yeah the engine doesn't matter at what valor it starts the car it's always at 100% also if you start with a 11%, 89%, 94%, 35%, etc.....but if this number start to going down than it said that you got a problem to the engine and when this number arrive to 0% you will se your engine firing.......just try to run some laps with radiator 1 and you will see that the number start do going now fast!!! also in some mods you will see that valor going down really fast if you don't drive properly!!!
about the tyres i think that XD is better than the pit menu and you can see your tyres wear while running that is very usefull for spare tyres during a race!!!
Ok, thanks for the clarification !