Quick rule of thumb. Things that are smaller-scale than the trains can be placed in the background and assist in forced perspective. Things that are larger-scale can't be placed in some kind of "pre-foreground" unless the trains are set pretty far back in the scene.
ANYTHING that goes on a layout should be toned down and lightly weathered. Anything that looks grossly improbable (like super-wide racing slicks on a street car) should be modified for improved appearance. Even showroom cars will be dusty after a test drive, and cars being unloaded from an over-the-road transporter can be downright dirty.
The one thing that jumps out and bites me are photos of quarries or construction sites where all of the heavy equipment is pristine! Any high-capacity machinery that is freshly painted and squeaky clean has just been assembled and has yet to turn a tire (or a track.) Historic equipment on display will have a coating of dust and sun-damaged paint.
On the other hand, there's no law that says inexpensive vehicles can't be kitbashed.
Chuck (who buys models to kitbash)
As I recall, the old Matchbox cars used to be marked on the bottom with the scale ratio. I have quite a few of them, plus other brands of cars close to the right size for HO scale. I try to find the models of common everyday cars, not the exotic ones. The thing to remember is that all these type toy cars are not made to a certain scale the way we would think of it. They are made to fit a certain sized box or bubble pack.
Use the close sized ones for the foreground and the off sized one in the background, especially if you can ge them cheap. When I see an HO (highway) car model for over $10, I say "you've GOT to be kidding."
Earlier today on a thread I posted about vehicles, ModelTrainman asked what was wrong with using Hot Wheels and Matchbox cars on your layout? I replied that the answer was enough for a separate thread. I also recommended an excellent article in the August issue of Model Railroad News titled "S, HO or No? The Diecast Dilemma."
What I didn't realize is that the "bcawthon" that participates on this forum is the same Bill Cawthon that wrote the article for MRN! Yes I am embarrassed to admit that, but at least I'm honest about it and admitted it.
I thought Bill brought up many excellent points and I certainly can't rehash his points as that would border on plagiarism if I remember correctly...Writing For News class was a long time ago. Anyway, I would make an effort to read Bill's article.
One of my pet peeves about HO layout photos has always been this: an immaculately detailed and weathered scale model of a locomotive passing by a Hot Wheels car sitting at the grade crossing. Or worse, a Tootsietoy. At least Hot Wheels and Matchbox cars aren't see through.
Most of them, as Bill points out, are not even close to HO scale. (Other than the trucks.) Now I know that we fudge a little with the HO scale cars, some might scale out to 1/86 or 1/90 and maybe we'll say that's "close enough." But 1/72 and 1/64th scales are just too big.
Bill did a good job of explaining the sizes of the toy cars compared to scale cars to real cars. One thing I would add is the tires. The tires on my S-10 are 9" wide. The tires on a Hot Wheels vehicle scale out at 1 and a half feet!
Also besides the scale factor, a lot of these toy vehicles don't receive even a little attention from the modeler that would make them look better. Taking off the toy-looking tampos with nail polish remover and giving it a more realistic paint job would be a huge step. Dulling the shine on the tires would help. The dead giveaway is the hubcaps or the wheels. The super shiney designs can be painted to look a little more realistic so they don't all have that "same" look. Painting in tail light lenses and windshield chrome helps, as do side reflectors.
A lot of the toy trucks, while not being super-detailed are, as Bill pointed out, close enough to HO in some cases that they at least would not look bad in the distance or parked in a warehouse.
And besides trying to fix these cars up with paint, try to weather them a little. Not every car on the highway looks like it just came off the new car lot or out of the car wash.
If 97 cents is what you can afford for a vehicle and you are comfortable with it being on your layout, then load them up. When I was a kid, all my vehicles were Matchbox or Hot Wheels until the godsend of Tomica Pocket Cars arrived. Sure they were more expensive, but look at that detail! And some were closer to HO scale.
But now I'm an old "serious" modeler and I wouldn't have any or many of these toy diecast vehicles on my HO layout. They are just too big and unrealistic-looking for a great scene that is otherwise to scale. Some folks who are so used to seeing Matchbox on a layout come in and try to tell me that these Busch cars cannot be HO scale because they are too small. It's the other way around, of course.
But thankfully this is the golden age for HO vehicle modelers. What a supply we have these days! Yep, they sure do cost more than a Hot Wheels but you get what you play for, uh, pay for. (sorry, my midwestern REO roots are showing)
So to summarize I think the toy cars can be fixed some but unless you are in S scale they really are not going to look right, especially when you start adding figures to the scenes. And I hope Kalmbach doesn't mind me plugging Bill's article on their forum, but if you are into vehicles it's a well-researched article.