My startup, Maintaino, is a web-based preventive maintenance reminder app. It’s fairly generic, but should work well for car maintenance based on mileage. I’ve thought about just the sort of direct car/phone integration you described, but unfortunately getting electronic odometer readings from the car’s onboard computer currently requires custom hardware, and the protocol isn’t fully standardized.

As an alternative, though, I believe it should be possible to use the phone’s camera to OCR the odometer reading and then transmit that back to Maintaino. That method should work on any type of vehicle (cars, airplanes, boats, etc).