[tropo]

For the units, do you mean US Customary or do you really mean Imperial? The gallons are very different.

There is even a difference in miles if you want to be picky, with the US using statute miles in some states and international miles in other states. The international mile is 1.609344 km, and the statute mile is about 1.6093472 km. This isn't a huge difference.

[sonofgod]

Given that's a difference in the sixth significant figure, it's probably not even a rounding error for these calculations.