The custom trucks turn a lot better than the stock trucks so I was wondering if runs could use them

I think that would be unfair if they have modified stats aswell :/

They don't. Everything stays the same as the other trucks, they just seem to turn better. That's all.

So it affects steering?

I guess so. I'll probably just stick with the stock trucks though.

It doesn't matter, back in the day teams would have custom trucks with their logo's on them. I use to make custom trucks. Any handling benefit you're feeling vs the stock trucks would be placebo. The trucks turn from a center point. There's a TRK file for every truck within it's POD, and the only parameters are cosmetic.

If it doesn't affect the truck in any other way than just cosmetic, then it's fine to use custom trucks of course.