RV Tips

Scratching your itch: RV fuel consumption

Scratching your itch is a software-developer euphemism about writing a program to solve a problem that you face personally. My itch was to understand and manage my RV fuel consumption. Here's some background:

I upgraded from a 1986 Class C (carburetor) motor home, that burned a LOT of gas, to a 2008 fuel-injected Class A that ALSO burns a lot of gas. I had hoped that newer technology would have reduced the fuel consumption and cost, but that was wishful thinking. There are two schools of thought about RV fuel consumption: 1) it's high, so get over it, or 2) it's high, so you should try to minimize it. I fall into the latter camp. However, I also knew that understanding what actually affected fuel consumption was necessary before trying to manage it, and this would require getting more accurate information than fill-up and odometer-based fuel consumption calculations. All modern vehicles have computers to measure fuel flow, and many use GPS to measure speed and position, so I sought to combine these two sources of information.

The result is a program called RVFuelMon. At its core, RVFuelMon combines the fuel flow information from the onboard diagnostic computer (OBD) with the vehicle's position and speed derived from the GPS (both recorded at one-second intervals) to give a very detailed view of fuel consumption. It shows the fuel consumption as affected by speed, grade, and other road factors in various maps and tables.

Using RVFuelMon, I determined the sweet spot for fuel consumption on various terrain classes (higher consumption at higher and lower speed). I also found that freeway travel is more fuel efficient than country roads, and the toad has negligible impact on fuel consumption.

Update June 2019: I wrote the previous sentence when I had a Ford Fiesta, and could not see a big difference between trips with or without the toad. However, fuel consumption increased by about 10% when I upgraded from the Fiesta (weight 2300 lb) to a Jeep Cherokee (weight over 4000 lb). I haven't yet made a trip without the Jeep, but it certainly increased fuel consumption compared to the Fiesta.

RVFuelMon also shows summaries of the overall road geometry, such as the amount of flat, rolling, or steep grades and how much straight or winding road is on a selected route. This information can be used to compare different routes. For example, is it more economical to take a short route that crosses a mountain range, or a longer, flat route? It's a complicated calculation involving distance, road class, and uphill and downhill grades, but using RVFuelMon, I determined that it's usually better to take the mountainous route, even though fuel consumption per km is higher. The longer route usually consumes more total fuel.

Being Canadian, I wrote RVFuelMon using metric measurements (km, l/100km), but converted some outputs to miles per imperial gal and US gal.

Here are some example outputs from trips taken in my 34-ft Class A motorhome.

This table of fuel consumption by speed and slope shows the sweet spot on level ground is 85 km/h, but is 95 km/h on a slight uphill grade. The slope classes (grade) are expressed in percent.

Of course, fuel consumption is lower travelling downhill than travelling uphill for any given grade class, as seen in the previous table. However, most of my RV trips involved travelling over each road section twice, once uphill and once downhill, unless I was following a circle route. Also, crossing a mountain range involves an uphill leg and a downhill leg. Does the lower fuel consumption while descending offset the higher consumption when ascending the hill? That calculation requires some additional parameters.

I defined seven Terrain Classes to calculate the combined fuel economy for up-and-down travel. Each Terrain Class is a combination of uphill and downhill sections of more-or-less consistent grade classes. For example, Rolling terrain is generally about 4% grade, but can include small amounts of higher or lower grades. I-5 from Vancouver to Bellingham has a lot of Rolling terrain. My RV will ascend a Cruise Climb section without slowing very much, but it noticeably slows down and kicks into a lower gear on a Sustained Climb. RVFuelMon automatically classifies every kilometer of every road section into one of these Terrain Classes using the GPS data.

RVFuelMon shows that the amount of uphill travel in each terrain class is roughly equal to the amount of downhill travel on each trip. This makes obvious sense for an out-and-back trip, but it also holds true when crossing a mountain range or travelling a circle route. In general terms, if a mountain range requires 20 km of Cruise Climb ascent, it will usually have about 20 km of Cruise Climb terrain on the downhill side. Therefore, the following table shows the fuel consumption for the various terrain classes when travelling equal distances uphill and downhill.

That's the detailed stuff -- what about overall fuel consumption? RVFuelMon "journeys" calculate the fuel consumption for each trip, as seen in the next table which shows the average fuel consumption for 12 major RV trips since 2015 These trips covered about 22,000 km, and the average fuel consumption was usually 32-34 l/100km. The detailed reports shown above allow me to find out what caused the increase or decrease in fuel consumption for the outliers. Without the detail shown above, there would be no way to determine what caused the outliers.

RVFuelMon can also show the fuel consumption, grade, terrain, speed, and other parameters on maps. Here are the 12 trips from the previous table.

It's been an interesting exercise to develop RVFuelMon and to really understand fuel consumption. For example, I found that different road surfaces on the Interstate highway near Phoenix AZ affected my fuel consumption by about 10%. Drop me a line at RVFuelMon@gmail.com for additional information.