Big tires, odometer error, and mpg

Recently a friend emailed this miles-per-gallon question:

I’m scratching my head here on fuel and mileage numbers. If I do the math for gallons of fuel consumed (fuel filter life % gauge) and odo I get about 16 mpg.

But the Ram’s EVIC dash display says 11 MPG.

With 4.30:1 gears and 37-inch tires, I know ALL the numbers are inaccurate.

How do I figure this out?

I’d be very surprised if you are able to routinely obtain 16 mpg with your Ram/Four Wheel Camper setup, unless you are driving 55 mph. I never trust dash displays. They are almost always overly optimistic, though in your case because of the tall, 37-inch tires, it may actually be a little pessimistic.  My built, 2006 4Runner mpg display is slightly low most of the time because the car travels further on taller tires than the ECM calculates.

The fuel filter life gauge is not a good source for mpg data, I think it’s really only useful for when to change the fuel filters. I’ve done similar calculations using the oil life percentage numbers (it probably uses the same ECM algorithm) after doing oil changes. The numbers literally do not add-up to the actual odometer distance traveled (neither indicated miles or the known and measured inaccuracy). To state the obvious, the only way to get accurate fuel economy readings is to have good numbers for the math; how many miles vs. how many gallons at fill-up.

I don’t think your gearing change matters. Most late-model vehicles calculate the speed from wheel sensors… as long as that’s the case on the new Ram (easy to test) then it is only the tire diameter that matters. I’ve tested many sets of 33, 34, and 35-inch tires on my 2014, but no 37s yet. My guess is that your odometer is about 8-10% slow, compared to the actual miles you are traveling.

My method of testing tire-induced odometer error is to compare actual miles traveled to odometer readings. California doesn’t use highway mile-markers most places anymore, but Nevada does. With two columns on paper, I reset my trip odometer at a mile-marker, and then log the indicated and mile-marker distances. Data gathered over more miles will be more helpful. For example, a 10-20 mile test can be better than nothing, but the initial error percentage will drop if you make a longer run, like 50-100 miles.

If you’re on a long trip you can use indicated GPS miles instead of mile-markers. Recently I compared GPS to mile-marker data over a 70-mile route, and was surprised that they were just slightly different.

Good luck.