I just made an attempt to measure the frame rails on my 04 Dakota and these figures are approximations as it's hard to do laying on the floor and with no help. Around 42 inches at the front of the frame and somewhere around 31 inches at the firewall. Hope this helps. FWIW, my Dakota is the 4.7 with cruise, air, front disc, automatic,r&p steering, and gets close to 20 on the highway and 15 in town if I keep my foot out of it.