The ScanGauge E that I have, is a plug in unit that monitors instantaneous mpg and average mpg, among other things. It records data on a daily, previous day, current journey, this tank basis, and can display two out of many criteria such as rpm, coolant temp, intake air temp, and so on.
This is an example of the data I record on longer trips:
Average mpg: 50.9 mpg
Max coolant temp: 84°C
Distance travelled: 61 miles
Maximum revs: 4377 rpm
Max speed: 57 mph
Average speed: 28 mphIt plugs into the OBD II port, above the clutch pedal, and I have mine mounted on its Velcro, on the steering wheel binnacle.
It switches on automatically and is completely passive. Every time I fill up, I input the fuel added and the cost per gallon. After a few top ups the accuracy gets pretty good.
It also reports any OBD codes and allows you to reset same.
Currently, Amazon are selling them for £92.20.
https://www.amazon.co.uk/ScanGaugeE-Ultra-Compact-Automotive-Computer/dp/B004SK5I46/ref=sr_1_3?ie=UTF8&qid=1518794320&sr=8-3&keywords=scangauge+e