Hi Dave,

Thanks for your help. Your guess is right, they come from the mid-early seventies. Here you are a more detailed description:
The cars are about 6-8 cm long, so the scale -not specified on them- may be somewhere between 1:50 and 1:32... maybe just 1:43. Anyway, since the wheels and bodies seem to be typically "distorted" to make the hot rod look, it's difficult for me to be more precise about it. Besides, the cement truck is only one centimeter longer than the others, so the scale must be slightly different for the different models to meet a common size.
The only identificative mark is the old "Tonka" logo carved on the cement drum surface (not visible in the photo). I contacted some toy specialists with no success; the seventies seem to be a mystery period.
The bodies are glossy coloured hard plastic made, no paint except for the chromed engines and little more. And the chassis looks like tin, with no screws, but some occasional round drills and stiffening folds.
The tires are rubber made, and they are joined to some sort of flexible axis; this feature kept them perfectly aligned for years, even when accidentally stepping on them.
Many years ago I saw another model from the same collection which I never had, but I remember it was a light blue Volkswagen Beetle inspired hot-rod.
Hope this helps.

Thanks again.
Jose