I would love to do exactly this. It's probably been done at various carmakers' proving grounds, but I've never seen a car power meter "in the wild".
Ideally, you would have strain gauges in each CV shaft to get driving torque, and read either the vehicle (i.e. transmission output shaft) speed sensor, or ABS wheel speed sensors (for higher precision).
You'd need an altimeter and knowledge of your car's mass if you wanted to account for gravitational potential energy.
Count fuel injector pulsewidth and divide by output shaft torque, and you'd have an instant engine+transmission efficiency readout.
If you can move this from "would love to do" to "have a proposal for a prototype, with bill of materials", then I'm very, very interested.
P.S. Second best is the ScanGauge. It's an excellent tool to coach yourself on driving technique, and to monitor your car's various sensors in real-time, but it's not a power meter.
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