[Low and Mean]

Hello, I have a 92 diesel box van and our company is on a mission to make this van as efficient as possible. To start testing different ways to make this van better we need a gauge that will show our MPG. This van has almost no electronic, there's no ECU/ECM, it's a 1992 E350 7.3L IDI no Turbo. I was wondering if there is a system available for our model, or something we can retrofit. Thanks for your help, we are very new to the forum, but I can see me spending a lot of time here.
Chris