Are GPAs in the US based on averaging all of the marks from all of the classes in a course?
If so I'd have don't rather badly rather than getting a First - the UK system generally puts the emphasis on performance in the final year (or it least it did in ye olden tymes).
At my (US) college, the first semester of freshman-level classes were taken pass-fail, so they didn't contribute to GPA. But that's quite a ways away, and still generally the exception. We will also sometimes report "In-Major GPA," which tends to represent later years more. I would argue the in-major GPA is more relevant than either total average or last-year average (as much as GPAs are relevant at all). This would be especially true at my college--people would finish all their major requirements early so their last semester was a nice relaxing victory lap of mostly electives.
Are GPAs in the US based on averaging all of the marks from all of the classes in a course?
Generally, yes. There are some ostensibly harder courses you can take that are weighted slightly higher (the highest grade in an honors course may be a 5, while the highest in a normal course is a 4, so it contributes more to your overall GPA).
That said, I haven't ever really heard too many complaints about this system. In my experience the level of effort required from year to year is similar, it's the type of effort that changes. For example, in a first-year math course, you might struggle because you just don't know what you're doing. By a fourth-year math course, you may know what you're doing, you just have to do a lot more, and do it better.
What kind of non-engineering classes might there be in a engineering degree? In the UK courses are (or at least used to be - some have gone all "modular") rather narrow in focus - in the 4 year CS course I did in the 80s only one elective class was included that wasn't specifically related to maths, engineering or CS (and that was a rather enjoyable "History of Science" course).
The specific nature of non-engineering classes varies, but most science/engineering degrees in the US will require about 16 1-semester classes spread across {history, sociology, literature, etc}.
This is reduced a bit if you go to an engineering school. I only had to take 9 non-science classes (+ 4 gym classes).
That's true, and it is a specific complaint I've heard that I did overlook in my statement.
That said, I think it's common to specifically point out your discipline-specific GPA (especially) if it is substantially better than your general GPA, whenever given the opportunity (like on a resume).
GPA calculation varies from school to school, and even from department to department within a school. Classes might be weighted differently across a variety of factors such as difficulty, hours, and course work, thus, a straight average, although common in many scenarios, is not always the hard and fast rule in the US.
I've never seen a US-based system that weights classes later in the curriculum higher than the beginning ones (which is too bad, since like many, mine rose towards the end).
Swarthmore has a system (the Honors program) where the degree granted (graduation, with Honors, with High Honors, or with Highest Honors) is based on the judgement of external examiners on the quality of examinations (written and oral) taken at the end of the senior year.
At my university, all classes weighted equally on a four point scale. My GPAs in my majors, CS and Linguistics, were slightly higher, but the heavy systems component of my department's curriculum kept it low.
If so I'd have don't rather badly rather than getting a First - the UK system generally puts the emphasis on performance in the final year (or it least it did in ye olden tymes).