Hello 7ru3blu3,
as far as i could find out through testing, you gave yourself the answer: at the end of the race, usually you're faster than at the start - ergo - you're a shorter period of time within the nitro zones, which mean less points...
I think it works like follows:
• you get points/money for beeing in contact with the nitro zones
>> slower drive through* = more points/money
• you get points/money for less time between hitting two nitro zones
>> less time between zone hits = more points/money
If there is a bonus given for the amount of nitro zones hitted within a race - which will be shown as NITRO-ZONE value at the race results - i couldn't recognize by now.
*I did try the following:
• took a good car (in my case 762PR) and raced a low chapter underground nitro race (chapter 3, race 2) This did give me a comfortable amount of race time.
• started the race and grinded on the walls (cause we don't have brakes lol - but kept an eye on the damage meter!)
• tried to drive through the nitro zones with as low as possible speed, but kept an eye on the remaining time!
You may encounter more points/money, as through fast and nitro accellerated racing while hitting every zone 😉
(Btw. that's the reason why i asked at the forums to give money bonus for the time leftover, after driving as fast as possible - which usually is the target of racing games...)
I did try to beat my snail driving 3.022$ NITRO-ZONE bonus on underground chapter 3, race 2 (see photo) with/through driving like hell ... lol - i wasn't able to: