Tire pressure and fuel economy

[bigoldthor]

I know there have been a number of threads about tire pressure. Not trying to start any arguments or open old wounds. Just wanting to share what I did and what I'm seeing as a result.

So the door jam recommended PSI is 36 front and back. I typically run mine over the recommended ratings by a few lbs to give me a slight bump in f.e. while trying not to affect ride quality and wear too much. I know my truck is not even "broken in" yet, but at 500 miles, I was seeing about 14.8 mpg in mixed driving. So this weekend, I bumped them to 40 psi to see what difference I saw. In roughly a 100 miles of mixed driving, mpg has jumped by 2, now at about 16.9. Ride quality is a teeny bit harder, but probably not enough to really notice if I didn't know about the tire pressure change. Time will tell, and maybe it's partly due to my use of 89 octane top-tier fuel vs. the dealer's cheap, initial fill-up of 87. I'll keep my eyes on it and let everyone know how it goes.

My point is that so far, I seem to like this pressure. Still a very good ride and (possibly) a ~10-15% improvement in f.e. As always, YMMV, literally and figuratively.

(P.S. And before anyone starts jumping on me about not following the manufacturer's recommendations, know that I worked for Michelin N.A. HQ for several years and spent a lot of time talking to engineers and designers. Increasing pressure above the mfg. recommendations is not a problem as long as you keep it within reason, and definitely do not exceed the maximum sidewall ratings under normal driving conditions. I personally won't go anywhere near that.)

 

[ForTehNguyen]

just dont want to go too much to negatively affect tire wear also over filling will wear the center tread faster.

I got better mpg and engine response using 89, it felt sluggish on 87. Guy on Ram facebook group said 87 was knock city when he was data logging the performance.

 

[bigoldthor]

just dont want to go too much to negatively affect tire wear also over filling will wear the center tread faster.

I got better mpg and engine response using 89, it felt sluggish on 87. Guy on Ram facebook group said 87 was knock city when he was data logging the performance.

Correct. I'm keeping a close eye on treadwear. I don't *think* 4 psi will make a difference, but who knows. Will keep you posted. And good to know about 89. I'll pay the premium for better f.e. and performance, even if it doesn't fully "pay for itself". There's more to fuel than which is the cheapest per mile.

 

[Cortesio]

The amount of force your tires need to apply to the road to pull the vehicle along is a function of rolling resistance. As your tires rotate, there is an amount of deformation where the tire contacts the road. This is a non-elastic loss of energy because the energy used to deform the tire is not returned to the system. The energy leaves the system as heat.

Your tire pressure directly relates to how much deformation the tires experience. A lower pressure allows more deformation which increases the rolling resistance and therefore the force the vehicle needs to apply to the road to rotate the tires. Imagine an aired down tire. It’s visibly deformed at the bottom where it sits on the road while it looks normal at the top. As it turns, the energy used to constantly deform and reform is this deformation loss we’re talking about. The vehicle has to use energy to squish and unsquish the tire that could have been used to instead drive the vehicle forward. That said, making the tire easier to deform allows it to mask changes in road surface which improves the perception of the ride from the passengers.

Conversely, a higher tire pressure is more resistant to deformation and therefore less energy is lost to deformation and less energy is needed to rotate the tires. This manifests as a rougher ride because road surface changes are more readily transferred from tire to suspension to passengers.

Deformation constitutes the vast majority of energy losses from your rotating tires. Friction against the road surface is another, but much smaller component. Your tires need friction against the road to work because your tires don’t merely roll across the surface. In order to move the vehicle, your tire has to “push” (via friction) against the ground. This push has to be of great enough magnitude to overcome or exceed the forces opposing the vehicle moving forward (like air resistance).

When you air down your tires, you increase the contact area of the tire against the road surface which increases the friction. Go too low and the pressure isn’t enough to push the center of the tread against the road and you see increased wear closer to the sidewalls. Air up your tires and you decrease the contact area which decreases the friction. Go too high and only the center of the tread is contacting the road, again causing uneven wear.

It’s important to reiterate that the deformation losses are an order of magnitude higher than the friction losses. You don’t need to air up your tires with the goal of reducing contact area and friction. If you want better fuel economy, you air up your tires to make them “harder” and therefore deform less as they rotate. When you start losing significant contact area, you’re going too high.


Sent from my iPhone using Tapatalk

 

[Cortesio]

Correct. I'm keeping a close eye on treadwear. I don't *think* 4 psi will make a difference, but who knows. Will keep you posted. And good to know about 89. I'll pay the premium for better f.e. and performance, even if it doesn't fully "pay for itself". There's more to fuel than which is the cheapest per mile.

If you want some information about fuel choices for our trucks, here’s something I wrote up a few months ago.

5.7 Hemi - 87 or 89 Octane

What I find funny is that buried somewhere in the owners manual it has this caution about using 85 octane fuel and a recommendation of 89. But no where in the gas door does it say what octane to use and no where on the display does it say what fuel to use. So say you rent one of these FCA...

5thgenrams.com

Sent from my iPhone using Tapatalk

 

[bigoldthor]

The amount of force your tires need to apply to the road to pull the vehicle along is a function of rolling resistance. As your tires rotate, there is an amount of deformation where the tire contacts the road. This is a non-elastic loss of energy because the energy used to deform the tire is not returned to the system. The energy leaves the system as heat.

Your tire pressure directly relates to how much deformation the tires experience. A lower pressure allows more deformation which increases the rolling resistance and therefore the force the vehicle needs to apply to the road to rotate the tires. Imagine an aired down tire. It’s visibly deformed at the bottom where it sits on the road while it looks normal at the top. As it turns, the energy used to constantly deform and reform is this deformation loss we’re talking about. The vehicle has to use energy to squish and unsquish the tire that could have been used to instead drive the vehicle forward. That said, making the tire easier to deform allows it to mask changes in road surface which improves the perception of the ride from the passengers.

Conversely, a higher tire pressure is more resistant to deformation and therefore less energy is lost to deformation and less energy is needed to rotate the tires. This manifests as a rougher ride because road surface changes are more readily transferred from tire to suspension to passengers.

Deformation constitutes the vast majority of energy losses from your rotating tires. Friction against the road surface is another, but much smaller component. Your tires need friction against the road to work because your tires don’t merely roll across the surface. In order to move the vehicle, your tire has to “push” (via friction) against the ground. This push has to be of great enough magnitude to overcome or exceed the forces opposing the vehicle moving forward (like air resistance).

When you air down your tires, you increase the contact area of the tire against the road surface which increases the friction. Go too low and the pressure isn’t enough to push the center of the tread against the road and you see increased wear closer to the sidewalls. Air up your tires and you decrease the contact area which decreases the friction. Go too high and only the center of the tread is contacting the road, again causing uneven wear.

It’s important to reiterate that the deformation losses are an order of magnitude higher than the friction losses. You don’t need to air up your tires with the goal of reducing contact area and friction. If you want better fuel economy, you air up your tires to make them “harder” and therefore deform less as they rotate. When you start losing significant contact area, you’re going too high.


Sent from my iPhone using Tapatalk

Well said, and makes sense. I've made mine a little harder and thus reduced rolling resistance. I'll continue to watch the tread wear but I doubt 4psi will make much difference. Will see.

 

[bigoldthor]

If you want some information about fuel choices for our trucks, here’s something I wrote up a few months ago.

5.7 Hemi - 87 or 89 Octane

What I find funny is that buried somewhere in the owners manual it has this caution about using 85 octane fuel and a recommendation of 89. But no where in the gas door does it say what octane to use and no where on the display does it say what fuel to use. So say you rent one of these FCA...

5thgenrams.com

Sent from my iPhone using Tapatalk

That is quite an interesting and informative write up. While I knew some of what you wrote, I learned a few new things. May I ask what you do (or what your credentials are) regarding automotive technology? You clearly know much more than the average enthusiast.

So your final takeaways are very helpful...best combination of power and f.e. is 89 straight gas (no ethanol). However, 87 may work nearly as well (or as well) as 89 under some circumstances, and the vehicle will adjust timing to account of any differences. So if f.e. is your primary concern, you may be money ahead using 87, though individual variables will affect that.

Thanks very much!

 

[LaxDfns15]

Just to let you know, the engine DOES take a bit to break in. Got my truck with 41 miles on the clock, and every tank of gas has increased 0.5-1 MPG. I drove it about 600 miles round trip this weekend to end up with 1100 on the clock and I've gone from averaging 14 to 16.5 mostly highway. This is with a Rebel, so if it keeps improving up to about 17-18 highway I'll be where I personally wanted to be with the truck. Since I've hit 1000 miles I also plan on changing the oil over to Rotella truck full synthetic as soon as I get a free weekend.

 

[bigoldthor]

Just to let you know, the engine DOES take a bit to break in. Got my truck with 41 miles on the clock, and every tank of gas has increased 0.5-1 MPG. I drove it about 600 miles round trip this weekend to end up with 1100 on the clock and I've gone from averaging 14 to 16.5 mostly highway. This is with a Rebel, so if it keeps improving up to about 17-18 highway I'll be where I personally wanted to be with the truck. Since I've hit 1000 miles I also plan on changing the oil over to Rotella truck full synthetic as soon as I get a free weekend.

Nice improvement...hope I see the same. The jump after the tire pressure change was instantaneous, so maybe I'll get even more improvement with some additional miles. Doubt I'll go full synthetic but haven't decided yet. Also not sure when I'm going to do my first oil change but definitely well before the OLM says I need it. Before 3K as well.

 

[Cortesio]

That is quite an interesting and informative write up. While I knew some of what you wrote, I learned a few new things. May I ask what you do (or what your credentials are) regarding automotive technology? You clearly know much more than the average enthusiast.

So your final takeaways are very helpful...best combination of power and f.e. is 89 straight gas (no ethanol). However, 87 may work nearly as well (or as well) as 89 under some circumstances, and the vehicle will adjust timing to account of any differences. So if f.e. is your primary concern, you may be money ahead using 87, though individual variables will affect that.

Thanks very much!

I’m about a year out from getting an engineering degree. I definitely don’t possess any specialized automotive experience, but I did pay attention in my physics and chemistry classes.


Sent from my iPhone using Tapatalk

 

[Cortesio]

Just to let you know, the engine DOES take a bit to break in. Got my truck with 41 miles on the clock, and every tank of gas has increased 0.5-1 MPG. I drove it about 600 miles round trip this weekend to end up with 1100 on the clock and I've gone from averaging 14 to 16.5 mostly highway. This is with a Rebel, so if it keeps improving up to about 17-18 highway I'll be where I personally wanted to be with the truck. Since I've hit 1000 miles I also plan on changing the oil over to Rotella truck full synthetic as soon as I get a free weekend.

While my experience is hardly empirical, I’ll contribute my sample size of 1.

I’m at about 9500 miles now and have had pretty steady fuel economy. Even with a few different blends of gas thrown in, there hasn’t been a significant variation in economy. I don’t drive to maximize economy and that’s definitely reflected here. I’m pretty confident that gains from “break in” have more to do with changing driving style than purely mechanical improvements.


Sent from my iPhone using Tapatalk

 

[Willwork4truck]

Other thing you have to consider is losing air pressure over time as well as in the cold. I was lazy and did not check the pressures for about 2 months (Dec-Jan) and it turns out the truck went down from 35 to 30-31. I only happened to catch it cause the tires "looked" softer than they should be and I checked it.

Since I did not have a 110v compressor (ordered one quickly) I went to a tire shop and they aired to 35. They won't exceed posted air pressures so you have to do that yourself.
Overall I'd agree that 38-40psi cold should be ok to run at, like was said, watch the center wear.

 

[jdmartin]

The only problem with raising your tire pressure to increase fuel economy is that you could be significantly shortening the safe life tread of your tires unless you are meticulous about measuring tread depth across the width of the tire. Most of the time people don't realize they've ruined their tires through over inflation until it's too late to do anything about it, i.e. the damage is already done. Considering the cost of tires versus the potential couple extra MPG you might get, it's really a Faustian bargain. I personally wouldn't want to trade $100 in fuel savings for $100 in extra tire wear.

HOWEVER, what tire pressure *you* should be running depends on your configuration and driving style and conditions. A truck that is always running with an empty bed really needs a little bit less air than the front end, since there's little weight to compress the tires. The opposite if you're running with a full load. But that gets to be too complicated for people (and computers, now!) to keep track of (anyone else here remember when front & rear tires had different PSI recommendations on the door tags?) so you get the flat pressure recommendation, which generally works out OK if you rotate your tires religiously. If you live in a really hilly, curvy area you can get away with a little higher tire pressure because you're going to be flexing the sidewall and forcing side contact anyway with the extra turns.

 

[bigoldthor]

Other thing you have to consider is losing air pressure over time as well as in the cold. I was lazy and did not check the pressures for about 2 months (Dec-Jan) and it turns out the truck went down from 35 to 30-31. I only happened to catch it cause the tires "looked" softer than they should be and I checked it.

Since I did not have a 110v compressor (ordered one quickly) I went to a tire shop and they aired to 35. They won't exceed posted air pressures so you have to do that yourself.
Overall I'd agree that 38-40psi cold should be ok to run at, like was said, watch the center wear.

It's easy to lose track of that. I would usually check it only occasionally on my F150s, but when I saw the lower-than-expected fuel economy on the Ram, I checked it out and saw that the air pressures ranged from about 33 to 37. My obsessive self couldn't handle that, so I adjusted them all to match and bumped them up in the process. I have a 20-gal compressor in my garage so it's no problem to do a quick top off as needed.

 

[bigoldthor]

Another observation. While psi obviously increases as the tires heat up and air inside expands, these seem to expand more than other vehicles I've had. These will go from 40 to 44 in only a few miles of driving, then will just stay there. So 10% increase and hold. Others I've had seemed to expand less. Maybe I'm just "misremembering" or maybe the tire size has something to do with it, but my last three vehicles had 275/55R20's I'm pretty sure.

 

[Dusty1948]

The amount of force your tires need to apply to the road to pull the vehicle along is a function of rolling resistance. As your tires rotate, there is an amount of deformation where the tire contacts the road. This is a non-elastic loss of energy because the energy used to deform the tire is not returned to the system. The energy leaves the system as heat.

Your tire pressure directly relates to how much deformation the tires experience. A lower pressure allows more deformation which increases the rolling resistance and therefore the force the vehicle needs to apply to the road to rotate the tires. Imagine an aired down tire. It’s visibly deformed at the bottom where it sits on the road while it looks normal at the top. As it turns, the energy used to constantly deform and reform is this deformation loss we’re talking about. The vehicle has to use energy to squish and unsquish the tire that could have been used to instead drive the vehicle forward. That said, making the tire easier to deform allows it to mask changes in road surface which improves the perception of the ride from the passengers.

Conversely, a higher tire pressure is more resistant to deformation and therefore less energy is lost to deformation and less energy is needed to rotate the tires. This manifests as a rougher ride because road surface changes are more readily transferred from tire to suspension to passengers.

Deformation constitutes the vast majority of energy losses from your rotating tires. Friction against the road surface is another, but much smaller component. Your tires need friction against the road to work because your tires don’t merely roll across the surface. In order to move the vehicle, your tire has to “push” (via friction) against the ground. This push has to be of great enough magnitude to overcome or exceed the forces opposing the vehicle moving forward (like air resistance).

When you air down your tires, you increase the contact area of the tire against the road surface which increases the friction. Go too low and the pressure isn’t enough to push the center of the tread against the road and you see increased wear closer to the sidewalls. Air up your tires and you decrease the contact area which decreases the friction. Go too high and only the center of the tread is contacting the road, again causing uneven wear.

It’s important to reiterate that the deformation losses are an order of magnitude higher than the friction losses. You don’t need to air up your tires with the goal of reducing contact area and friction. If you want better fuel economy, you air up your tires to make them “harder” and therefore deform less as they rotate. When you start losing significant contact area, you’re going too high.


Sent from my iPhone using Tapatalk

Excellent!!!

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 17 July 2018. Now at: 029656 miles.