. A 195 wide tire is 195 /25.4 = 7.68 inches wide at the tread. The load on one tire is about 900 pounds. At 40 psi, the area of the contact patch is-- A = 900 lb /40 psi = 22.5 square inches. Divide that by the tread width to get the other dimension-- 22.5 /7.68 = 2.93 inches long. To make 2.93 inches of curve flat on a 25 inch diameter tire, you have to deflect the center of it by 0.11 inches. The harder it is to get the tread surface to move in that amount, the more energy the car loses as it rolls. That means soft rubber is better, and a thin tire is better. "Low friction" rubber and tire construction is also better, and that's what you are paying for in an "energy tire." Ernie Rogers Quote:
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IME it really depends on the tire. My passenger car tires have shown no wear after thousands of miles, but my light truck tires definitely showed the lower tread in the middle and alla that.
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