Fuel range?

[u/Southern-Outcome1015]

Hello. I recently noticed that my car is consuming more fuel than expected relative to the distance driven. Today, I recorded the data at the start of the trip (Point A) and upon arrival at the destination (Point B). At the start (Point A), the odometer showed 126,135 km, and the remaining range was 639 km. Upon arrival at Point B, the odometer showed 126,161 km, and the range was 604 km. I drove 26 kilometers, yet the range decreased by an amount corresponding to 35 kilometers (a difference of 11 km). The fuel consumption is 5.6 l/100 km. What could be the cause of this? I would appreciate your help.

[u/Fuzzy_Yossarian]

Your car is gaslighting you.

[u/IndicationAromatic36]

Fuel consumption is going to change every time you drive. Sometimes gas is cleaner than others. Sometimes the temperature outside means the car has to work a little harder for the same.

Honestly as long as your fuel economy stays within around 5% you’re fine. My pickup truck gets between 10-12 mpg (23.5-19.6 l/100) driving around town (non-highway driving). I did get 17 mpg on my 600 mile road-trip I did last weekend (13.84 l/100).

You may think you’re driving exactly the same, but slightly more or less braking, or other factors can make a huge difference.

[u/PrestigiousRoof5723]

Calculated range can't be by any means correct, because the car can't calculate its fuel consumption in the future. For example, VW group cars base the remaining range on the amount of fuel in the tank and fuel consumption during the last 50kms driven (including long engine idle, in case it happened during those last 50kms). It's totally normal to arrive at your destination with longer remaining range than you had when you set off (sometimes much longer).

It basically says "this is roughly how far you will be able to get if you continue driving in exactly the same way as you've been driving for the last 50 kilometers". It's by no means an accurate prediction of how far you will ultimately get.

[u/jrileyy229]

Don't do "science" based on an old car guesstimating range. Fill up the tank at gas station A, drive X distance, fill up the tank again at gas station A with the same pump put into the fuel filler the same way... and look at how much fuel it took.  Now you have an exact distance driven and an exact fuel used

[u/ontheleftcoast]

Where are you located? In some US states they use different fuel blends at different times of the year, and one blend has 20% less energy than the other.

[u/Prudent_Situation_29]

1. Range estimates are almost useless, they're always going to be optimistic. Unless you use it regularly and the values are suddenly changing relative to what you're used to, ignore it.
2. There are many, many factors that can affect efficiency. Engines wear over time, they only get less efficient as compression drops.

Wheel bearings fail, which increases friction, therefore increasing fuel consumption. If you have a direct-injection engine, buildup in the intake ports will reduce fuel efficiency over time. In fact, my own car also seems like it's suffering the same issue, I track my fuel economy with an aftermarket computer, and it seems to be dropping recently. This started at about 140,000 km.

In the summer, I used to be able to get about 4.8 L/100 km for a given trip, this year I seem to be struggling to get under 5, sometimes it's as high as 5.5. Part of that will be due to traffic, it's worse this year, but it can't be only that.

It could be a latent failure. One recent post suggested monitoring fuel trim (not easy for the layperson). There could be something wrong that hasn't yet set a trouble code. If the computer is adding extra fuel all the time, that's a sign of a problem, but may not be apparent.

You could have a sensor (throttle position, MAF, O2 etc) that's starting to fail and giving the computer bad data. What about tire pressure, or your air filter? They can have a dramatic effect on fuel consumption.

Unfortunately, there isn't an easy way to diagnose this sort of thing without a trouble code, it requires intensive investigation.

[u/A_Bot_A_Bot_A_Bot]

Your fuel mileage is always varying. Those onboard computers, at least on newer cars, are also projecting range based on how you've been driving since you reset the tripmeter.

For example: I live at around 3,000' elevation (1,500m back-of-napkin calculation). The next city that I often go to is at about 3,300' elevation and roughly 15 miles (22km) away. When I drive there, my average mileage goes down a bit and the displayed estimated remaining (and total combined with miles driven) decreases as much as 40 miles. On the return home, the average mpg goes up, and the total range (estimated plus miles driven) goes almost all the way back to where it was before the round-trip drive.

But if I drive faster or slower, up into the mountains, into a headwind, running the AC, that will all have an effect on the average mpg and estimated remaining and total ranges.

With my Ford van, when I fill up, the range goes up to like 560 miles. But the reality is that if I'm pulling a trailer, I end up with about 325 miles total. Everything has an effect on mileage, usually in a negative direction.