calculations

I want to make a tow bar and need some help with calculations to make sure I choose suitable steel profiles. The tow bar won't be for towing, but only for attaching a bicycle carrier. The current idea is as follows: 1) two pieces of channel pointing front to rear. These will have the...

How exactly is this calculated (in my case, on a W208)? For instance if I do a 40 mile trip non-stop and it says 38mpg is that 38mpg over the whole trip or just the last x number of miles. What about if I stopped for a coffee half way for 15 mins? I'm sure you understand what I am asking!

Now some of us are unfortunate (or perhaps fortunate in some cases) to not have a dash display teslling us avg MPG, overall MPG, etc ... and therefore we have to resort to manual calculations (using the good old trusty Lotus 1-2-3. However - Depending on which way I caculate my data I get...