Why do European cars have much better fuel economy? I dug into this a bit with the VW Golf. The best European Golf gets 69.2 mpg, the best American one gets 42 mpg.
Europe's best Golf has this 1.6l turbo diesel "bluemotion" engine that isn't available in the US. Fuel economy is 69.2 mpg, curb weight 1314 kg.
The best Golf in the US has a 2.0l turbo diesel, rated at 140 hp. EPA rated fuel economy is 41 mpg on the highway (the automatic gets the 42 mpg rating), curb weight 1358 kg.
What I think is the same 2.0l engine and setup (6 speed manual) gets 57.4 mpg in Europe's extraurban test and weighs 1351kg. There are several 2.0l engines, and I picked 1 of the 2 that has "140 PS". PS is German for horsepower, but is slightly different, to fit the metric system. 1 pferdestarke = 0.9863201 horsepower.
The different engines account for much-- a 20% hit to FE when moving from the 1.6l to the 2.0l. But there is still a huge difference between 57.4 and 41. The European extraurban test averages 39 mph. The US highway test averages 48.3 mph. Is that enough to account for the rest? I'm not sure. Taking a wild guess, I'm supposing that FE ought to be about 10% better at the lower speed, which still isn't enough to bridge the gap. What else might account for it? The Europeans add 8% to the numbers to account for the improvement in FE after break in, and I don't think the US does anything equivalent. That might cover it all, but there are more possibilities.
Could they use taller gearing in Europe? I couldn't find info on that. Perhaps the European cars are lighter? In the Golf's case, looks like there's no significant difference in weight. Does European diesel have more oomph? I doubt it. The auto manufacturers are more experienced and better at rigging cars for the European test? I read that there could be ways to cheat, but unless someone is caught, it's hard to guess whether it happens and how effective it would be.
The Golf TDI wasn't available in 2008 when I got mine. I couldn't have afforded it anyway, but I'd like driving one. Mine is rated for a pitiful 22/28, though if you check my gaslog I'm squeezing a lot more out of it.
It came with a 2.5L I5 gas engine which makes decent power, especially low-end torque. It also came with ridiculously short gears. It's got plenty of torque for highway speeds going up and down hills at a lot lower RPM than it actually uses. At 70mph it's running 30000rpm. I bet it could do the job at 1500, but 2000 ought to be a decent compromise.
I think it can reach its maximum speed in 4th, so it's not like there's any reason 5th can't be a whole lot taller than the rest of the gears, still giving people the close ratios they want for all that drag racing that's done in VW Golfs...yeah, right.
Anyway, to answer your question, I think it's all in the test procedure. A few years ago the US test was redesigned to represent more wasteful driving, to better represent real US drivers. It was probably already more aggressive than the European test.
I would venture to guess that the power difference is probably really noticeable. in europe, they are much more tolerant to lower HP than us americans.
also, they can build their engines differently since they don't have to meet anywhere near the emissions regulations that they do in america. I went to india about 7 years ago and most of the cars there were diesels (especially anything with any size). the taxis ran off of propane and most of the motorcycles there would classify as a beginner dirt bike over here. they can build it differently because people are OK with it.
most of this is my opinion but it is something to think about.
also, a side note: are the european numbers in US gallons or imperical gallons?
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