Now I created a quite simple model, which fits quite good, when I use real world data. I made an example with the 2012er Honda Accord 2.4i with 6-speed manual:
You can see the gears (1st to 6th) and their weigthing (in this case percent).
Out of this I calculated an average gear ratio. Combined with the final drive, the engine life and the wheel circumference I calculate the predicted car life. I was very surprised, that the result fits perfectly into the real world, where the typical car life of a passenger car is between 150.000-200.000mi.
I calculated this with an Excel spreadsheet, so if anyone wants to have it to play with weighting, gear ratios etc., I can send it...