Tire pressure question

[Lowell]

I reviewed all the manuals that came with my TT and was unable to locate any information on what pressure I should use in the tires.  I was very surprised that 5 psi would make that much difference in MPG.  I know it's really very difficult to establish MPG performance due to one variable such as this.  Other factors could be temperature, moisture content of the air, gasoline quality, etc, etc. But the TT tire pressue was the only thing that I know was changed. It didn't seem like I was going downhill both ways, especially on that 10 mile grade back up I-10 out of Indio.  Going both ways, I don't think I went over 2500 RPM very often on the down shifts.  Only when passing.  According to my GPS, my maximum speed was 68.2 mph and my average moving speed was something around 52 mph.  I just love the statistics you get from the GPS.

 

[Tom]

I just love the statistics you get from the GPS.

As the saying goes, there are lies, damned lies and statistics  ;D  The first thing the CAT tech does when he comes to our boat is plug his laptop into the two engine computers and look at the "stats". One thing he's looking for is the percentage of time spent at idle and full open throttle. He's always "happy" with the results, but they don't say much about how the captain actually "drives" the boat.

 

[BernieD]

Lowell

I usually get about 1 mpg better fuel economy driving east bound compare to driving westbound. Prevailing winds. Mileage drops when you drive into a head wind and increases with a tailwind.

 

[Ned]

There are too many other variables to attribute any difference in MPG to tire pressure.  The 1.7MPG difference could easily be due to winds, grades, driving methods, etc.  If you drove the exact same course several times at the different pressures, you MIGHT be able to derive a difference due to that one factor, but such a test is almost impossible under real world driving conditions.  How did you measure the MPG?  If by using the fuel fills, that by itself could account for that difference.  You need to average over several hundred gallons to get any meaningful measure.

 

[Lowell]

How did you measure the MPG?

Ned,

Each direction was over 530 miles.? Milage was measured by the truck's computer system.? Maybe Bernie nailed it. It could be that wind was a factor.? I wonder if there is a web site I could go to and see what the prevailing winds were on the days in question.  Actually, I was pretty pleased with the milage in both directions.

 

[Ned]

The prevailing winds in that area are westerly, so you most likely had headwinds going west and tailwinds coming back east.

 

[rbell]

I took my bicycle out the other day for a ride. After about a mile I noticed how it was moving a bit hard and not coasting very well. I went back home and checked the tires, the back was 18 lbs low and the front was down 15 lbs. They should have 65 lbs according to the sidewall. It wasn't noticeable to look at though. It's a Trek Navigator with fat tires. So you can really tell how much difference a bit of pressure makes like that. Also riding into the wind is a lot harder than riding downwind.

 

[Gottasmilealot]

Here are two articles worth reading. Don't forget that RV/truck tires may have two recommendations for the same tire... one for dual wheel applications, and one for single wheel applications.? Exceeding the manufacturer's maximum is not recommended.? If you have to go that high with pressure, then you should probably be going to a higher rated tire.? Your tires should be rated to the rating of your axle so that if you max out on the axle, your tires are still OK.? There's a tendency to shop for the best price on tires, but be sure you're getting the weight and speed rating you need.?

Then again, RV's never overload their axles or tires, right?? ;D

http://www.nhtsa.dot.gov/cars/rules/TireSafety/ridesonit/brochure.htmlhttp://www.nhtsa.dot.gov/cars/rules/TireSafety/ridesonit/brochure.html

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=21http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=21

 

[Lowell]

I finally found some tire pressure info in my stack of documents that came with my trailer.  I had put it in with the wiring diagram :.  My trailer came with Carlisle tires.  While the tire booklet contains a trailer tire load limit table it states the following just below the table:

" The above table does not endorse the reduction of pressure.  It is to be used as an example of the relationship between air pressure vs load or lack there of.  Carlisle Tire and Wheel only recommends and only warrants tires maintained @ the max air pressure while in service. This table (above) shows the relationship between tire pressure and load capacity for popular sizes of trailer tires.  The capacities apply equally to radial and bias versions.  The letters following some capacities indicate the load range." 

So according to the quote, I should be using 50 PSI as called out as max pressure on the side of my tire, regardless of the load on each axle.

 

[rsalhus]

Our motorhome manufacturer, Rexhall, recommends 75 PSI for each tire (front and back, back have duals).  I checked the maximum PSI for my tires (G159 Good Years, 245/75 R22.5) from Good Year's website, and it is listed at 110 PSI.  I talked to the guys at the local Good Year dealer for trucks, and they say they would inflate these tires to 100 PSI if they were installing them.  When I told them that my RV manufacturer recommends 75 PSI, they say "Of course, they want you to have a nice, smooth ride!"  My tires were all inflated to 75 PSI when I bought it used last year, but I'm thinking they should probably be closer to 100 PSI when I'm carrying my maximum load rating (22,000 lbs.)  With 25K miles on them, the front tires are wearing around both edges more than the rear ones, but is that because they are steering tires or because they are underinflated?  And who's right, the RV manufacturer or the guys at Good Year?

 

[Ned]

Neither, probably.  Use the Goodyear inflation table to determine the correct pressure.  The tire people will always put the maximum pressure in the tires, they don't seem to know about the tables at all.  Of course, you should weigh your coach, at least by axle, better by wheel position, to use the tables correctly.

I would attach the Goodyear table but the attachment is over 120kB and the forum software won't accept it.  You can find the same table on the Goodyear web site here.

 

[Gottasmilealot]

Side wear is usually consistent with underinflation, center wear with overinflation.

 

[Carl L]

Side wear is usually consistent with underinflation, center wear with overinflation.

True, but side wear is also charateristic of trailer tires owing to the geometry of their axles.  I think it should be noted in this thread which started out as a trailer discussion, has mixed up motorhome and trailer issues.  They are not the same issues.

 

[Jeff]

Our motorhome manufacturer, Rexhall, recommends 75 PSI for each tire (front and back, back have duals).  I checked the maximum PSI for my tires (G159 Good Years, 245/75 R22.5) from Good Year's website, and it is listed at 110 PSI.  I talked to the guys at the local Good Year dealer for trucks, and they say they would inflate these tires to 100 PSI if they were installing them.  When I told them that my RV manufacturer recommends 75 PSI, they say "Of course, they want you to have a nice, smooth ride!"  My tires were all inflated to 75 PSI when I bought it used last year, but I'm thinking they should probably be closer to 100 PSI when I'm carrying my maximum load rating (22,000 lbs.)  With 25K miles on them, the front tires are wearing around both edges more than the rear ones, but is that because they are steering tires or because they are underinflated?  And who's right, the RV manufacturer or the guys at Good Year?

As Ron and others have said the ONLY proper way to know is measure the four corners of your RV and use the load/inflation tables for your tires. There is no other "right" way and in addition to saving your tires proper inflation almost always improves handling.

 

[BernieD]

Our motorhome manufacturer, Rexhall, recommends 75 PSI for each tire (front and back, back have duals).  I checked the maximum PSI for my tires (G159 Good Years, 245/75 R22.5) from Good Year's website, and it is listed at 110 PSI. 

Neither is right. The pressures that Rexhall puts on it's placard and literature is the amount of PSI needed to support the FULL GVWR of your coach, based on maximum load rating of the front and rear axles. Goodyear gives you the figures for the maximum cold PSI that the tires are designed to support. Neither of these have any relation to your actual weight or weight distribution. As Ron recommends, get your coach weighed and pressure up accordingly.

 

[rsalhus]

OK, thanks for the tire presssure advice everyone.? I downloaded the Load/Inflation Information for Good Year tires per Ned's link and will keep that info for reference.? Now all I have to do is to weigh the motorhome.? I called three of the big grain elevators nearby to see if I could weigh my motorhome on their scales, and one number was disconnected and the other two didn't answer the phone.? Also, I read somewhere that if you look in the Yellow Pages under 'Scales - Public' you can find a public scale to weigh your rig on.? Not true, at least not in my Yellow Pages.? So, weighing a motorhome is not quite as easy as it sounds.? Kind of reminds me of the winterizing advice to 'put a gallon of RV antifreeze into your water tank'.? Sounds easy, but in my motorhome the only inlet to my fresh water tank is via a garden hose.? (Don't bother telling me how to get antifreeze into the water tank, I've crossed that bridge already.)? ;D? It's just that it's not as easy to do for some people as for others.

Next question.? Does anyone rotate tires on their motorhome?? And if so, in what direction if you have duals in back??

 

[Ned]

I have never seen the need for rotating motorhome tires.  The tires will wear out from age long before the tread is gone.

 

[Howard R]

Now all I have to do is to weigh the motorhome.  I called three of the big grain elevators nearby to see if I could weigh my motorhome on their scales, and one number was disconnected and the other two didn't answer the phone.  Also, I read somewhere that if you look in the Yellow Pages under 'Scales - Public' you can find a public scale to weigh your rig on.  Not true, at least not in my Yellow Pages.  So, weighing a motorhome is not quite as easy as it sounds. 

Another option I have used is a landscape yard, usually sell rock, etc by the ton and have scales.  You may or may not be able to get individual corner weights depending on their layout around the scales, but I have used these before.

Howard

 

[Gary RV_Wizard]

Here's a link to locations for Cat Scales

Drive by those grain elevators - sometimes they leave the scales on and the readout in a window where you can see it.

 

[Gnuman]

Our motorhome manufacturer, Rexhall, recommends 75 PSI for each tire (front and back, back have duals).  I checked the maximum PSI for my tires (G159 Good Years, 245/75 R22.5) from Good Year's website, and it is listed at 110 PSI.  I talked to the guys at the local Good Year dealer for trucks, and they say they would inflate these tires to 100 PSI if they were installing them.  When I told them that my RV manufacturer recommends 75 PSI, they say "Of course, they want you to have a nice, smooth ride!"  My tires were all inflated to 75 PSI when I bought it used last year, but I'm thinking they should probably be closer to 100 PSI when I'm carrying my maximum load rating (22,000 lbs.)  With 25K miles on them, the front tires are wearing around both edges more than the rear ones, but is that because they are steering tires or because they are underinflated?  And who's right, the RV manufacturer or the guys at Good Year?

The guys at Good Year are correct. Tires have improved greatly over the years, and the rating on your motorhome is for the inferior tires it came with. It is as simple as that. Go with what is on the tire.