Higher tire pressure -> lower MPG??
 

Just switch from Dunlop Enasaves A/S to General RT-43H, and looks like there is 5-10% MPG penalty. There are other factors in play, such as break in, lower ambient temperatures, stations switching to winter gas mix, etc, but subjectively car does not roll as well as it was on LRR Enasaves.

The only thing I found out of order is that RT43s were overinflated to 49psi. (This is 2000lbs car).

General wisdom of ecomodder says higher pressure = to better MPG, but the question I am contemplating, is there such thing as too much? RT43s are non-LRR tire, but they were designed with "Low surface abrasion technology (LSA®)" belts to reduce wear/friction. Would this also mean that at some point higher pressure will change the shape of the contact patch and increase the abrasion?

It's likely a combination of the new tires, a less rolling resistant tire & the winter fuel blend.

Higher pressures will always result in improved rolling resistance, especially between 30-50 psi.

Well this is a 2000lbs car. 50psi would be equivalent of 75psi in 3000lbs car, correct?

I am seeing MPG improvement after dropping pressure down under 40psi

Too many varibles, old bald smooth tires are roll easier than new tires.

Double check your brakes for drag

Mpgmetro posted the most though test I can think of
http://ecomodder.com/forum/attachmen...1&d=1212424018
http://ecomodder.com/forum/showthrea...tire-2721.html
The tires tested were rated 44psi max sidewall.

I wouldnt say "too much", but a point of diminishing returns.

since you don't say what you drive or where you live, i say give more info and get a better answer.

I just put some Enasaves on my car, from non-LRR tyres, and was surprised at the noticeable increases in rolling distance and mpg.

I reckon it's going from LRR to non-LRR tyres that's the sole cause of the effects you're noticing.

While I don't think anyone disagrees that there is an upper limit (after all, the tire will burst due to excessive pressure!), there have been long and at times acrimonious discussions on how high is too high.

Among the things that come along with excessive inflation pressure is a reduction in traction, groove wander, harsh ride, and uneven wear.

I am of the opinion that tire pressure needs to be referenced to the vehicle spec pressure, not what it says on the sidewall and I think anything over 5 psi is excessive.

Sort of. Increasing inflation pressure changes the shape of the footprint regardless of who makes the tire and what technology they employ.

No that is NOT correct. While the load curve on tires is fairly linear, it is NOT proportional.

Being the one who changed the OP's tires, a job I think I will never do again, I refrained from posting any response.

Marks post hits the nail on the head, my 2015 Mirage is one of the best coasting cars I have ever driven, including a 1st gen Insight on RE 92s.

I can not remember a single instance where it was confirmed that increasing pressure reduces mpg.

Sorry to the OP for overinflating his tires, he observed it and the gauge read 40psi, versus his requested 38. Having a pressure gauge in my glove box, I figured he could reduce the pressure any time, like myself. In the meantime he could drive the car with (what I thought based on the gauge reading) was 40 psi.

he (cyclopathic) even advised me on placement of the tire in relation to the valve stem while I was changing his tires and the last two (after taking his advice) needed no weight and, as far as I know, were riding fine when we drove back to my house.

We made a deal on his Ensaves, which measure exactly as new 8/32nds in all treads at 3300 miles. I have them for when mine wear out, waiting in the garage. 7300 miles on my Mirage since 5/9/15.

regards
mech

This bring up an interesting point, which pressure reading is correct. No two of my gauges read the same.

I bet they were 40 psi cold and then the PO measured 49 psi after driving. The 40 psi cold is correct and measuring the tires after driving is a worthless/meaningless number. So many people make the mistake of measuring pressure some other time then first thing in the morning before even the sun has a chance to heat up the black sidewalls and before driving the car which also heat them up and raises the pressure.
This is why I also believe under inflation is the only real possible point for pressure related tire failure. The tires can be inflated to max sidewall in sub-freezing Montana and then driven across the country to 110 death Valley in the sun where I bet the pressure reading would be almost off the charts and yet still correct as being set cold.

I see 3-4 degree swing in my car with TPMS that indicates real pressure. My Cambell Hausfield chuck indicates 28 when real pressure is 40 yesterday(stick gauge and car).

Throw it in the trash.

Actually, you did the right thing. Tires grow a lot in the first 24 hours. Our test garage used to over inflate new tires by 2 to 3 psi, which came out really close a couple of days later.

Makes me wonder if there is a correlation between the pressure rise and rolling resistance of the tires. Makes sense that there would be one.

regards
mech

lol!!!

Having just gotten my VW up! back from servicing, they also "adjusted" the tyre pressure.

Down to the manufacturer recommended 2.2 bar / 32 psi, coming from 2.8 bar / 41 psi
(3 bar / 43.5 psi is excessive, as the rear gets bouncy and tail-happy)

They need not have stated the fact - it was noticeable right away.
The ride was sluggish, wallowing (65% profile is rather high these days), understeering, but comfy-soft .
FC was up compared to what I'm used to seeing @ the speeds driven.

Car manufacturers clearly use low-ish tyre pressure to hide flaws in their suspensions, at their customer's expense.

My tyre wear has been lower since I started using higher pressures.
Wear evened out on Hagär the Volvo.
No uneven wear issues on the up! which has been using higher than recommended pressure since day 1.


Sure does.
I've used "excessive" pressure to correct uneven thread wear caused by
using "correct" - manufacturer prescribed - pressure.

Old tires had 3,000mi, brand new.

Brand new car brakes are fine

They were measured early in the morning, and the difference btw tires on sunny and shady side was less then .5psi; below gauge accuracy.

On m/c tires I used to pump them to 60psi while sitting in garage (operating pressure 42psi for rear) then drop pressure to 28psi and go for short 1/2hr sub-50mph ride. It would take several cycles to completely break in tires this way.

M/C racing tires are said to grow so much that by the end of the race people would need different gear ratio. So the trick was to get it on lower side so it would be more or less ideal in the middle of the race.

don't worry it is expected for gauges to differ. I've gone through dozen of them, and there are only 2 which seem to agree on the pressure. One of them is attachment for inflation and another one is a premium pencil type which I think I had as long as I remember.. 25 years? And it is not that any loss of MPG is a big deal, I'd like to understand what is going on.

As others mentioned there are too many variables, for example starting Sept 16 they started switching to winter blend, cooler temperatures, getting gas from no-brand gas station (not that station may have been cheating, but different pumps have different shut off sensitivity). Not to say that some gas station pumps may have issues, I've put 5.3gal in 4.8gal m/c tank in the past. Bike was still running fine w/o switching to reserve, so there should have been at least .5gal.

RT43 are non-LRR tire, and Enasaves are LRR, so some loss is expected.

Now, what I am interested in the actual physics of rolling resistance. This is what Continental is saying about "Low surface abrasion technology":
From what I've read there are several venues tire makers explore to make LRR tires. For example on LRR tires Bridgestone or Michelin use special more flexible rubber compound in sidewalls, and probably best LRR tires in US market Michelin Energy Savers have the thinnest sidewalls I've seen. Other makers (Enasaves would be a good example) make sidewalls stiffer, to reduce flexing and flex-related heating.

For this very reason (sidewall heating/flexing) higher profile tires of the same make tend to be more fuel efficient. Taller sidewalls = less flex = less heating = less losses.

The question becomes that if the tire was optimized for specific contact patch shape, could over-inflating and changing shape be detrimental to wear/rolling resistance?

On a side note alignment is a big variable of the rolling resistance, and some tire designs seem to be more sensitive to small misalignment than others. Then again, less inflation = softer tire, which in theory should help to negate less than ideal alignment.

If anyone came across any articles on tire rolling resistance, could you please post a link? thnx

I look for tire gauges at yard sales, then match them to those I already have and throw away the ones that are way off. Older made in the USA gauges are best from my experience.
What everyone is experiencing is the "China effect", cheap tools that might be adequate for a very occasional use, nowhere near their design limits. I have tools that I bought in 1969, USA made and still working fine. The most frequent purchase at yard sales are tools.
Anyone know what a "fountain brush" is without looking it up?

regards
mech

cyclopathic,

Below are 2 of my webpages on RR and Fuel Economy. I'll bet most of the answers to your questions are there:

Barry's Tire Tech

Barry's Tire Tech

Thanks for the links, good summary. Unfortunately it does not answer my question on contact patch. I remember seeing a high speed camera video of contact patch wave a while back, with explanation on how this impacts rolling resistance.

Also have you looked at EU efficiency labeling? Could not find tire database compilation with info on tire ratings.

here is info on EU labels:

Full Guide To The EU Tyre Label

This is a speed related phenomenon and common to all types of tires. Yes, it does impact RR, so to reduce this phenomenon, keep your speed down. There's a chart around somewhere, but you'll have to dig for it, if you are truly interested. But, again, it is common to all tires, so it isn't something you need to worry about.

I am not aware of anyone who has compiled a summary on the EU labeling. NHTSA publishes a summary of the UTQG ratings, but that does not include RR as a rating point - perhaps in the future.

I think speed matters not as you still have the same number of revolutions per mile. So your total loss on RR per trip will be the same regardless of speed

The EU Energy Savers will still roll noticeably lighter @ higher pressure.


Before joining ecomodder, I originally started increasing tyre pressure to cure or reduce wallowing and tram lining effects due to too low OEM pressure recommendations.

This says otherwise.

https://www.tut.fi/ms/muo/vert/7_raw...p_image008.jpg

https://www.tut.fi/ms/muo/vert/7_raw...p_image008.jpg

And there is other data to support that RR is relatively the same until you hit about 60 mph, then it increases dramatically - which is probably where the video you looked at was coming from.

I'm curious of the physics involved in this. On the surface, you would think tire resistance would be linear. What is causing the curve, is some sort of internal dynamics on the tire causing the change? Is is something like heat, or a harmonic vibration or something? Do speed ratings have any effect on when that curve occurs?

Best guess = standing wave that grows as the speed increases.

I'm not sure what effect speed ratings have - or more precisely, what affect the change in construction has - but I suspect that the increased RR you get to achieve a higher speed rating is only slightly offset by the softening of the speed effect.

Put another way:

1) Higher speed rating = higher RR (all other things being equal)

2) Higher speed rating reduces the effect speed has on RR, but does not eliminate the effect.

What about centrifugal force throwing the tires out at higher speeds? You can definitely see that effect on Top Fuel dragsters. Then again 60 mph is past in a fraction of a second with those things.

On dragsters you don't have steel belts and the increased diameter is a part of the overall gearing.

regards
mech

I would think that even if diameter expansion was to blame, I would still expect it to be more linear. It's that flat before the curve that has me curious. Its like comparing it to metallurgy. That curve is the beginning of the yield curve. I know they are completely different things, I'm just trying to get an idea of a similar physical process a material goes through that has such a drastic change.

I am still not sure the graph above compares oranges to oranges. It shows that RR force increases with speed, but OP never argued this. The argument is that in the range of speeds tire used the increase is liniar, so driving one mile at 25mph and 50mph will require to overcome XYZ kJ of RR

I think what is going on that at higher speeds there more slippage as tire doesn't get enough time to bend into optimal shape (is it also a reason why too much pressure could be bad?) Or higher hysteresis at higher speed

Here is more on physics: https://en.m.wikipedia.org/wiki/Rolling_resistance

Amen on slippage. The iMiEV has skinny LRR tires that I run a bit tight (44 psi). High speed sweeper turns with the ASC turned off feel just fine with no howling tires and don't feel loose to me, but switch the safeties back on, and the nanny systems intervene! I didn't think the car was loose enough on a 60 mph turn to kick in the calipers, but the wheelspin sensors don't lie! Tire wear has also indicated slippage, as though the 145 mm fronts scrub hard in turns, it is the 175 mm rears that wear faster on this RWD car! Weight distribution is 45/55 and the rear has 20% more rubber, so that's proportional, but the relationship between load and traction is non-linear. Perhaps the relationships between load and wear and contact area and wear are also non-linear.

Slippage?

I think there are 2 different concepts being discussed.

1) Slippage in the direction of travel. As tires are required to generate more forward force to overcome wind resistance, the tire has to slip relative to the direction of travel to generate that force. This MIGHT have implications on RR - BUT - when RR is measured in a lab, there is no wind resistance. The only thing being measured is pure RR. That's why I think the standing wave is the reason for the increased RR.

2) Lateral slippage - as in a turn. Another way to look at this is grip. And, yes, slippage due to loss of grip is going to affect the energy loss a vehicle experiences, but I don't think that is impacting what that graph is showing.

The cause of the splippage could be different but the process is the same: you have a segments of rubber in contact patch moving in relation to road surface and causing abrasion. Standing wave, lateral turn load, ?too much?/insufficient pressure could be causing it.

If an issue with wave was an issue, than there would have been a drop/increase at resonance frequency.

Wheels are rotation at relatively slow speeds to have the wave make all the way around wheel. At 60MPH wheels rotate at 700-900RPM.