My area of expertise...

Anything about delivering pizza, I can tell you. Well, anything relating to Papa John's, anyway.
I don't know about $40K/year or even $30K/year, but I think you could certainly clear $20K. You could maybe do $30K.
This is how it works at Papa John's. I got minimum wage ($5.15 in SC), paid every 2 weeks. Working my normal 35-45 hours/week during the summer, that check was usually about $300-$350 or so, if I remember correctly. We also got a "commission" to cover our gas costs. It was 5% of our sales for the day. This can easily add up, especially if you're working the day shift and there's a few company or school lunches. I remember one day I did more than $500 in total sales. 5% of that... bam, that's $25 right there and you've not gotten a single tip. Our scale had a few variables in it. If you didn't put a car top on your car, you only got 3%. If it was dark and you didn't have your top lit up, you only got 4%. If you closed and had more than 20 orders, you got 6%. The people who closed on busy Friday and Saturday nights could make some serious bank with that extra percent.
As far as most people tipping, "most" might be a little bit of a stretch, but I'd say greater than 80% of people give you at least a dollar and the remaining change on the order (like it's $18.60, a lot of people will give you a $20 and call it even). There's of course the nicer people that would give you $21 or even $22 in that case. It's not out of the question for someone to give you $25 in the above scenario, either. Not often, but it does happen. A lot of the bigger orders will give you $10 or so, too.
That was probably very poorly thought-out and hard to read, but what it adds up to is this: every two weeks, I'd take about $750 to the bank. That's roughly $20,000 if I would have worked ~40 hours/week, 52 weeks/year. Probably not going to happen. I don't see how you could make more than $25K/year. At least, working at the exact location I worked at. It might be possible downtown where most of the deliveries are probably to dorms and you can make 75 deliveries in one night. Even with cheapskate college kids, 75 deliveries would add up. I knew a driver at that store and he said he'd make $100 in a night, easy. Add in the minimum wage and you might be able to do $30 or $40K in a year.
If that was unreadable because I'm a scatterbrain, let me know and I'll clarify
