2010 Ford Fusion hybrid-- terrible gas mileage

The chances of you driving to match the government specified "city" cycle, well it won't happen...

Your city drive may not have the variety that the "city cycle" assumed ie X percent of highway, 3 stops etc etc.

The mileage you are getting is down to the route you drive and the time you drive it.

I used to have a long commute and if I left early ie on time then the mileage was fine, but if I left late by just 10 minutes, the mileage was 20% worse as the traffic was then so heavy... (M25 around London: one of the "best" car parks around...:) )

The MPG figures for cars need to be taken with a large pinch of salt, especially city driving as the definition of city driving used is pretty optimistic. 32mpg instead of 36mpg is only a 10% difference, which is actually not bad. I would say there's no problem that needs to be looked at, just make sure your car is well maintained, your tires are at the right inflation levels, and you don't drive like a nutcase.

This is not a bug; it's a feature.

I have a 2016 Toyota RAV4 hybrid. According to NEDC, it's supposed to get 5.0l/100km highway and city. I can match this in the city if driving very carefully and driving long non-highway distances during a warm day (not so hot that A/C is needed), but the highway mileage cannot be matched at real highway speeds.

I actually get 6.5l/100km. It's 30% more fuel consumed than advertised.

You are using about 17% more fuel than advertised. Far closer than mine.

I guess the difference between your use and my use is that the old European NEDC (used to mean "new" European driving cycle, but now it's old and not new, new being WLTP) was very very unrealistic. In comparison, the US driving cycles are far better than NEDC but still give slightly different readings than WLTP.

The WLTP mileage of my car is actually 6.5l/100km, exactly what I've been getting.

For your case, the car is so old that it probably doesn't have WLTP measurements.

TL;DR

If your car is advertised for 36 city and you are getting 32-33 then the discrepancy is only about 10% which is apparently really good. Even at 31 mpg it is just a 14% loss.

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Fuel economy is derived from dyno testing in a controlled facility, at controlled temps, controlled load, controlled resistance, controlled dust, and immaculate shifting.

This is a well known discrepancy. Your mileage may will vary.

Per https://www.fueleconomy.gov/feg/coldweather.shtml

The effect on hybrids is worse. Their fuel economy can drop about 31% to 34% under these conditions.

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https://www.fueleconomy.gov/feg/why_differ.shtml

https://jalopnik.com/how-fuel-economy-is-measured-and-why-you-get-different-1716232721

I drive very carefully and efficiently, accelerating slowly and using the EV mode wherever I can. But I can only drive so carefully before I start getting honked at by other folks for driving too slowly.

I think that's your problem. You should drive like an average person, because that's what the car is optimized and tested for.

As an example, you would think that driving slowly saves fuel. But if you drive the slowest possible, 0, that is the most inefficient condition, because the engine is idling and you are going nowhere.

The efficiency of an internal combustion engine and drivetrain peaks at some point, for which engineers have optimized. That point is usually cruise conditions. Low powers are inefficient, and so are high powers. (This is demonstrated with the Prius vs BMW M3 efficiency as others have mentioned; they are optimized at different points.)

There are various technical reasons. For example, when an engine is going at too low of an RPM, the gases stay too long in the cylinder before being expanded to do work, and heat is lost through the cylinder walls.

The idea of a hybrid is to widen the efficiency peak by bringing the system closer to the optimal point at all times. For example, if the engine is producing less power than optimal, we can store some for later. If the engine needs to run at a higher level than optimal, we use stored energy to relieve some load.

What you should do in most cases is to get the car to cruise conditions quickly and keep it there. You should accelerate at an average to slightly aggressive rate to cruise speed, then back off and keep it at a constant speed as long as possible.

Where you should be gentle is deceleration. Slow naturally as much as possible, by rolling, or at a rate that does not require mechanical brakes.

The skill of actually getting the mileage on the EPA sticker is called "hypermiling". Spend an evening with Google on that topic, and you will learn the crux of what you need to do. It will need some adaptation to the functioning of the hybrid, but you can handle that.

I mention that because "driving slow in the city and getting honked at" is not a hypermiling strategy.

I am not saying accelerate briskly to speed. It's not that simple. You need to listen to the drive system and figure out where it is operating most efficiently.

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Generally your #1 enemy in the city is friction brakes. Every time you use brakes, you are destroying kinetic energy. That energy didn't come from nowhere; you paid for it with gasoline.

I must disclaim what a moron would do: refuse to use the brakes when prudent, bullying his way through intersections etc. with too much kinetic energy (speed) to safely deal with other drivers or pedestrians, like this jackass. Obviously, we don't mean that.

The precious resource is not kinetic energy, it is forethought. The way the hypermiler wins that the other guy doesn't, is 1/4 mile back from the light the other guy has the cruise control at 45, and he will continue to apply power until he jabs the brakes 300' from the light. The hypermiler will do that too, but only if she was caught by surprise. Normally at the 1/4 mile mark she would have already lifted of the throttle back at the 1/3 mile mark and be using up the car's kinetic energy to approach a light she knows is red.

What's the difference? The other guy was motoring for an extra 1/4 mile only to promptly waste it with a brake jab. How much gas was that? 1/4 / MPG is the rough answer, and that was just one light. Do that every mile and you're talking real savings.

The forethought and attuned situational awareness needed can be quite a change for drivers long accustomed to driving in "tunnel vision" with their attention locked forward and only looking braking distance ahead.

Ford programmed their 2010-2013 fusion hybrids such that after 9-10 years, the Electric Vehicle mode drops off dramatically. Possible a precaution to prevent HV battery failures. Check Forscan forum for tutorial on how to reset the battery age.