A rule of thumb is that in complexity we have three systems in a car. Body. Engine. Transmission. The rule of thumb is that you only introduce one new one of these at a time, if possible. We have a discipline called program size, and the amount of change in each of these is multiplied together to give you an idea of how long it'll take to develop. New engine*new transmission*new body =3*3*3=27, then divide that by some number for you organisation's development speed, 3 being typical then add a year, because. Hence 27/3+1=10 years from scratch for an all new vehicle (say Prius), or two years to put a new engine in an old car with the old trans (3*1*1/3)+1. Or to put an updated engine and gearbox into an existing shell, say 2*2*1/3+1= 2 years
These timings aren't rigorous, but they are robust. The multiplicative effect means that big changes to all three systems at the same time are not encouraged. Small teams working on low production volume vehicles can easily beat these times.