Tilt compensate accelerometer & gyrometer? [MPU-9150]

Hi!

I need help to find a way to tilt-compensate the accelerometer and gyrometer of the MPU-9150.

The IMU will be part of a bigger system for monitoring car performance and because of the noisy enviroment of a car the magnetometer won't be used. I would like to tilt compensate the accelerometer and gyrometer reading to allow the customer to mount the central unit any way they want.

I would like the customer to send a calibration command to the microcontroller interfacing the IMU when the unit is mounted and the car is on a flat surface. The microcontroller will then read values from the IMU and calculate the compensation formula[with help from gravity] to make the axis with highest value the emulated Z-axis. The X- and Y- axis will then be compensated with the same relation to form emulated axes [outputing 0,0,0.98]. The formulas will be stored in microcontroller memory and when bridging IMU data to the rest of the system the readout will pass this "filter" to emulate a flat mounted IMU.

I know that X- and Y-axis won't be able to know if they're mounted in the direction of travel but this will be compensated by another part of the system with either GPS input or external input from user.

Is this possible without having to do heavy calculations? It's just a simple 8-bit ATmega interfacing with the IMU so toying with vectors and matrices is preferably avoided.