accelerometer's can be had for cheap, so if it going to the CPU instead of the ECU is ok, I'd look into finding one with a range and accuracy that meets your needs.
Here's one place that sells some
This page lists some ranges for typical, sports and F1 cars to give you an idea of the sensor range you would need. (A hard crash is bound to peg any sensor you get). So I guess the biggest factor would be sample time.
Automotive Acceleration (g)
event typical car, sports car, F-1 race car, large truck
starting 0.3 - 0.5, > 0.9, 1.7, < 0.2
braking 0.8 - 1.0, > 1.3, 2, ~ 0.6
cornering 0.6 - 1.0, > 2.5, 3, ??