I do n't go down your way often, but north of Denver the cheapest (and good quality) diesel is at a Conoco and Texaco truck stops at the I-25 highway 119 Longmont exit. I filled there an hour ago at $1.519, that's 20 cents less than their regular gas. Once you get into Cheyenne, diesel is running $1.429 at the Flying J at I-25 exit 7.
Does anyone recommend certain diesel locations for purchasing fuel? I am in the Castle Rock area, and diesel prices vary quite a bit in Denver south. Don't know if there are differences.
I live in Parker and a buddy of mine is from CR. I'd suggest checking Elizabeth prices and maybe Franktown. Anything south of CR I couldn't speak to, but my buddy mentioned Elizabeth as a decent place to fill up.