Just expanding on a few points.
Firstly, Leg is right to say that faces are what impact the rendering performance (since they are what gets drawn, it follows that more ploygons will slow it down). But the vertices are what cause the errors.
If you have under 1000 verts on any given model (we usually talk about the body model since it's the most prevalent component of the truck), Shiny faces work perfectly "as designed" in the game. It's nice if you can stay under the 1000-vert limit, but most people don't pay it much attention anymore since it's just too restrictive when making a detailed truck.
Above 1000, you get a "Can't environment map model" error if you use Shiny faces. To get around this you can either set your face types to non-shiny (which generally looks bad), or as BD says go into the game's 3D graphics options and turn off "Truck Reflections" (which lets you keep your nice shiny faces, and the graphics effect you lose by turning it off is really no great loss). Nowadays, virtually everyone who downloads trucks has the "Truck Reflections" option disabled, so you can pretty much safely assume that anyone who will want your truck has already turned the effect off, and just set your truck to Shiny faces anyway.
Going further, somewhere around 1500-1600 (I think) vertices we encounter the "Noah Phenomenon", a bizarre bug where the track floods with water anytime you drive your truck near a water feature. The solution is to turn off another 3D graphics option, "Reflections", which is what makes the water nice and mirror-like... or alternatively, just don't drive on a track that features water. Some (maybe most) people have this option disabled as well - I personally don't since there are not that many trucks over this limit that I like to drive (especially after Rep Fan trimmed down many of his trucks
), and I just like the nice water.
I personally think that keeping below the Noah threshold (ie. around 1400 verts) is a good target to aim for, and not just because I stubbornly hang on to my Reflections: because if you go much further, you'll almost certainly start seeing lag issues, especially with two or more trucks on the screen at the same time. Technically, the game can still run fairly smoothly (subject to computer specs) with a truck detailed to 2000 vertices or beyond... reliable so long as it's the only truck in the game. Once you get to around 3K, you start getting really <i>really</i> bad graphics glitches (and trust me, you really don't want to go there - have a look at <a href="">this example</a> to see the sort of thing I'm talking about).