Getting Mileage in a Motorhome

[Beaver]

On the skid plate and exhaust pipe issues, the study I mentioned in an earlier post would indicate the more that air flow is limited underneath the vehicle the less of a factor the roughness of the underside will be. In other words, if your rig has a low front spoiler, side skirts and wheel pants, the air flow (and thus turbulence) under the rig should be reduced. I also keep a record of mileage and I have noticed sustained improvements in mileage each time I added ground effects. However I did not do dedicated before-and-after tests, so I can't definitively say anything about them. The side skirts and wheel pants may have their greatest effect during cross winds, which are a component of most of the winds you'll face. Still, smoothing out the undercarriage can't be a bad thing, particularly if the skid plate is made of plastic. I don't know about your rigs, but mine is front-heavy by a couple hundred pounds. I hesitate to hang too much weight near or in front of the front axle.

[chefdave]

Hi beaver
sump guard could be fitted to a surf or 4 runner. can be found on ebay.

[chefdave]

Hi
beaver altered timing to 12-13 degrees btdc. pulls even better and brakes have more bite????? Fuel consumpsion on last tank was 14mpg. ummmmmm but a coolent and starting probs engine running stationary for cooling and timing. had to keep engine running up until normal temp once aday to avoid starting issues. timing has (fingers crossed) solved starting issue. also air temp is now down to 0c two weeks ago was 15+c.

[Beaver]

Yikes! That sounds cold! Where the heck do you live?

[chefdave]

Hi
live in john o groats far north of scotland. on latitue i'm further north than olso in norway. approx 2000 miles south of north pole. northly winds come direct from there. wind speeds can reach 100+mph. normal is 70-80 mph when a storm hits. so chill factor makes it colder than a freezer. salt roads from nov to march. ummmmmm plus side 22 hours of sunshine in summer 20c for most of summer.

[joe24658]

New as a member of the site. I have read many of the posts over the years, but just signed up after reading the comments in this post.

I've been an Owner operator truck Driver the last 15 years. As you can imagine Fuel mileage really affected my profitability as a trucker. Also I am always looking at cost/benefits ratios in making upgrades.

One of the big components of fuel mileage is rolling resistance of the tires. In the trucking industry each individual tire is tested and given a number with respect to rolling resistance. Typically the ribbed tires are better than the drive/traction tires. Recently many have been replacing the dual setups with extra wide singles as they typically have better rolling resistance. Regardless of the type of tire, you must know the rolling resistance, as not all tires follow the typical rules. Michelin tires has a good chart for truck tires rolling resistance.

The components which determine rolling resistance are weight, tread design, rubber compounds and sidewall flexing.

In this post one person stated that he wanted to go to a heavy duty van tire so he could run higher tire pressures. While I understand his reasoning, I have to wonder how much more the van tires weigh and what their rolling resistance is.

When I bought tires for my car, I tried to find out as much information as possible. Consumer reports had a very limited chart regarding rolling resistance and this helped me make my decision. I have to wonder if there is information available for RV tires.

I also wonder how much swapping from rear duals to a wide single would help mileage. There would be less weight and two less sidewalls to flex.

Anyway. I won't run on forever on tires. I realize this is an old thread already.

Think Vortex generators (Google Airtabs). I had them on my truck and they do work. They are a lot cheaper and lighter than a boat tail and will help MPG and stability. Also you can do a lot more with them than just place them on the rear of the unit. I am amazed I haven't really heard about them more on this site. I also strategically placed some of them ahead of my doors, so that they would improve the airflow as the truck become wider behind the door. The Toyota motor home would probably benefit from this placement as well.

Last but not least there are alternatives to the big A/C unit on the roof. Call me crazy, but installing a home window unit gives you a lot more options on power consumption and therefore generator consumption and choice. At the same time, you'll get the unit out of the air.

It all depends on how you plan to use the RV and how often you run the A/C. You could find a way to mount the unit in the rear wall. (Yes think outside the box). In my truck, when I needed A/C to sleep, I would put the air conditioner through my passenger window (on the door) and close off the gap. It saved me a lot of fuel, because I was running it off a Honda EU2000 generator. I had planned to one day mount it in the rear wall.

The roof top A/C unit is typically 13K BTU. window units are as low as 5000 BTU, 6500 BTU, 8000 BTU.. you name it and up and up and up.

I can give more details on any of the above if anyone wants.

I was obsessive with MPG on my truck and am considering getting an RV and trying the same. The Lesharo would probably be the best starting point, but from what I've read it wouldn't make much sense unless you replaced the drive train.

Years ago I had a Toyota Dolphin and may get a similar unit. I think 20 MPG would be an attainable starting goal.

[Hersbird]

Welcome trucker Joe.
My Hi-Lo had a big roof A/C that contributed to all of it's water damage when the roof buckled a bit and cable tear outs from the rot and heavy top. I agree the normal window A/C mounted in a cabinet in the wall is better. Many of the new inexpensive models of travel trailer have these. For my size a 5k btu is plenty, they cost much less, don't add to drag, and can be powered by a single small quite generator (like you said) if need be. The money you save compared to buying a roof one about pays for 1/2 the generator.

[joe24658]

Thanks, I appreciate the welcome.

It's amazing all the devices I ran off that Honda generator.. but perhaps that's best left to another forum.

I did some more reading and indeed there is mention of Airtabs.. Oh well seems like some readers don't believe they improve mileage. I have no proof they do.. but it seems like my mileage went up and up and up as a combination of many changes including the Airtabs. I do believe they improved the stability of my rig.

[Beaver]

Welcome Joe, and thanks for the good info! I will check on the airtabs. I've considered something like the 4-panel "boattail" that I see on many long haul trucks on the freeway, but it would be kind of big and weird looking on a Toyota MH. I'm pretty sure it would be helpful, since truckers use them (they are also evaluated in the study mentioned earlier in this thread). On the air conditioner, I took mine off, and we try to always be somewhere where we don't need one (or a heater either). These little MH's have 14" tires on a very unique wheel that would make it difficult to switch to super singles, though I'd love to. I'd also like to find more info on low rolling resistance tires in this odd light truck tire size. There is very little selection in this size and they can be hard to find. Your comment about having only two sidewalls to flex is a good point.

[chefdave]

If only toyota had fitted 15" rims we in europe would be spoilt by purposed designed rv tyres. LLR and high mileage tyres with low noise and good steering feed back. On roof a/c unit have removed mine handling improved on corners and cross winds but mpg stayed the same. Fitted smaller mirrors on an L bar and ditched door scaffolding for orginal mirrors. Freebeard aero expert who posted answers on my aero questions he suggested fitting a half round to camper body by cab doors. 4" guttering fits perfectly also fit this around all four sides on the front of cab overhang. thinking of doing same on rear body to smooth airflow into huge drag wake. This is similar to rear of many modern motorhomes made in europe. Hope some of this helps also google chef dave and look for my postings on mh aerodyamics.