Any Data on Tire Inflation Benefits?

Great info! I didn't know that. Thanks.

 

And to answer the OP's question:


from Barry's Tire Tech

The curve is flattening near 40 PSI

 

The result of overinflation is not small if you assume anything over manufacture placard is overinflation and not just above tire max pressure listed on the tire.

Tire pressure is one of the easiest methods available to increase mpg. Most other modifications require a drastic change in driving techniques, which most dedicated hypermilers already do, or a lot of money. The only other method to significantly increase mpg is severe weight reduction but turning your car into swiss cheese is not what most people are interested in doing. Many of us have done this for racing purposes and I'll tell you from experience that this ruins the car and the driving experience.

Rolling resistance is indeed a major figure in the total mpg calculation. You are correct that tire pressure plays a minor role in the total mpg outcome but that is only once you hit the point of diminishing returns. Drastic underinflation not only drops mpg significantly but it is very unsafe. So choosing a good LRR tire and inflating them to near sidewall max pressure if the biggest bang for your buck.

 

LOL agreed!

 

Mathematically speaking, diminishing returns happens everywhere along that curve. Any point of apparent flattening is in the eye of the beholder, and can be moved to any arbitrary location by changing the graph scaling.

This is a value judgement, a compromise between competing factors, and you can speak only for yourself as to the value of the tradeoffs. Hardcore hypermilers place a high value on small incremental improvements compared to average drivers. As a mild hypermiler, I fall in between. Numerous others place little value on fuel economy and prefer driving higher performance muscle cars or fuel-guzzling monster SUVs or luxurious land yachts.

 

+1

It should be obvious to anyone with an engineering or science background that those values were put on the chart to illustrate the trend, not as actual data. The % savings and position of the corresponding data points on the vertical axis of the graph aren't even to scale.

 

Yes, the chart is not perfect. Are you saying that invalidates the data?

 

I'm saying it is only useful as a generic example. Do you have some source that says it is useful as anything more than that?

 

Strong work!
It's refreshing to see empiric evidence rather than worthless opinions.

 

In a quick skim of this workshop publication from 1977, I see that for the truck tires illustrated on page 108**, nominal pressure 80 psi, the minimum rolling resistance (peak rolling efficiency) occurs over the range of 100 to 160 psi, depending on the parameter of each particular line.

For the passenger car tires illustrated on pages 130 and 153, the graphs stop at 50 and 64 psi, respectively, without reaching a limiting pressure of minimum rolling resistance.

**(page numbers are Acrobat file page number, not article's original page number.)

 

What is correct pressure? My Australian handbook says 36/35. I believe the US handbook is different and the British different again. My tyre dealer says 34 all round for the Michie XM2s. I run at 40/40 get even wear and better handling. I don't care as much for MPG because without strict controls all opinions are subjective and even arbitrary. I did notice what seemed to be an increase in L/100ks when I first installed the new Mitchies but I seem to be back to my average of 4.8l/100ks.

 

Over the last two years, I've been experimenting with overinflating Yokohama Avid S33d ( 35/33 psi to 40/38 psi to 44/42psi to 50/48 psi) that came with my 2010 Prius III. There are other equally as important factors other than a tire's rolling resistance that affect the Prius' ability to get very high MPG. As I increased the Prius tire pressure over max sidewall pressure setting of 44psi on my Yokohama Avid S33d - the fuel efficiency gains were incremental and required me to work harder to get more MPGs. Like all things I had to know how and when to take advantage of the lower rolling resistance for it to translate to higher MPGs. Sofar I'm getting +60 mpg over +20000 miles but I'm hypermiling to get there. Certain third party LLR tires have a max sidewall of 50 psi and can be over inflated safely up to 60psi. The most signficant factor to getting high MPGs with LLR tires is the road you are driving on -- you need a dry smooth road. When the county was doing road construction on my commuting route they made the normally smooth road surface uneven and rough with continous swirling grooves on the surface. It was like all driving over rumble strips for two miles for about a month or so. My MPG dropped by about 8 mpg on that 2 mile stretch of road while it was rough and uneven despite going slower and trying every hypermiling trick I knew. The only solution to getting better MPG was to change my route to avoid county road construction. When the county finally resurfaced that same 2 mile stretch of road back to a smooth asphalt surface my fuel efficiency jump back up 8 mpg and my overall MPG improved the following month. When I have to drive in rain/ice/snow then my MPG also drops despite overinflationing them higher (44/42psi to 50/48 psi) but when the roads are dry again my MPG shoots back up again. MPG returns are situational. Last Friday morning, I went in to the dealership for an oil change. After the oil change, I drove 12 miles from the dealership to my job. The route I selected from the dealership to work had a max speed of about 35mph over series of hilly roads with an overall decrease in elevation -- so basically it was a downhill race. The road was dry and smooth. The traffic was light. Using a combination of hypermiling techniques DWB, DWL, Rabbit Timing, Stealth warp mode, and a pre-warmed up engine, the ScangaugeII calculated I was able to get about 150 mpg for that 12 mile trip. However, the ScangaugeII calculated I got about 15 mpg on the Prius that previous night when I drove from a cold start a 1 mile trip to the local grocery store. It is not a simple answer but a simple relationship. Smooth roads make it easier to get better fuel efficiency.


Walter Lee
aka HyperDrive1 on cleanmpg.com
2010 Toyota Prius III Blue Ribbon/Dk grey, oem floormats
Yokohama Avid S33d (50 psi front/48 psi rear)
100% grill blocking
ScanguageII
odometer +20158 miles/ 60.9 mpg

 

Thanks for that last post, Walter. I have the Yokohama AVID S33D as well and the car feels like it doesn't want to roll well even when gliding. I considered trading them in for a set of Energy Saver A/S but I'd like to give the stock tires a chance and your results give me hope. I can't help but think the Energy Savers would give me a 3mpg boost. What do you think?

 

I know but after they wear down their value will drop significantly so I likely wouldn't receive any real credit for them and selling them would be harder. I just dislike the thought of starting off with a possible 3mpg deficit and having to stomach that for 30k-40k miles until it is time to replace them. If I could sell/trade the AVIDs for a set of Energy Savers and spend less than $150 I would come out well ahead in the long run and enjoy higher mpg.

 

We ran our stock Yokohama S33Ds for nearly 43,000 miles with elevated pressures. Most of those miles were clocked at 40F / 38R because that seemed to provide a good balance between fuel economy, handling, and ride comfort. At the end of all those miles, there were no unusual signs of wear to indicate that the tires had been overinflated.

Keep in mind that there is no "perfect" inflation pressure. The recommendations that came with your vehicle are a compromise between economy, handling, ride comfort, and safety. Many of us here on PriusChat.com are running with elevated inflation pressures and have lived to tell the tale.