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...has been around since OBD-1 days, as far back as 1984. OBD-2 programming systems are available for anything from 1994 through 2010. There are even scanners that allow you to enter the PIDs of your choice (obtained from monitoring the data line while performing operations with a scantool).

Since newer vehicles control nearly everything via CANbus, it's no surprise that someone has taken the time to monitor the bus and inject various commands. This sort of hacking has been around for over 20 years (despite auto manufacturers' attempts to protect their hardware with security keys and seeds). I don't see them "solving" this "problem" anytime soon...unless they come up with a way to make a "secure" bus (perhaps using fiber optics).

It's not that difficult to see the throttle position sensor values, via OBD-II with something like a ScanGauge.

My car's ECU reports the TPS values, and I don't think it even has electronic throttle control.

My Scangauge http://www.scangauge.com/ will show something like 30-40 MPG while coasting in neutral, but shows 999 MPG when in gear, engine braking, since the fuel injectors have been shut off.

1) Get a Scan Gauge. You can monitor fuel economy 'live'. You can easily pick up 5 MPG just by watching how you take off from stop lights and such.

2) In my TDI I get 61 MPG at 45-50 MPH, 55MPG at 55-60. 40 MPG no matter how else I drive it.

3) When I say 55 MPH I don't mean 'mean' of 55 MPH flooring it out to 60 then dropping down to 50. Set cruise control and lock it.

Nearly every car made past 1996 has the ability to measure mileage, manufacturers just choose not to make a gauge for it. If you want to see what kind of mileage you are getting, you can read it off of your car's diagnostic port with one of these

It's entirely possible that a conventional car might still get better mileage just by showing this data to the driver so, for example, they learn to avoid racing up to red lights and mashing on the brakes. Not sure if there's a way to safely retrofit this type of thing into a conventional car, though.

Enter the Scangauge II. Not only does it give you a choice of 4 readings to display at the same time, it also reads the maintenance codes when your Service Soon light goes off. Works on all cars after 1996 or so which have a standard ODB-II (On Board Diagnostic) system.

I would like to get one of these myself when I have a few spare hundred dollars, but just haven't bought one yet.

Are you talking about add-on mpg displays like this one? It used to be $130, so that's probably true. While searching for this, I also turned up a post that said the law required any cars sold after 1996 to have an on board diagnostic computer connection in the dash. So these things just plug into that connection. This means the gap between the current law and what I'm talking about is even lower than I expected. And while these are $170, I'm sure that's not the actual cost and is especially not the cost of the generic MPG one that I always see in the cheapo cars.

Try a ScanGauge. I don't have one, because my car's too old, but a Yaris (actually, most 1996 and newer cars) will almost certainly work with it.