To expand a little on Jon's excellent analysis of the situation, I have spent a considerable amount of time road testing vehicles with a portable 4-gas exhaust analyzer in the back seat, powered by a 115 volt inverter. The results of those test drives were pretty enlightening, particularly while climbing hills, towing trailers, or doing stuff other than light throttle cruise on a straight and level road. All it takes is a slight upgrade or a couple of extra passengers to completely change the power/cruise/economy fuel mixture. Doing anything in the shop except setting the idle mixture is a total waste of time and effort, unless there's a chassis dyno available to simulate road conditions.
The equipment we used on those road tests cost somewhere in the $5K range. You get what you pay for, equipment-wise!
Jerry
