MPG changes on long trips

I have a 2001 Suburban, 5.3 liter, 2 wheel drive, automatic transmission.
I often take 250 to 600 mile trips and track my MPG between gas fill ups/rest stops about every 2-3 hours.
I use cruise control and maintain the same speed as much as possible on each trip.
What happens is my MPG varies from 16.8 to an all time best, 24 MPG between fill ups.
What could be causing this?

Elevation my dear Watson, elevation. Going down hill you get better mileage than up hill. I used to get that on my 200 mile trip to school. Mileage was always better one way than the other. Though seemingly flat, there was an elevation change.

Not to mention summer vs winter gas! Winter gas has less BTU’s per gallon - but it also vaporizes easier at cold temperatures.

Did we talk about ethanol? That changes the BTU content as well!

If the pumps shut off at different times, that will make a difference. Averaging over a few tanks would be better.

Driving west into a headwind and driving east with a tailwind can also make a difference.

Might you be using the air conditioner more on some trips than others?

I used to travel cross country a few times a year driving my Jeep Cherokee. I averaged about 24MPG going west to LA and about 26-27MPG traveling back east to Ohio. My wife kept meticulous records and we just attributed the anomaly to differences in the terrain and the wind. I always travel at 72MPH on the interstate using the cruise control. The little Cherokee seemed to prefer that speed.

Your measurement of fuel economy at every fill-up is part of the problem. You couldn’t possibly measure your fuel usage and mileage precisely enough to give you an answer as precise as 16.8 MPG.

First, you aren’t filling up at the same pump, so when the pump stops could affect the numbers. Second, your odometer isn’t very accurate, but even if it was, it only gives you a value as precise as 1/10 of a mile. That being the case, you really only know you got about 17 MPG, not 16.8. Your answer can’t be more precise than the data you used to calculate it.

I could name at least a dozen additional factors that could make your calculated fuel economy fluctuate, but instead, I recommend you measure your fuel economy over several tankfuls and average the results. That will give you a more realistic perception of your car’s fuel economy.

And you might want to frame that 24 mpg receipt. You must have had everything going your way! My '95 4wd Suburban was lucky to get 15 on the highway.

On my last road trip from Wash. DC to Chicago MPG varied from 31 to 35 mpg (stickshift Corolla/Matrix).
It was lowest on the hills and curves of the Pennsylvania Turnpike, highest zooming across flat Indiana and Ohio.
Same thing coming and going.

I had a relative who claimed to get better mileage on his full size 1965 Mercury using Shell gasoline than he did with any other brand. I’ve never noticed any difference whether I used a brand name gasoline or an independent label. However, I have noticed on long road trips if I am facing a strong headwind, my mileage will drop, although not as drastic as claimed by the OP.

60mph on flat road. Reset mpg. What does it say in 1 miles and than 10miles? 17mpg? That’s ur real, avg mileage that day. I would bet a ton, in 1 yr if you drove on same road u would get same mileage.

I have found very significant differences in mileage in my SUV that were much more minor in compacts I have owned. The biggest differences evolved around speed. Small drops in mileage in my compacts were big differences in the SUV as the speeds over 60 mph were increased. I believe trucks and truck based SuVs have such poor aerodynamics, especially uneneath, and gearing, you can have a drop of 5 miles per gallon with increase in speed in an SUV where a drop of just one or two occurred in the compact over the same speed interval.
Now, if you had a Fusion or a Camry that exhibited such a swing, that would be more puzzling.
I also agree with Texases, 24 mpg is better then you can get being towed with a Suburban.

Measuring true mpg only works if you tank at the same station, same pump, and stop at the first click. The shutoff adjustment on each pump differs.

Also, as pointed out, if you drive into the mountains, as I do often, you get very different mileage as driving OUT, since you are mostly going downhill.

Small changes in speed can make big differences in SUVs. Last one I drove would return 20-21 mixed if I limited it just under 60, and 16-18 at 65.

Tire pressure can change as well. If you’re not checking the pressure every morning, that could change mileage, especially on a heavy vehicle.

24mpg explained:

  1. Overfilled tank by continuing to fill after the nozzle clicked off
  2. You drove far enough to use only part of one tankful, maybe with tailwind
  3. Underfilled by stopping when nozzle clicked off
  4. Did the math for that short drive and unequal fill ups.

Better procedure:

  1. Fill up and stop when nozzle clicks off
  2. Reset trip odometer
  3. Record gallons used for three, four, or more fill ups.
  4. Fill up and stop when nozzle clicks off.
  5. Do the math - miles divided by total gallons. That’s your actual mileage.