MPG and Tire Pressure

KIX · 11-08-2007, 04:53 PM

A friend has a 34' (I think) with a gas engine and 22.5" tires. He was getting something like 7mpg. He weighed the coach and reduced tire pressure per the tire makers inflation chart. His MPG dropped to something like 5MPG. From what he says the tire pressure is the only discernable difference.
I have Michelin XZA3's and per the inflation table I run 95 in fronts and 87 in rears and get about 7 -7.5 MPG. Total weight is just under 30,000. We normally run about 60mph. See sig for coach info. Those that have similar rigs and weight what tire pressures are you running and what MPG are you getting?
What I'm trying to determine is would 5 -10 psi more per tire make any real difference in MPG.
Thanks.

KIX · 11-08-2007, 04:53 PM

A friend has a 34' (I think) with a gas engine and 22.5" tires. He was getting something like 7mpg. He weighed the coach and reduced tire pressure per the tire makers inflation chart. His MPG dropped to something like 5MPG. From what he says the tire pressure is the only discernable difference.
I have Michelin XZA3's and per the inflation table I run 95 in fronts and 87 in rears and get about 7 -7.5 MPG. Total weight is just under 30,000. We normally run about 60mph. See sig for coach info. Those that have similar rigs and weight what tire pressures are you running and what MPG are you getting?
What I'm trying to determine is would 5 -10 psi more per tire make any real difference in MPG.
Thanks.

Garbageman · 11-08-2007, 05:06 PM

The simple answer is yes. The more air pressure the less rolling resistence against the road surface. Do not "over" inflate a tire for the sole purpose of increasing mpg, always follow the manufatures inflation recommendations.

In drag racing we over inflate our front tires to gain the least amount of rolling resistence to go faster, but not enough pressure to cause worry about blowing the tire out.

Paul

KIX · 11-08-2007, 05:22 PM

So, Garbageman, what model UA do you have and what do you weigh and what tire pressure do you run and most importantly, what MPG do you get at what speed? Thanks

Garbageman · 11-08-2007, 05:35 PM

KIX, we have 36C and we haven't had it weighed yet. I run 85lb in the fronts and 80lb in the rears and this seems comfortable. Funny you should ask about mpg because we are leaving on a weekend trip tomarrow and I (finally) will calculate the mpg. We have only owned our UA since Jan 07 and most of our trips are dry camping at race tracks where we us our generator. Using the generator throws of my mpg calculation so I haven't done it yet.

How long have you owned your UA and how do you like it? We love ours but could use more engine power.

Paul

KIX · 11-08-2007, 05:45 PM

We bought it in '05. And we do like it too. I don't know what genset you have but ours is rated at .5 gph at half load and .75 gph at 3/4 load. Ours is an Onan 7.5 Quiet diesel. If yours is diferent you may try going to the
makers site and get some fuel consumption ideas.
The pressures you are running seem a little low but probably are very comfortable. When you get it weighed you can go onto the tire mfg site and check the load inflation tables for the correct press. to run.

Garbageman · 11-08-2007, 06:07 PM

Thanks KIX, I will do that. I am basing the pressures on what we weighed our old coach at. I also have the Onan 7.5 genset but when I'm on the track I don't know exactly how the DW leaves the genset on or what she's doing at the time.

Paul

AFChap · 11-08-2007, 06:31 PM

KIX

Difficult to really calculate fuel mileage with the generator mixed in there. We can really tell the difference when we check our fuel mileage on a fillup just after a week-long rally where we were boondocking and recharging batteries daily w/the generator.

We are running right at 31k. I have Michelin XZA2's on the rear, and Continental HSLs on the front. The charts say 105 for front & 95 for rear, but I "plus 5" the pressures for a safety margin and run them at 110 front and 100 rear. We normally run at 60-65 mph nowadays. Before retirement 2 years ago I generally ran around 70, and always at least 65 mph. We tow a Honda Odyssey (apx 4,100 lb) and often run the generator while driving. Our avg before fulltiming and driving at the faster speeds was 6.55 mpg. Avg for the past two years driving at apx 5 mph slower is 7.26 mpg. Note those figures include 660 hrs of generator use over the four years, about 360 of it the first two and 300 the second two years.

-------
Paul

Unless you are running with very little "stuff" on board, I suspect your pressures may be on the low side ...but I'm not that familiar with weights of the shorter coach either...

Trek-Cyclist · 11-10-2007, 01:00 PM

I concur with what was said about increasing MPG using the proper inflation for the tire maker.. after getting the coach weighed. However, the safety margin of an increase of 5 pounds concerns me a little..
If you inflate your tires lets say in a cold climate with plus 5 pounds by the time you get to a warm/hot climate you could be over inflated by as mush as 10 pounds maybe even more... that wouldn't be good not only for handling but you would also tempting tire failure.
Question: Why the 5 plus lbs for safety? Wouldn't it make more sense to invest in a accurate tire guage and inflate to the proper inflation?
I know I wouldn't over-inflate my tires for safety. So far my average MPG is 7.7 at 62-65 mph for this past 5 months.

AFChap · 11-10-2007, 04:52 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">However, the safety margin of an increase of 5 pounds concerns me a little..
If you inflate your tires lets say in a cold climate with plus 5 pounds by the time you get to a warm/hot climate you could be over inflated by as mush as 10 pounds maybe even more... that wouldn't be good not only for handling but you would also tempting tire failure. </div></BLOCKQUOTE>
When I "plus 5" psi, the tires are still well within the maximum cold pressure. I would not inflate them beyond the maximum rated psi. I have more load carrying capacity on my tires than on my axles, so have a margin of safety there. I presume you are not talking about gain in PSI when the tires warm up while driving as that does not apply. Cold psi is what the tables specify.

I check my tires daily when driving. If I did experience the rare/very unlikely situation of being in a cold climate one day and a hot climate the next, I adjust the cold tire pressures accordingly. I do adjust tire psi in the Fall, adding air as required to stay at the "right" psi, and letting air out in the late Spring.

I have read the "plus 5" recommendation in several RV references (nope, can't recall where or find such a reference now). My understanding of "why?" is that RVs tend to "gain weight" rather than loosing it as we take stuff on board more than we take stuff off. That "plus 5 psi" (again, the tires are still well within their maximum psi rating) allows for some "weight gain" over the last weigh-in. Bottom line, since the weight on board changes as times goes by, it is much better to err 5 psi "over" than to be 5 psi or more under-inflated.

We are full-timers, and have had our coach weighed (4 wheel weights) twice in the past four years. Last time it was weighed, we were still below our maximum axle weights, but were a bit heavier than we thought we would be. We plan to get a 4 wheel weight again at a rally in January, just to be sure we aren't gaining too much weight.

Tom N · 11-11-2007, 03:35 AM

I have one comment concerning "overinflating by 5#s for safety" and I've quoted Michelin's RV Tire Guide below. I do realize many think "more is better". If adding 5#s for safety was prudent it would be included in the Michelin Tire Pressure/Load charts.

Quoting Michelin: "Overinflation, on the other hand, will reduce the tire's contact area with the road, which reduces traction, braking ability, and handling. A tire that's overinflated for the weight it's carrying is more prone to a harsh ride, uneven tire wear, and impact damage."

-Tom

CC38EL · 11-11-2007, 08:27 AM

We are running 29,600lbs. Our tires, replaced last December, are Continenal HSL's Eco Plus. I run air pressure according to the maximum axle weight rating and the load/inflation tables for my tire.

We have a 12000# front axle and a 20000# rear axle. I put 115psi in the fronts (equivalent to the pressure needed to handle 12740#) and 95psi in the rears (equivalent to the pressure needed to handle 22040#). We get 8mpg pretty much in any configuration. Plus or minus .5mpg for extra genny use, out west highway speeds around 70, or just loping along with a tail wind. East of the Mississippi, we run 65mph or less.

Rims for our coach are rated 7300# each at 120psi maximum. Which is also the maximum air pressure for the tire.

All that being said, I'm always concerned with underinflation, rather than overinflation. I've found that higher pressures (but within the range for the tire) yield better performance. The Michelins I replaced had 42000 miles on them and except for the cracking on the sides, you couldn't tell they were used. Excellent tread depth and wear. Uneven wear was not indicated. We ran the Michelins in a similar fashion to the way we run the Continentals.

I appreciate Michelins warning about overinflation, but I believe in safety margins for tires just as I do when driving a car or flying a plane. If I'm staying in the performance band of the tire as specified by the manufacturer, then I will overinflate to achieve better fuel economy and tire longevity. I do not exceed maximum recommendations of the manufacturer.

This is just my data point. Your experience will vary.

richierich · 11-11-2007, 03:10 PM

Have a 39QD, 02 horizon,approx 55,000mi. Just put 6 new Michelin tires on. Ran from NY, to FL, on I-95. I averaged 9mpg, kept speeds at 63 mph,with cruise on most of the time, conditions permitting. Had dealer put 100psi on all tires. Had the cross over air system installed on rears. Can leave on the trip, check the pressure before driveing,have 100 psi all around. Stop for fuel, maybe 300 miles of driving, and check the psi, it increases to 105psi. Tires before I replaced them,had 52000mi on them,showed no wear. They dried out, before they wore out. Kept them at 100psi too.
Anyway, in my driving history, I have great luck keeping psi's at 100,and, watching the speeds,road hazards we find,and washing the tires with soap and water, no tire conditioners.
Have had the coach weighed,and manage to keep the gvw under winnies recomendations.

Garbageman · 11-12-2007, 11:48 AM

Just got back from a weekend in Portland with the MH. Before leaving I checked my air pressures (tires) and misspoke in an earlier post. Fronts are 90lb and rears are 85lb.

The trip was 175 miles round trip through two lane curvy coast range highway, slight city traffic with stop lights in Salem and the rest interstate I5. My UA 36C w/CAT 3126, pulling our two door convertible Suzuki (2300lb) averaged 9.46 mpg.

Shes going in for an M3 service soon. The air filter needs changed for sure plus all the other M3 items I will also have the valves adjusted. I'll be curious on my fuel mileage after the service.

Paul

Trek-Cyclist · 11-12-2007, 12:13 PM

Was that pencil and paper fuel mileage or computer readout?

Garbageman · 11-12-2007, 12:35 PM

Ed, that was pencil and paper. My MH doesn't have the onboard computer.

I filled the tank at my shop, wrote down the odometer reading, filled it again upon my return and noted the odometer reading.

Paul

Trek-Cyclist · 11-12-2007, 01:36 PM

Well knowing that I would have to say, great mileage!

Nothing is better then pencil to paper in a computer world.