Google around for OBDII cables, software, and hardware. The most popular product on this forum is the ScanGauge II, which doesn't hook up to a computer but does give you 4 pieces of live data (you choose from dozens but can only display 4 at a time) as a gauge.
The cheapest would be an ELM327-based product, which you can get on eBay for ~$30. It works with various open-source, freeware, shareware, and old-fashioned payware programs, and can be used with computers, PDAs, even some phones now (if you get one with bluetooth, that's probably more than $30).
There's also code scanners, engine tuners, and a million other products that can give you that information via your OBDII port.
Many won't calculate your MPG, though; and IMO they don't really help you fine-tune your hypermiling once you've gotten a little practice, because they all suffer from one major shortcoming of the OBDII standard for this purpose -- they do not measure fuel flow rate. This can conceivably be a problem when you do weird things that aren't expected by the software/hardware, such as P&G, DFCO, and EOC. OTOH, folks here really like their ScanGauges.