Really bad desync/speed difference with my friend

Please describe the issue you are having in as much detail as possible:
my friend and i always have really horrible issues with my car being at least 100mph faster on his end
example: he was driving a GTS Scintilla and i was driving a v6 Sport Pessima and i gave him a head start by 5 seconds and i still caught up. he tabbed over to me said i was going 190mph on mine i was going 100