Well, first of all, I don't think your average customer really cares a whole helluva lot whether we pull up in a pickup or eco car. I think most of them expect a contractor to pull up in some sort of truck. But one of my biggest competitors in town has these little Prius cars for all their salesmen/estimators. They grew by 18% last year and broke the $5Mil. in annual sales mark for the first time. So I'd have to say that just because they pulled up in a car rather than a truck didn't seem to effect their ability to land jobs at all.
I drive a full-size truck because I need a full size truck. With 28 employees, I'm obviously not the one out there installing landscapes or mowing lawns very often. But I am the one checking on jobs in progress. And often times I need a certain tool or a 12" sprinkler head, or a SAM head, some planting fertilizer bags, or any number of tools or parts that the crew may have run out of or forgot to bring. So having a lot of tools and parts in the back of my truck makes my company run a lot more efficiently. I also have a fair amount of gear like measuring wheels, material samples, etc. in the back. All this stuff wouldn't fit in anything else.
As long as you look professional and have a reason for driving what you pull up in, I don't think the customer really cares or thinks about it too much.
Now, if you pull up riding a Moped or Scooter, I think you may notice your % of sales decreasing. That would be because you would just look too odd. Similarly, if you pulled up in a Corvette, Mercedes, Hummer, etc. I think you'd also lose jobs. For one, they'd think you are definitely making too much. And for two, you'd come off like a pompous braggart. Same goes for some of these gigantic F-350, lifted, loud exhaust, dually, quad cab, mini monster trucks that I've seen a couple of landscapers in my area driving. I don't think that gets you too much respect. It makes you look like you are wanting attention a little too much.
Just keep it normal and professional and something close to what the client expects to see you in. Other than that, doesn't really matter.