Tire inflation: MPG versus comfort

[belfert]

How much difference does tire inflation make for MPG?

I am running 90 PSI front and 80 PSI rears as required for my axle weights.  I had been running them at the maximum 105 PSI.  Am I killing my MPG by lowering the PSI?

[Sean]

Brian,

Generally, the higher the tire pressure, the better the fuel mileage.  Opinion (and testing) varies on how much mileage changes with pressure, but you will find lots of it if you Google around.

All is not roses, however.  In addition to ride comfort, you will also be sacrificing safety by running your tires over the recommended pressure, and the tires will also wear out faster.

The safety aspect comes from the fact that there is less rubber in contact with the road.  That means increased braking distance on all surfaces, and also less cornering traction in emergency handling situations.

The tire wear has to do with the fact that the smaller contact patch will mean the center of the tread will wear faster than normal, and the shoulders will wear more slowly.  The uneven wear will get you to sub-legal tread in the center in fewer miles.  For many with bus conversions, this is a red herring, because the tires need to be replaced from age (7 years maximum, generally) long before the tread wears down.  (We wear our tires out every four years, give or take.)  It becomes very difficult to calculate whether the improved mileage offsets the increased tire wear in lifetime cost-of-operation; obviously, the more expensive fuel is, the more lopsided the equation gets.

Personally, I would not trade the safety off for minor fuel savings.  YMMV, as they say.

-Sean
http://ourodyssey.blogspot.com

[buswarrior]

Over-inflated steer tires have been known to contribute to wandering.

Proper contact patch is pretty important for lateral traction in the rain.

Pounding the crap out of the coach suspension and frame with harder tires might have some issues down the road.

Adjusting one's driving style and terminal velocity are the surest way of moderating fuel use.

Stop putting the pedal all the way to the floor.

Best to count anything over 5 mpg as a bonus and be done with it?

happy coaching!
buswarrior

[Lee Bradley]

The conditions that influence hydroplaning include speed, tire design, tread depth, water depth on the road, load on the tires, and inflation pressure. At low speeds (less than about 50 mph), if your tires are under-inflated, you actually have more tire touching the road. However, hydroplaning does not occur very often at speeds below 50 mph, unless there is deep water (usually standing water) on the road. As you get to about 55 mph and the water pressure going under the tire increases, an under-inflated tire has less pressure in it pushing down on the road and you have less tire-to-road contact than a properly inflated tire as the center portion of the tread gets lifted out of contact with the road. As speed increases to 70 mph and above and water depth increases due to a severe local storm with poor drainage, the under-inflated tire could lose 40 percent of the tire-to-road contact area compared to a properly inflated tire. The higher the speed (above 50 mph) and the more under-inflated the tire is, then the lower the tire-to-road contact and the higher is the chance of hydroplaning.

Tread depth has a substantial impact on the probability of hydroplaning. If you make a simplifying assumption that the water depth exceeds the capability of the tread design to remove water (which most likely would occur with very worn tires), then an approximation of the speed at which hydroplaning can occur can be estimated by the following formula:

Hydroplaning speed? = 10.35 x  inflation pressure [25]

Under this assumption of water depth exceeding the capability of the tread design to remove water:

At 30 psi, hydroplaning could occur at 56.7 mph

At 25 psi, hydroplaning could occur at 51.8 mph

At 20 psi, hydroplaning could occur at 46.3 mph.

This is presented to show the relative effect of inflation pressure on the possibility of hydroplaning.


Skidding and Loss of Control

Table of Contents

For loss of control crashes, speed is the most critical factor. Excessive speed alone can cause a loss of control in a curve or in a lane change maneuver. Tread depth, inflation pressure of the tires, and road surface condition are the most notable of a long list of factors including vehicle steering characteristics and tire cornering capabilities that affect the vehicle/tire interface with the road. In the Indiana Tri-Level Study, under-inflation was not considered a contributing factor to a crash when there was high speed involved. It was only considered when the tires were significantly under-inflated (an undefined term generally taken by the investigators to mean at least 10 to 15 psi below recommended pressure). Still, it is hard to know whether correcting this one problem area could result in the collision being avoided or reduced in severity. That is one reason why under-inflation was never cited as the definite cause of a crash. We tried to consider this by comparing under-inflation as a percentage of all of the probable causes in crashes. Certainly, reducing under-inflation is an important area and a move in the right direction. However, it is difficult to determine what the effectiveness of increasing tire pressure would be on these crashes. The following discussions describe how inflation pressure affects these crash types to the extent known.

Skidding and/or loss of control in a curve

Low tire pressure generates lower cornering stiffness because of reduced tire stiffness. When the tire pressure is low, the vehicle wants to go straight and requires a greater steering angle to generate the same cornering force in a curve. The maximum speed at which an off-ramp can be driven while staying in the lane is reduced by a few mph as tire inflation pressure is decreased. An example provided by Goodyear shows that when all four tires are at 30 psi the maximum speed on the ramp was 38 mph, at 27 psi the maximum speed was 37 mph, and at 20 psi the maximum speed was 35 mph while staying in the lane. Having only one front tire under-inflated by the same amount resulted in about the same impact on maximum speed. But, the influence of having only one rear tire under-inflated by the same amount was only about one-half of the impact on maximum speed (a 1.5 mph difference from 30 psi to 20 psi).

The agency also has run a series of tests to examine the issue of decreases in tire pressure on vehicle handling. A 2001 Toyota 4-Runner was run through 50 mph constant speed/decreasing radius circles to see the effects of inflation pressure on lateral road holding. We examined lefthand turns from 0 to 90 degrees handwheel angle for tire inflation pressures varied from 15 to 35 psi. The data indicate to us that in on-ramps/off ramps, tire inflation pressure is a critical factor in vehicle handling. The data show how much friction the vehicle can utilize, in terms of lateral acceleration (g?s), before it slides off the road. The more lateral g?s the vehicle can utilize, the better it stays on the road. So, if you are going around an off-ramp and need to turn the wheel 50 degrees at 50 mph, you can utilize 0.27 g?s at 15 psi, or you can utilize 0.35 g?s at 30 psi.

[JohnEd]

Lee,

That ought to do it!

Nice write up.

John

[belfert]

Best to count anything over 5 mpg as a bonus and be done with it?

I must be getting a huge bonus as I've been getting right at 8 MPG on long trips.  That 8 MPG was with the tires at 105 PSI.  I'm hoping not to lose a lot of MPG by going to lower PSI in line with the actual axle weights.

I already keep the MPH at 65 MPH to decrease fuel usage.  I discussed perhaps even going to 60 MPH last summer, but everybody else on the trip didn't want to spend an extra 3 hours in the bus each way, plus being 15 to 20 MPH slower than other traffic out west gets dangerous.  (Speed limits on I80 from Nebraska to California are mostly 75 MPH.)

[buswarrior]

So, for your 2340 mile trip:

36 hours at 65 mph
39 hours at 60 mph

Question needs to be: How much fuel did it cost extra to save the 3 hours?

Perhaps the single biggest issue that lets the fuel companies empty our wallets, car companies sell us powerful SUV's and who knows how many collisions happen... is the inability of the human mind to get its head around the relationship between time, speed and distance.

If just one vehicle passes us, we "feel" we are being left behind. Which bothers that other head that periodically thinks it is in charge....

We are the most educated masses in recorded history, yet we still can't stand it when others pass us on the highway, and feel we are being left behind. And we let ourselves get fleeced at every automotive related turn because of it. anyhow....

My coach gets 6 mpg at 70 mph and gets 7 mpg at 60 mph.

MC8, 8V71 at the 270 hp settings, HT740, 3.7:1 diff gear.

To travel down for Jack's party:

For fuel costs, we'll use my preferred cheapest-fuel-on-the-trip stopping point, the Flying J at Wytheville VA, where fuel this afternoon is said to be selling for the low price of $2.019 a gallon....
(where did $4.50 go?)

1500 miles

70 mph = 21.5 hours = 250 gallons = $504.75
60 mph = 25 hours = 214.3 gallons = $432.67

so, saving 3.5 hours costs us 35.7 gallons/ $72.08 one way.

(At $4.50 a gallon diesel, the savings would be $160.65)

If there was a way to earn more than $20.60 per hour for those saved 3.5 hours, then the economics of driving faster and burning more fuel works.

I can choose to spend some of those savings on "back home beverages" for the ex-patriots and snowbirds.

Of course, in any road trip, and making these kinds of calculations, we need to consider that the whole trip is not made at the calculated speeds, any city driving will negatively effect fuel economy as will all the accelerating to highway cruise speed. The actual driving time is longer than our math.

If the family is inspired to get in and get out when we stop, the "tortoise and the hare" story can easily be told again. It would be pretty easy for the people on the "faster" coach to burn through 3.5 hours of sitting still during two long days of road travel. If you haven't done it, try running a log book like the commercial drivers do, you'd be amazed at how time can get burned up sitting still while you are on the road, and, relatively how little the slower moving traffic through an urban setting slows your pace across the nation.

On your bus, the differences in tire pressures you are considering will not make a difference you can accurately measure at the fuel pump with your odometer. (Hot fuel will do more to you, I might suggest) Too many other variables in operation and driver behaviour, especially when you are thinking harder about fuel consumption now.

THINK versus FEEL....

only a few will get it, the rest will subsidize?

happy coaching!
buswarrior

[Sean]

I must be getting a huge bonus as I've been getting right at 8 MPG on long trips.

That's the advantage your Series-60 and B500 has over those of us with the older powerplants and trannys.  Also, the Marcopolo Viaggio is a more aerodynamic body than many.

I already keep the MPH at 65 MPH to decrease fuel usage.  I discussed perhaps even going to 60 MPH last summer, but everybody else on the trip didn't want to spend an extra 3 hours in the bus each way, plus being 15 to 20 MPH slower than other traffic out west gets dangerous. 

FWIW, That is an oft-quoted but completely unsupported statement.  AFAIK, no traffic safety studies have ever found that vehicles traveling below the posted limits (within reason, also usually posted) are less safe.

BTW, we normally drive 55 for safety and fuel economy, but we also stay off the Interstates as much as possible.  When forced to be on an Interstate, we drive 60, unless on our way to a disaster, when we will push it to 67-68, just a couple MPH below our top speed.

(Speed limits on I80 from Nebraska to California are mostly 75 MPH.)

The top speed limit for all trucks and any vehicle towing anything in California is 55 (the limit for cars tops out at 70).  Nevada, Utah, Wyoming, and Nebraska are, as you say, 75.  Illinois, BTW, is 55 again for RVs.

We have found that the US highways are a much more relaxing drive.  It's hard to say for certain, but my sense is that we save way more fuel driving at these speeds than we use due to grade, alignment, and traffic controls.  YMMV, of course.

If you really want to feel like the slow guy on the road, parts of Texas have 80mph limits, and even "farm to market" roads are often posted at 70.

-Sean
http://ourodyssey.blogspot.com

[belfert]

If just one vehicle passes us, we "feel" we are being left behind. Which bothers that other head that periodically thinks it is in charge....

We are the most educated masses in recorded history, yet we still can't stand it when others pass us on the highway, and feel we are being left behind. And we let ourselves get fleeced at every automotive related turn because of it. anyhow....

At 65 MPH on a highway signed at 75 MPH we get passed pretty regularly.  It doesn't bother me if vehicles pass me.  It is less stressful as a driver of a large vehicle to have others pass me than for me to regularly pass other vehicles.  I just stay in the right hand lane and let others pass if they want to go faster than me.  It was quite the rare event to pass a vehicle on a 4,000 mile round trip last year.

We always drive straight through to our destination because we have limited time for our trips.  We are always going to an event so we have no choice on how far to drive if we want to attend the event.  We can't choose to drive to the next state if the event is 2,000 miles away.

[pvcces]

Brian, there is a safe operating range for your tires, as there is for ours. We found that we had more than 1 mpg difference in operating at the low pressure end from the high pressure end.

For us, it was worth running the higher pressure.

For what it's worth.

Tom Caffrey

[Eric]

Not to step on any toes here being a noob, however you guys are missing one very very important factor with Tire Inflation and that's LOAD! Every tire manufacture has a chart for there various product that will give you Tire pressure in reference to axle weight! Most you can google for those you can't hit your local truck shop. As Sean said under / over inflation will end up leading to premature tire failure and possibly even a blowout! OK I'm done rambling!

[Sean]

Not to step on any toes here being a noob, however you guys are missing one very very important factor with Tire Inflation and that's LOAD! Every tire manufacture has a chart for there various product that will give you Tire pressure in reference to axle weight!

There was no need to bring it up, because the OP's very first post indicated that he had already consulted the load and inflation tables for his tires:

I am running 90 PSI front and 80 PSI rears as required for my axle weights.

(emphasis mine).

Also, we've discussed this topic here many times before, and so most of us took the question at face value.

-Sean
http://ourodyssey.blogspot.com

[WEC4104]

.....hydroplaning can occur can be estimated by the following formula:

Hydroplaning speed? = 10.35 x  inflation pressure [25]

Under this assumption of water depth exceeding the capability of the tread design to remove water:

At 30 psi, hydroplaning could occur at 56.7 mph

At 25 psi, hydroplaning could occur at 51.8 mph

At 20 psi, hydroplaning could occur at 46.3 mph.

This is presented to show the relative effect of inflation pressure on the possibility of hydroplaning.

Sorry, but I need to point out that the above formula contains an error.   The hydroplaning point is not 10.35 times the tire inflation, but rather it is 10.35 times the square root of the tire inflation.   You can see that 10.35 x 30psi obviously does not equal 56.7 mph.

I believe this equation originated from Samual K. Clark's work entitled "The Mechanics of Pneumatic Tires".

[Lee Bradley]

Well that certainly invalids that study.

I knew there was a reason I was posting less and less here. Now I think zero.

[WEC4104]

Oh, the study itself is just fine. I am simply pointing out that it appears the special character for the square root symbol did not survive the cut & paste process.  It might be important to someone who wonders why the rest of the math in the document doesn't work, or for anyone who wants to do the calculations on higher psi figures.