Reiza lately included "Vehicle Reliability" as another driver's value to determine the odds of a DNF. In my newest update I have included this value for all driver's based on their 1986real-life results and using Reiza's formula.
Tip: Put "Fuel Consumption" to "Off" or "Authentic" or allow Refueling (though not accurate) to prevent the AI from DNFing due to running out of fuel. ATM the tank volume is too low for these cars to complete a full race distance like in 1986, eg 78 laps at Monaco ...