low power & speedo issue: 20002 Kenworth T300 cat3126

I recently bought a 2002 kenworth T300 S/A day cab. Allison 5 speed, cat 3126 250HP 181K milesruns like a top as a bobtailhas terrible power with an unloaded 28' pup trailer going up hills. my regular mechanic who services our small fleet of medium duty trucks was surprised as he said the 3126 was as good a runner as he'd seen just as far as the engine goes. So the obvious choice is to take it to CAT and have it dynoed and see what's up. my reservation with this is that the speedo reports the ground speed of the truck as 75mph when it's going 50mph.also, there's a dash light saying the "engine fan" is on (green picture of the engine and a fan) which comes on and stays on as soon as you fire the truck up. So my question is, could this be a kenworth dash/chassis problem (in which I'd want to take it to KW) or is this more likely a cat problem, or is it unlikely that these 3 things are related? fyi, the truck starts great, even in freezing temperatures, and doesn't smoke at all. it's running 22.5 rubber, and I doubt it was converted down from 24.5. It's past life was a beverage truck. so, start with KW or go directly to CAT?thanks