OP:
First of all, I don't think the "ECO" rating should be taken too seriously. It's probably some formula with an arbitrary unit Garmin invented to help you compare one trip/average to another.
Second, all Jeremy Clarkson did was state the obvious in an elaborate and misleading way. Any automotive engineer will know... hell, any behavioral psychologist will know, average people drive their cars in relatively the same manner regardless of its total horsepower. If anything, those with more horsepower will be tempted to USE more horsepower, exponentially reducing their mileage. In everyday use, a Prius would undoubtedly be the winner here.
That's not to say it's the more environmentally-friendly of the two. But that issue could take up a whole thread of its own...
Moving on. In my best estimation, I don't think accelerometer-based apps are reliable; more toy than tool. A phone's accelerometer is for basic movements and gestures... I have doubts about their ability to discern inertial changes at speed. Different brands might use different sensor manufacturers, too.
Also...
All engines increase in RPM as the speed is increased so this makes your car less efficient. This is also where different car profiles would come in useful. Some cars can get away with good gas mileage at 75mph but others wont.
I understand what you're trying to say, but this is incorrect. Or rather, you over-simplified. Engine speed is not proportional to vehicle speed, and vehicle speed is not proportional to gas mileage. I am not traveling 10mph at 1000rpm and 65mph at 6500rpm; likewise, I might net a higher mileage at highway speed than at town speed. Also, vehicles greatly vary in mileage depending on their coefficient of drag. For some cars, CoD is even more influential to mileage than engine size or speed, or even the cylinder count!
I use a true ODBII trip computer (ScanGaugeII) full-time, and it's quite accurate; I always verify the mileage with a manual check. It wasn't accurate with the default settings but I've tweaked it over the years to be accurate within ~0.1 GPH.
There are many small ECU quirks that are individual to makes and models. For instance, my Honda has a fuel injector voltage cut-off instruction when decelerating with a closed throttle until it reaches 850rpm. In easyspeak, my engine doesn't use any gas at all when decelerating. My ScanGaugeII's accuracy suffered for a long time until they released a software update that made this a modifiable setting. What sort of variables to you plan on using for your app?
I don't know much about programming... I'm wondering if you might get any use out of the ScanGaugeII algorithms. It uses what looks like an Ethernet port for the OBDII cable. If you think a system dump would be useful (and know how to extract it), I'd be happy to help. There is also a wealth of information on sites like CleanMPG.com (databases of vehicle CoDs, etc).
Good luck! We need more car-related apps for Android!
