1:18 Diecast vehicles & plastic figures: not the same size?

[CB2001]

Now, I know a lot of scales in vehicles and figures exist. But not too long ago, I remember reading at a forum (I can't remember if it was a G.I. Joe forum or another customizing forum), that some people had issues with using 1:18 diecast cars and trucks with G.I. Joe-size figures, which are of the same scale of 1:18, explaining that despite that the scale is suppose to be the same size, the vehicles a bit smaller than the action figures. But while here, I've seen at least one person using a 1:18 scale diecast vehicle with a G.I. Joe scale custom figure.

So, my question is this: are the vehicles slightly smaller than the 1:18 scale action figure size or are they able to be able to include 1:18 figures inside of them if you wanted to use them? I'm asking about cars and trucks only, as I know that when it comes to motorcycles and ATV diecasts of 1:18 scale, they do seem to fit the same size. Let me know what you guys think and any opinions and experience you have with this topic.