Why is my MPG so inconsistent?

I have a 2012 Ford Fusion with a V6 and record my mileage & fuel info every time I fill up. For the first 45,000 miles I was getting 26mpg consistently every time I filled up, around 45,00 miles it started hopping around. Some fills I get as low as 20mpg others I get up to 33mpg. Over several fill-ups it is averaging close to 27mpg for the past year, but the volatility concerns me. I haven’t seen something like this on past vehicles and not sure what it could be. Any ideas?

What method you using to determine gas mileage?

Consistently getting 26 MPG every fill up for 45000 miles. Doubtful.

Did your route or routine change? I got 20 ±1 mpg on my car over 100k, because I used it for commuting, same trips every week. On the few road trips it got 28 mpg or so.

How far down do you run the tank down before you refill?
It could be inconsistencies in how full the tank is filled, which will be more significant if you don’t run it down much.
This would also lead to a low mileage tankfull to be followed by a high mileage one and vice versa.
Inconsistent filling could point to some issue with the EVAP system.
If the issue doesn’t lead to release of raw gas fumes it wouldn’t trigger the check engine light.

Another possibility is a sticky thermostat that doesn’t keep the engine at a consistent temp, or a flaky sensor.
I’m assuming you’re accounting for changes in driving conditions.

"Over several fill-ups it is averaging close to 27mpg for the past year, "

+1 to @circuitsmith

This tells me that your mileage has not changed much, but the way the tank is getting filled has changed, sometimes the pump clicks off sooner-sometimes much later. One gives you great mileage, the other less than great, but the average is in line with your historical mileage.

I think I would tend to rely on the mileage the computer is reporting for average and instantaneous rather than using a calculator at each fill up. I’ve found them pretty accurate.

Since your overall mileage is about the same it may just be different stopping points for fillup at the pump.

Just be happy with the mileage you are getting.

Concur, this doesn’t seem like a problem worth loosing any sleep over. MPG changes unrelated to driving styles are most often caused by temperature. Either the air temperature is different, or the engine coolant temperature is different than before.

Everybody’s mileage including my own is all over the map. Anyone who claims "consistent " exact MPG for an extended period is either on a cross country highway trip at steady speed, is lying, or just guessing.

Consumer Reports, Motor Week and car magazines test cars all the time, and they get a wide range of gas mileage during a typical test. They normally report the average of careful highway runs, the average of city driving to give the readers/viewers and idea of what to expect. They then compare that to EPA sticker mileage and seldom get the same result.

I think most people that check their gas mileage have a particular number in mind as to what mpg their getting . Of course that number isn’t going to be exact for various reasons .

@Sloepoke Right. Some years ago a person was trying to sell a friend of mine a Dodge Ram pickup and told him it got 35mpg!! My friend had never owned a pickup and asked my opinion. I told him it was possible on a very long downhill run at low speed. But expect more like 20 or so under good conditions.

This person might have measured his truck’s gas mileage one time (probably inaccurately) and got that figure ONCE. In other words, most people don’t really know how to measure gas mileage accurately and are ever hopeful of getting a high number. When selling a vehicle they often just don’t tell the truth and make things up.

When I consider all the factors that can affect fuel economy, what comes to mind are:

  1. Fuel formulation - I think the winter mix gets worse fuel economy than the summer mix, when all other factors are equal.
  2. Tires - not only proper inflation, but changing tire models with different tread patterns - Some are designed for better fuel economy and some are designed for better traction.
  3. Different routes - Unless you drive the exact same route each time you measure fuel economy, you can’t expect the same results.
  4. Weather - Aside from winter/summer fuel formulation, low temperatures lead to worse fuel economy. Also, your air conditioner compressor has to work harder in hot temperatures than moderate temperatures.

Then there are factors of measurement to consider:

  1. Are you always fueling up at the same pump? If not, your measurements of fuel dispensed might appear to be more precise than they are.
  2. Do you always dispense your fuel at the same time of day? The temperature of the fuel in the tank can make a difference.
  3. How precise are you measuring the distance traveled? If it’s only to the nearest 10th of a mile, that isn’t very precise.

If my calculated fuel economy went from 27 MPG to 20 MPG, and then to 33 MPG, I would suspect the reason is an error in measurement or calculations rather than a problem with the car.

“If it’s only to the nearest 10th of a mile, that isn’t very precise.”

1/10th mile out of 200 is a variation of 0.05%
I think that’s a lot more precise than most of the other tolerances, including absolute accuracy of the odometer.
IMHO even measuring to the nearest whole mile is close enough in most cases.

@circuitsmith, Maybe this is just a matter of perception, because the last time I walked more than a mile (on anything other than a treadmill), 1/10 of a mile seemed like a long enough distance that it didn’t seem very precise to me.

It’s pretty amazing how much farther away things seem when you have to walk, and how having a vehicle at your disposal really seems to shrink the world.

1/10th mile error would be nothing for an mpg calculation.

I keep a meticulous log and so does my wife. Every time we fill up we record the miles and gallons added. Also all maintenance items and service.

With respect to mpg - it’s fairly consistent because my commute is fairly consistent. I’ll see fluctuations when I deviate from my routine or days where we had adverse weather. Also big drop starting in Oct and lasting til Apr due to winter gas.

s.nelson64: As so far noted there are many factors affecting average MPG. If you are filling until the pump clicks off, that can vary between pumps. I continued using the odometer/fuel used method for a few fill-ups on my new 2010 then realized the average MPG readout was quite accurate. Has your state recently mandated 10% ethanol (E-10) gasoline? That is a 3% reduction in MPG. Ambient air temperature on initial startup is another factor. I briefly idle my car until 1,500 RPM drops to 1,000. At 40F and above it takes 30 seconds or less and my average MPG readout drops up to .3 MPG. At 40F down to 20F with increased fuel/air enrichment and taking up to a minute it drops up to .8 MPG. With todays remote start feature with drivers warming up the vehicle while they finish getting ready for work the average MPG reduction could be very significant. The only other factor I am going to attempt to address is the federal mandated Summer/Winter gasoline formulation. Winter formulation results in about a 1.5% reduction in average MPG. Of course the blend of Winter verses Summer gasoline depends on how much is left in the storage tanks when the different fuel arrives.

@Whitey I agree 1/10 mile can be a significant distance to walk.
It’s 528 feet; almost two football fields.
However, it takes a car 6 seconds to travel that far at 60mph.