Bob's Guides

Driving For Mileage

It's been said that the best way to get great mileage in your RV is to lie about it. When any RVer announces that they get much better mileage than you do with a similar rig, take their claim with a grain of salt.

That said, there are a lot of factors that influence your mileage and it helps to know a little bit about them. My rig comes with a miles-per-gallon indicator on the dash. It almost always indicates that I'm getting better mileage than I really am (some old-timers refer to it as the "lie-O-meter)."

On the other hand, it's fairly consistent. After watching it, I know if my mileage will be better or worse than usual. Here are some of the things I've learned about mileage from watching it for many years, and calculating my MPG at every fill of the tank. But first, a word about calculating your MPG.

Calculating MPG

I don't want to insult your intelligence here, so I'll assume you know that "miles per gallon" means you take the number of miles you've driven and divide it by the number of gallons of fuel you've used. If you've gone 100 miles and used 20 gallons, your MPG is 5 (100 / 20).

If you want to keep track of this (some people prefer not to — I have a friend who never even looks at the number on the pump), it's important to fill the tank to the same level each time. I always fill the tank until the pump clicks off the first time. If you continue past that point, it will affect the accuracy of your calculations. You'll also risk damage to your emissions system that happens when you overfill the tank.

I should mention that your fuel gauge is next to useless in this process. I once met an RVer who claimed to get 12 MPG on a rig that I would expect to get no more than 8. When I asked him how he calculated that, he said he always filled up a half a tank, then divided the miles driven by half of the tank's holding capacity.

The odds that your tank is exactly half empty when the gauge says you have half a tank left are near zero. The gauge in my rig moves very slowly for the first half a tank, then drops like a rock.

Tires and Tire Pressure

Getting low-rolling-resistance (LRR) tires can help a little with your mileage, but less so than a car because of the rig's comparatively bad aerodynamics. Since most LRR tires are not suitable for an RV and might compromise emergency handling, they're probably not a good choice.

Your tire pressure does affect your mileage, but not as much as you might think. Some people jack up their tire pressure to get better mileage, but I've tried it on several long trips and it didn't make much difference. I do over-inflate my tires somewhat because an overinflated tire will run cooler and then an underinflated one and heat leads to blowouts. You have to balance this, though, against the harsher ride you get.

The harsher ride is not only unpleasant for you, it's also less pleasant for the electric and mechanical components in your engine, your fridge, your plumbing, your furnace, your water heater, and the body of the rig itself. My old Minnie Winnie had such a harsh ride that I had to tighten up all the screws in the bed over the cab at the end of each day of driving.

Gas Brand

There are people that claim better mileage with certain brands of fuel, but I've never seen any actual evidence of this. I do run my rig exclusively on Top Tier gas, but not because of the mileage, I do it because I believe it's better for the engine.

While higher octane (premium) gas doesn't help your mileage unless your rig requires it, lower octane not only can harm the engine over the long term, but can also lower your mileage and your power. My rig requires 87-octane gas and the worst mileage I ever recorded was when I accidentally filled it with 85-octane gas. The conditions were the same as the previous tank, but my mileage went down, and my rig slowed down dramatically every time I went over an overpass.

Hills

People usually assume that you'll get lower fuel mileage in hilly or mountainous terrain, but my experience is that what you lose going uphill, you regain coming back down. One of the best mileage days I've ever recorded was going over the continental divide. The mountains didn't lower the mileage and I gained some because of the slower speeds (less wind resistance).

Overdrive

If your rig has overdrive (an extra top gear) you can help your mileage a lot by staying in it. This means letting your rig slow down when going over an overpass or small hill if that will keep the rig from downshifting. Some people claim that too much use of overdrive can harm your transmission. This was probably true many years ago before there were sophisticated computers controlling your transmission. Modern transmissions (especially the Allison transmissions used in many RVs) won't let you do anything that might harm the transmission.

Cruise Control

Using cruise control can help your mileage, but *only* if the cruise control will work without dropping your engine out of overdrive for every overpass and small hill. If you have a powerful enough engine or a good tail wind, overdrive will generally work for you, but if your engine keeps dropping out of overdrive, you're better off turning off the cruise and letting the rig slow down a little on the hills to keep it in overdrive.

When watching the "lie-O-meter" on my rig, the MPG will drop by as much as 30-40 percent when the engine downshifts out of overdrive. If I'm in a hurry, I'll use the cruise in those conditions, but I know I won't be getting very good mileage.

Wind

This is a big one, especially in a class A rig with its flat front end, which is kind of like pushing a large garage door down the road. A stiff headwind can cause a drastic drop in MPG. Of course a good tail wind will do the opposite, though the difference is not quite as dramatic.

You might think that a straight side wind would have no effect on your mileage, but it does. It does because it breaks up the laminar flow around your rig. That causes turbulence, which increases the drag. A gusty side wind is worse because it not only causes more turbulence, it makes you weave back and forth on the road, increasing the distance you travel.

You might think there's nothing you can do about the wind, but that's not always the case. There are a lot of days when the wind increases as the day goes on. You can help your mileage by leaving early, though often it's offset by driving at a colder temperature.

If you have a weather app on your phone (e.g., WeatherBug), you can check the wind forecast for each hour of the day and plan your trip accordingly.

Temperature

Colder air is much more dense than warmer air, so even if it's not windy, you'll get significantly better mileage in higher temperatures. If you're a snowbird, you'll notice a steady increase in your mileage as you travel south, and the reverse when you head back north. If your schedule is flexible, you can wait for a warm front to make your trip.

Humidity

Contrary to common sense, moist air is less dense than dry air (ask any pilot). Baseball announcers almost always get this wrong. You will get better mileage on a moist day as long as it's not really raining. The rain droplets will lower your mileage, but in my experience, you'll get better mileage on a day with a mist, or even a light rain, than you will on a dry day, other conditions being equal.

Speed

Other than high wind from ahead or behind, speed is the biggest hazard to your mileage. That's why during the gas shortages of the 1970s, the government set a national speed limit of 55 mph. Everyone complained, but their mileage went up. In my rig, I've often seen the "lie-O-meter" drop from around 7.5 mpg to the low 5s when I increase from 60 to 70 miles-per-hour. Over 70, the drop is even more dramatic. The obvious cause is wind resistance. You have to weigh the decrease in driving time against the extra fuel you'll be using

Some people like to say that it's not worth speeding up to save a few minutes off your trip time, but on a long day, driving slowly could mean as much as an hour of extra driving. I do an off-the-cuff calculation and consider how tired I am, how far I have to go, the status of my pocketbook, and my current mood to decide how fast I'll go. It's also worth considering that driving at high speeds is much more tiring, especially with a gusty side wind.

Driving Style

The most gas you'll use at one time will be for getting your rig from a dead stop up to cruising speed. The effect depends on the weight of your rig.

I can get my rig up to speed without seeing the "lie-O-meter" go below 3 to 4 MPG if I'm careful. I pretend there's a fresh egg on the gas pedal that I'm trying not to break. If I'm not careful, though, I can see the "lie-O-meter" go to 2, 1, or even 0 (which means I'm sucking gas so fast it can't calculate the mpg).

Stomping on the gas to pass someone will also suck gas like crazy. Of course you have to weigh the loss of mileage against any safety or courtesy concerns involved. If you're on two lane highway, the person behind you isn't going to be happy about you creeping away from a stop. You also need to get up to speed on an on-ramp fast enough to merge safely into traffic.

You can help offset the cost of accelerating from a dead stop by traveling lighter — not bringing stuff you won't need, dumping your tanks more often, and not carrying more fresh water and fuel than you need, Though some rigs have a much harsher ride when the tanks are empty.

I tend to ignore this guideline on fuel much of the time, because the more air there is in your fuel tank, the more condensation you'll get in your tank when the temperature goes down overnight. I try to camp with a full fuel tank and always have it full when the rig won't be driven for a while.

Towing

Pulling a tow vehicle behind your rig will lower your mileage, but much of that effect will occur when you accelerate from a dead stop or speed up to pass someone, depending on the weight of your tow vehicle. In stop-and-go traffic the effect can be big, but it's less so at steady cruising speeds. There's not much you can do about it other than avoiding jack-rabbit starts, or getting lighter tow vehicle.

I recently got to make a roughly 2,000 mile trip without my tow vehicle. I was surprised at how little difference it made in my mileage. It made me suspect that having a tow vehicle behind your rig improves the rigs aerodynamics by reducing the turbulence at the back of the rig, like the attachments you sometimes see on the back of semis.

Wrapping Up

Once you realize all the things that can affect your mileage, you have to decide how important good mileage is to you. Once you've done that, you can decide what to do about it after weighing the pluses and minuses of the various methods for improving your mileage.

There are lots of trade-offs. Higher tire pressure will give you a rougher ride and shake up your rig. Driving slower will mean more driving time to your destination. Carrying less fuel will increase your chances of running out. These are personal decisions and what you do will depend on your particular rig, who you are, and what's important to you.

Copyright © 2011-2023
Bob Ray