ScanGauge II 3-in-1 Compact Multifunction Vehicle Computer for $139.95 shipped.  Here is a screenshot for your reference.  It requires no tool for installation and serves as a programmable 3-in-1 automotive trip computer, diagnostic scan tool, and shows fuel consumption, cost-per-mile, coolant temperature in real time, along with Diagnostic Trouble Codes (DTCs), and more.  Most places have it for about $159.