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Old 06-18-2008, 05:06 AM   #1
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I have always heard that truck drivers like to lower the air pressure in their tires to get a softer ride, but that most companies want to run higher pressures to get better MPG. Is this true or just an old wives tail? If it is true, how much of a different in pressure to change the MPG? I am looking for every little bit of increase now a days.

I had my rig weighted and based on the Michelin charts, I can lower the tire pressure 10 to 15 pounds from what is on the Winnebago's door sticker(90 front 85 rear). I would rather have the better MPG than a softer ride.
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Old 06-18-2008, 05:06 AM   #2
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I have always heard that truck drivers like to lower the air pressure in their tires to get a softer ride, but that most companies want to run higher pressures to get better MPG. Is this true or just an old wives tail? If it is true, how much of a different in pressure to change the MPG? I am looking for every little bit of increase now a days.

I had my rig weighted and based on the Michelin charts, I can lower the tire pressure 10 to 15 pounds from what is on the Winnebago's door sticker(90 front 85 rear). I would rather have the better MPG than a softer ride.
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Old 06-18-2008, 06:43 AM   #3
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It's not safe to inflate motorhome tires over the pressure required for the load.

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Old 06-18-2008, 07:36 AM   #4
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I believe, and have read, that keeping the tires at the required pressure is the best way to go both for safety and MPG. Lower pressure equals more drag and lower MPG.
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Old 06-18-2008, 02:26 PM   #5
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">It's not safe to inflate motorhome tires over the pressure required for the load. </div></BLOCKQUOTE>
AFAIK the only downsides are a bit harsher ride and uneven tire wear.

It is often safer to err on the high side up to the max pressure indicated on the sidewall of the tire. That pressure is determined with a tire at ambient temperatures. On the road, the tire temperature may get to 120F to 140F. If it gets higher than that, you need more air in the tire.

Higher tire pressure reduces rolling resistance and should increase fuel efficiency.

A primary failure cause for tires is under inflation. The guides in your owner's manual or door jamb are for the tires that came with the vehicle. Other tires may need different pressures for various loads. Check with the manufacturer. Again, one of the easiest ways to tell is by checking tire temperature on the road.
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Old 06-18-2008, 04:49 PM   #6
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BryanL:
AFAIK the only downsides are a bit harsher ride and uneven tire wear.

It is often safer to err on the high side up to the max pressure indicated on the sidewall of the tire. That pressure is determined with a tire at ambient temperatures. On the road, the tire temperature may get to 120F to 140F. If it gets higher than that, you need more air in the tire.

Higher tire pressure reduces rolling resistance and should increase fuel efficiency.

A primary failure cause for tires is under inflation. The guides in your owner's manual or door jamb are for the tires that came with the vehicle. Other tires may need different pressures for various loads. Check with the manufacturer. Again, one of the easiest ways to tell is by checking tire temperature on the road. </div></BLOCKQUOTE>

It is not safer to "err on the high side". It is safest to adjust your pressure to your weight.

Overinflation will reduce the tire's contact area with the road, which reduces traction (ie: hydroplaning), braking ability (ie: skidding), and handling (ie: loss of control). A tire that's overinflated for the weight it's carrying is more prone to a harsh ride, uneven tire wear, and impact damage.
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Old 06-19-2008, 08:10 PM   #7
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I believe Tom N gives the best advice here. The tire manufacturers publish those charts for a reason. The motorhomes manufactures charts are written for maximum load. I know from personal experience I get the best mileage, ride, and handling when the pressure is set to the actual coach weight.
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Old 06-19-2008, 09:37 PM   #8
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IMEO of 25 years in the business, Tom N has hit the nail on the head


Here is a little information on the accuracy of checking tire temperature on the road. This reponse from Bridgestone speaks mostly to "contained" air temperature.

I asked:

With air pressure and tire temp sensors installed in aluminum wheels, what kind of temperatures should be observed while driving in 90 to 100 degree temperatures?

What air temperature reading would be too high? I am using the Smart Tire system.



Thank you,

Tom Dietrich

A BFS distributor in California .







Dear Tom,



Thank you for the opportunity to be of assistance.



The answer to your question depends on a number of factors, and quite frankly, there is not a hard and fast rule.



There are basically (3) types of tire temperature measurement

Probes inserted into the tire

Contained air temperature (which your system uses)

Tread (surface) temperature



A probe inserted into the tire into either the belt edge or the bead area the hottest points of the tire is the most accurate method; however, it can only be performed under controlled conditions.



The contained air temperature method is the next most accurate, however, it is affected by the mounting system of the sensor if the sensor is attached to the wheel, it will pick up heat from the wheel (which is picking up heat from the brake drum); and if it is attached to the tire interior, it will pick up heat from the casing.



Tread (surface) temperature is the least accurate, since measurement is normally performed by a hand held unit, thus hampering repeatability, plus the question of where do you measure? The ribs will be cooler than the grooves, and the center will be cooler than the shoulders, etc.



So, while all this has so far done little to answer your question, hopefully it has shed some light as to why I am being a bit reserved in my answer.



Now, what can I say to try to address your question?



While this is not set in stone: A very general rule of thumb is that a properly inflated/loaded tire, when up to operating temperature one hour+ operation - will typically run about 60 degrees F. hotter than the ambient temperature. Anything above 200 degrees F. could lead to tire degradation and you need to investigate for a problem.





I hope this has answered your question to your satisfaction; if not, or if you have any other questions, please feel free to contact me.



Regards



Greg
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Old 06-21-2008, 12:33 PM   #9
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Hydro planing occurs at 9 times the square root of the tire pressure. Therefore if the tire pressure is 100 psi you will start to HP at 90 knots. Lowering the tier pressure actually decreases the spped at which you will HP.

Now on a different subject a higher tire pressure will cause an increase in stopping distance due to the reduced contact patch.

An excessive high or low Tire Pressure is dangerous.

Excessively High tire pressure will have a significant negative effect on tire wear. any money saved by getting better milage will be more than used up when you go to purchase new tires.

Bill
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Old 06-21-2008, 03:18 PM   #10
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This past spring I replaced my tires with the same tire that was on there due to them being 6 years old. They are Michelin XRV 235r80/22.5. The tires are rated for up to 110 psi. For five years I ran 90 in the front and 85 in the rear per the door sticker. I never had any problems with hydro plaining. I weighted the motorhome set up for a weekend trip and used that weight to check the charts. The charts say I can lower the pressue to 80 in the front and 70 in the rear. After reading all the problems that the XRVs had with zippering due to low tire pressure, I hesitate to run them that low. Also I don't know if the next trip out I might weight more or less that the one time I weigthed the motorhome. With 20-25 psi to spare on the tires, I feel I would be safe running them at the recommand pressure on the door sticker.

My original question was about the effect a change in tire pressure might have on MPG. I did not intend to start a be deal about the safety of running tires above the recomended psi. I am sorry for the misunderstand in my question.
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Old 06-21-2008, 07:05 PM   #11
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Most tire experts recommend that you run 5-10 psi over the minimum required for your load. That gives you a margin for a bit of extra load, the tire runs a bit cooler and with a bit less roll resistance and is not enough extra pressure to cause loss of contact with the road or any of the other negatives cited here.
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Old 06-21-2008, 07:07 PM   #12
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My manufacturer's sticker reads 110 lbs all around for my coach. That made for a rough ride and some poor handling. I've weighed the coach and dropped the pressures to 93 in the rear and 102 in the front. That's adding in a 5 lb extra as a safety margin. The coach rides and drives much better. I have not noticed any decrease in fuel mileage since adjusting the pressures to that recommended by Michelin. It has made quite a difference to my old tired saggy butt. You might want to try driving your coach each way for several tanks of fuel and see if it makes a difference.
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Old 06-21-2008, 07:26 PM   #13
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RV_Boy:
My original question was about the effect a change in tire pressure might have on MPG. </div></BLOCKQUOTE>

The maximum pressure printed on the tire would give you the maximum mileage because this would give you the least contact with the highway.

But, adjusting pressure for maximum mileage is not an option because RV tires have to aired to the correct pressure to carry the load according to the tire manufacturer's load/pressure tables. Adding or subtracting from the tire manufacturer's tables can place you in an unsafe condition.

There are no pressure tables showing maximum mileage for a given pressure. There are tables for pressure to weight.

But, there are those that believe more is better.

I only use Michelin tires and what I post pertains to Michelin and is not my opinion but material from Michelin's RV Tire Guide.

-Tom
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Old 06-22-2008, 05:08 AM   #14
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I read a recent article on getting the best mpg out of a vehicle, now called "hypermiling". They said that keeping your air pressure at what the manufacture states is the best way to go. Their testing did not indicate any measurable increase in mpg by having the air pressure in the tire higher than recommended by the manufacturer. I have since driven various long trips with higher and lower air pressures and did not see any differences in mpg.
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