Well, one of my gift from Christmas is a ScanGaugeII. It's a small little unit, but packs a great deal of info from the vehicle's computer (PCM OBD-II). For the Ram Van, that is 1998 and newer.
Realized that the van gets so-so MPG when the engine temp is below 170. Once above 184, the MPG jumps up really nice.
The unit plugs into the OBD-II plug just above the brake pedal. I have the cable zip tied securely and the unit sits above the radio trim piece.
This unit will also display the Pcodes and allow you to clear them.
There are other features too. It displays the TPS voltage, MAP info, RPMs, engine temp, IAT feedback,
Somehow, it knows the MPG and trip information all to well. It asks for engine size, fuel tank size and knows the injector flow to provide real time data for your MPG. It's very much right on for averaging it all.
JC whitney had these on sale. Look around for a deal.
PS, it can easily be transfer to another vehicle to check codes and clear them.