I'm not saying that manufacturers are blameless in making their figures look good, and VW are certainly dishonest to criminal levels, but as you've cited the GLA250, let's look at those figures. My 2014 GLA250 brochure shows these fuel consumption figures:
Urban: 34.0
Extra-Urban: 50.4
Combined: 42.8
These are EEC Directive laboratory tests to ensure that every engine is tested in the same way (in theory and cheat devices notwithstanding), so MB are just doing what they are told to do in publishing these figures.
Speaking as an engineer, a pilot and a racing driver, as it happens, I know that there is a *vast* difference in an engine's fuel consumption over a range of revs and power demands, as well as over a range of temperatures and weather conditions, which is all to do with how much oxygen is present in the air for a given temperature and humidity. Pilots have to understand these things to a fine degree, as do racing drivers. By this I mean that you need to put your foot down harder to reach a given speed on a hot day, than you do on a cold day (I’m generalising here), which in turn uses more fuel.
Then there’s traffic: are you cruising down an empty motorway at a constant 65mph, or start-stopping and accelerating and braking your way through heavy traffic? The two environments will lead to *vastly* different consumptions.
So given all these complex factors, and the biggest one which is the differences in driving style (use of the accelerator and brake, and the ability to anticipate demands on the engine), no wonder the EEC has come up with a laboratory test for publishing figures.
So let’s look at my real-world use of fuel. I live in the sticks of North Wales so I have to drive 6 miles to my nearest shop, pub or sports facility. I do this journey a lot, always on a cold engine, and it involves a lot of hills and sudden up-hill overtakes (because I don’t like crawling behind tourists). I occasionally do a long motorway journey, but most of my journeys are very short.
After 14 months of ownership, my overall consumption is showing as 34mpg, which is exactly the same as MB’s Urban figure, and which is what I would expect my driving to relate to. Don’t forget, by the way, that the GLA250 is a quick, turbocharged, 4WD, hot-hatch designed for high performance; not a super-green city car designed for sandel-wearing tree-huggers!
So what do I get on ‘combined’ journeys? I can get 40mph without any effort other than NOT flooring the accelerator at every opportunity. 43mpg is easily achievable if one doesn’t break the speed limit, one anticipates braking (so that you don’t accelerate and then immediately brake), and one uses the cruise control where one can. I’d say this figure is spot-on.
Could I get 50mpg on the motorways (Extra-Urban)? Yes, if I sit at 65mph on a flat road in no wind with the cruise control on. Try it.
I’m sorry Protea, but speaking with experience and a bit of engineering knowledge, I won’t be joining you on your well-meaning crusade. I’m happy that MB were not trying to decive me.