Non-dealers can make a bona fide "profit" on some cars but by and large the entire "investment" rationale is something we have invented to convince ourselves and our wives that we aren't making silly purchases. I paid $160k for my GT 4 years ago. I can make 10% all day long on relatively safe investments. Even assuming the GT is still worth $160k (it isn't), I have "lost" about $64,000 in interest (non-compounded interest, no less) and about $7,500 in insurance, registration fees, etc. Although I have not done the calculation, I bet the GT would need to be valued at about $500k 10 years after purchase (it won't) to equal what I could have made in a typical investment, assuming annually compounded interest.
I looked long and hard at an original 289 street Cobra when the prices dipped into the low-mid $300k range several years ago, but I could not justify the purchase. I believed the opportunity cost was simply too high. Even with the increase in the Cobra's value since then, I probably would have only matched my safer investment that yielded 10%/year. Of course I never would have sold the Cobra, and that makes it the WORST investment possible.
Never view cars as investments. Buy and keep them because you love them.
I'm not saying 10% a year cant be done easily, but safely like clock work??? I trade for a living, I own junk bonds that pay that, but they arent safe. Over time they should average a little above treasuries even though the current yield is 10%. I do get your point though. I was doing the math on some of these 70's muscle cars that sold for 10x their original msrp from 40 years ago, the avg rate of return in only 5.5%, and this doesnt include maintenance, restoration, or insurance. The return has to be horrid. Regardless, 5% can be easily outmatched, and safely. So yes I agree, its better to "invest" in something else.