There are some pre-OBDII mileage monitors but they may all be DIY electronics projects. There's a link in my sig (do I sound like a broken record?) that is very easy and not too expensive (<$40) that can at least easily show you fuel rate. It's not combined with speed so it won't show MPG, but right at the top of it I recently added links to two threads about MPG monitors based on similar technology.
when the Dashdyno is calibrated it predicts my weekly tank fill, which is usually very close unless I fill up at a pump that rips me off with bad gas!
Thanks for mentioning that.
I think the OBD2 approach (what's used by DashDyno and Scanguage) probably works well for most people most of the time. But I think there are certain circumstances where errors might creep in. I also think certain types of errors might tend to balance each other, which means you won't notice them when you're doing your calibration on a strictly per-tank basis (which is the main means of calibration that is typically used).
I don't want to overstate the problem, and I think there is no perfect solution that is superior. I just think it's good to be aware of this possible limitation.