Dear Tom and Ray:

I just purchased a 2014 Mazda3, and I am thrilled with getting 32 miles per gallon, city and highway combined. Then I spoke to my brother-in-law in England this weekend, and withstood 15 minutes of browbeating. He says a new car in the U.K. simply would not sell at 32 mpg. So I did a little research, and lo and behold, the U.K. version of this car, which looks identical and has the same engine and about the same weight, is being advertised at 50.4 mpg combined. With the diesel, that jumps up to 72 mpg. Why does the European version of this car get much better mileage, and why can't we buy these European car configurations in the U.S.?

-- Robert

RAY: You're comparing crumpets and oranges, Robert.

TOM: They're not the same exact car, not the same mileage tests and not even the same size gallons!

RAY: European cars often do get somewhat better mileage than similar U.S. cars. But that's because the engines tend to be smaller.

TOM: Europe places a high value on low carbon emissions, and puts strict regulations on them. And the best way to lower carbon emissions is with a smaller engine.

RAY: And that's the case with the Mazda3. If you got the smallest available engine in your U.S. Mazda3, you'd get 155 horsepower.

TOM: Get the largest available gasoline engine in a Mazda3 in the U.K., and you'd get about 120 hp.

RAY: Most car manufacturers consider U.S. drivers power-obsessed, so they don't sell their smallest engines here. Whereas Europeans are content to accelerate a little slower in exchange for using less fuel.

TOM: Another factor is the gallons they're measuring. The U.K. uses the imperial gallon, which is about 1.2 U.S. gallons. So for each U.K. gallon, you get about 20 percent more fuel!

RAY: So if you were to fill your Mazda here in the states with imperial gallons and calculate your "miles per imperial gallon," your combined mileage would instantly be 38, not 32.

TOM: And the other major factor is the mileage test itself. The European test tends to use slower acceleration, lower top speeds, and fewer starts and stops than the U.S. test. It also doesn't take into account things like air conditioning use and cold-temperature use.

RAY: So if you took the same exact car, with same exact engine and same amount of fuel, and ran them through the two different tests, the European mileage number would be, on average, about 20 percent higher, just because the test is different.

TOM: So if you convert for imperial gallons, and then convert for the difference in testing methodology, your car would be rated at about 45.6 mpg. That's pretty close to the 50.4 advertised in the U.K.

RAY: Of course, you can't count on getting ANY of these numbers in real life. While the U.S. test is more realistic than the European test, neither of the tests really mimics your own driving.

TOM: So they're always best used as relative measures, to compare one car with another.

RAY: The best measure is the one you do at the pump. You've done that, and as you say, you're "thrilled" with your 32 mpg combined. So enjoy your car and tell your brother-in-law to sod off.