Using GPS to Accurately Calculate RV Fuel Economy

So there I was, driving my RV down the interstate on a beautiful summer day. The sun was shining, the traffic was light, and I didn’t have a care in the world. That is, until I noticed my fuel gauge rapidly dropping and realized I had no idea how many miles per gallon this beast of a vehicle was getting!

As an RV owner, accurately tracking your fuel economy is incredibly important. Not knowing your mpg means you don’t know your vehicle’s range, which makes trip planning and budgeting nearly impossible. But calculating mpg in an RV isn’t as simple as resetting a trip odometer like in a car. The size and weight make fuel economy highly variable, and the large fuel tank capacity means you need a lot of data to get an accurate average. This is where using GPS can come in handy!

The Basics of Calculating MPG

Figuring out miles per gallon is pretty straightforward – you take the number of miles driven and divide it by the gallons of fuel used.

Miles Per Gallon = Miles Driven / Gallons Used

Of course, the challenge is accurately measuring the miles driven and fuel used, especially over longer distances. Small errors can throw off your calculation.

For example, let’s say you drive 300 miles between fill-ups and it takes 30 gallons to refill your tank. 300 miles / 30 gallons = 10 mpg. But if your odometer was off by 3 miles or your fuel gauge was off by 1 gallon, your calculation could be as high as 11 mpg or as low as 9 mpg – a huge difference!

This is why having precise measurements is so important when calculating fuel economy. Tiny inaccuracies get magnified over the course of a long trip.

Using the RV’s Odometer Readings

The easiest way to estimate mpg is to simply use your odometer readings. When you fill up, note the total miles on the odometer. After driving for a while, fill up again and note the new odometer reading. Subtract the first reading from the second to get total miles driven. Then divide by the gallons used to get mpg.

This method works pretty well for shorter trips where you fill up frequently, like a few times a week. But odometers can be off slightly, throwing things off on longer trips. My RV’s odometer seems accurate to about 0.3% (a bit under 1 mile per 300 driven). Over a 1000 mile trip, that could throw mpg off by up to 3%.

Small tire wear and inflation changes can also impact odometer accuracy over time. And if you ever need to replace a speedometer sensor, the new one may read slightly differently.

So while the odometer is a simple way to estimate fuel economy, it may not be precise enough if you really want to dial in your RV’s mpg.

Using the RV’s Fuel Gauge

Another option is to use the onboard fuel gauge instead of filling up completely. When you begin a trip, note the fuel gauge reading. Drive for a while, then fill up to the same gauge reading. Divide miles driven by gallons added to calculate mpg.

This eliminates potential odometer inaccuracies. But fuel gauges themselves can be inaccurate, especially for the large fuel tanks found in most RVs. The sensors aren’t precise and the displays usually round to the nearest 1/8 or 1/4 tank.

My RV’s gauge seems to consistently underreport by around 1 gallon per 16 gallon fill-up. Over a long trip, that could throw off mpg by 5% or more.

The gauge also may not be perfectly linear, with more or less error at different fill levels. And it’s dependent on having a level RV, since tilting the tank can alter the reading.

Like the odometer, the fuel gauge gives a ballpark mpg estimate but may not be ideal for truly dialing in your fuel economy.

Using GPS Tracking

For the most accurate results, GPS tracking is the way to go. A GPS unit precisely logs your actual miles driven down to within about 30 feet. No errors from tire wear or odometer inaccuracies.

And instead of using potentially inaccurate fuel gauge readings, you can simply log your fill ups in the GPS. Enter the exact gallons added at each stop. Now you have incredibly precise mileage and fuel data for calculating mpg.

Most RV-specific GPS units like those from Rand McNally have built in features to make this really easy. You can view and record mpg averages over various timespans to see how it changes under different conditions.

Besides accuracy, these devices allow you to track mpg over much longer timespans. I can check my average over the last month or year – something impossible to calculate from just odometer readings.

Some GPS units also estimate mpg on the fly based on changes in your RV’s geographic position. While less accurate than the fill up method, this allows you to see mpg trends emerge in real-time on a given trip. Pretty neat!

GPS Tracking Tips for Best Accuracy

To get the most precise mpg results from GPS tracking, keep these tips in mind:

  • Carefully measure fuel at fill up – Use the pump’s tally device if available. Don’t rely on the nozzle auto-stop which may click off early. Every ounce matters over a big trip!
  • Fill up consistently – Stopping just shy of a full tank can throw things off. Always top off for best accuracy.
  • Check tire pressure often – Underinflated tires increase rolling resistance which hurts mpg. Keep them properly inflated.
  • Note the fill-up location – The GPS won’t know you refueled unless you tell it where and how much. Entering the data is key.
  • Use a quality GPS device – Inexpensive units and apps may have accuracy limitations. Invest in a good RV-specific GPS.
  • Compare mpg to expected values – Does your calculated mpg seem realistic for your RV? Cross-check against published estimates.
  • Make sure the GPS has the right specs – Engine size, wheel/tire size, and other details improve accuracy. Program it correctly.

Following these tips, you can trust the mpg calculations from your RV GPS. I tell people to rely more on the GPS reading than your instincts – sometimes your RV is more efficient than it feels behind the wheel!

Keeping a Fuel Economy Log

Once you’re collecting mpg data with a GPS, it helps tremendously to keep a log. Note the mpg each time you fuel up along with details like:

  • Location
  • Elevation and terrain
  • Weather conditions
  • Driving speed
  • Tire pressure
  • Generator/accessory use
  • Road grades

Reviewing this log allows you to pinpoint the conditions that affect your RV’s mpg, both positively and negatively. You can then optimize future trips to maximize fuel economy.

I’ve found my RV’s mileage drops by 20%+ in mountainous areas. Cold weather and high winds hurt mpg. But keeping it under 60 mph and using cruise control boosts it. Adjusting driving habits, routes, and timing to limit these mpg drains really helps stretch my fuel budget.

Without the detailed records from GPS tracking and logging, I never would have learned how to improve my RV’s mpg this much.

Recent Examples From My RV GPS

To really demonstrate the value of GPS mpg tracking, here are some recent examples from my own RV travels.

I have a 30′ Class A motorhome built on a Ford F53 chassis with a 362 HP V10 engine. It has a 100 gallon fuel tank and gets 6-10 mpg depending on conditions. I log every fill-up in my Rand McNally RV GPS to track mpg.

Driving across the plains of Eastern Colorado last fall, my GPS calculated 9.1 mpg over 240 miles between fill-ups. This was at 60-65 mph with no wind and gently rolling terrain. Shows what my RV’s capable of in ideal conditions!

Later on that same trip while climbing 6% grades on I-70 through the Rockies, mpg plunged to 6.2 over a 175 mile segment. Elevation and terrain make a huge difference!

On a springtime trip through West Texas, mpg averaged just 7.4 thanks to 30-40 mph headwinds blasting me all day. You can really feel the impact of wind resistance.

And driving through hilly Western Arkansas in the summer heat, mpg dropped to 6.5 over a 200 mile leg. Lots of steep uphill grades worked the engine hard.

As you can see from these figures, RV fuel economy varies wildly but GPS tracking captures it all. This lets me fine-tune future trip planning and driving to maximize mpg as much as realistically possible.

Apps and Accessories For RV MPG Tracking

Modern apps and accessories make GPS mpg tracking easier than ever:

  • Dedicated RV GPS units – Brands like Garmin and Rand McNally have models designed specifically for RVs with mpg tracking functions built-in. Easy to use right out of the box.
  • OBD2 plug-in adapters – These gadgets plug into your RV’s OBD2 port to capture mpg, engine data, and more. Pair with a phone app to monitor and log the numbers.
  • GPS navigation apps – Many popular navigation apps like Waze and Google Maps have added mpg tracking features. User friendly and ideal for partially tech-savvy users.
  • Spreadsheets – For the data nerd, building a detailed mpg tracking spreadsheet from your GPS logs lets you slice and dice fuel economy stats galore.
  • GPS logger devices – Small standalone GPS loggers can provide automated mileage reports without having a navigation screen.

No matter your budget or technical abilities, you can find great options to leverage GPS to precisely calculate your RV’s mpg.

Crunching the Numbers

Here’s a hypothetical example to show how GPS mpg tracking can illuminate your RV’s fuel economy.

Let’s say you drive 450 miles between fill-ups and it takes 50 gallons to refill your tank. Your RV GPS shows you actual drove 449.7 miles due to slight route variations. Using the odometer, you’d calculate 450 / 50 = 9.0 mpg. Close, but not perfect.

With the GPS, you know precisely 449.7 miles were driven. And you top off so have an exact 50 gallons added at fill-up. Therefore, your true mpg is 449.7 / 50 = 8.994 mpg – essentially 9.0 mpg but with more accuracy.

That tiny difference of 0.006 mpg sounds meaningless. But over a 10,000 mile year of RVing, it would add up to 60 miles – enough to make extra stops for fuel if you didn’t account for it in planning. Small deviations get amplified over the long haul.

GPS tracking provides that level of dialed-in precision to truly optimize your RV fuel economy and spending. The difference can really add up.

Improving MPG Over Time

The real power of GPS mpg tracking comes from observing fuel economy trends over weeks, months, and years of RV trips.

Plotting your mpg figures over a map visualizes how factors like elevation, terrain, and weather affect efficiency. Comparing legs with tailwinds vs headwinds shows the dramatic impact of wind resistance. You quickly learn how to optimize route selection and timing.

Seeing mpg fluctuate seasonally helps you plan ahead. I know to budget 15% more fuel costs for a summer trip through the mountains vs a springtime drive across the plains.

And tracking mpg over the years illustrates how variables like engine wear, added weight, and deteriorated aerodynamics gradually sap fuel economy. This data helps decide when upgrading to a new RV could pay for itself through fuel savings – often sooner than expected!

Without comprehensive GPS tracking, you miss out on all these insights for enhancing performance and savings over your RV ownership. The precision and continuity of data is invaluable.

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