I took the liberty and the pain of measuring a straight shot down the highway at top speed. Seeing as how we are provided the metric system instead of the imperial for some reason even though the game is based in America i had to translate it over to the good ole stars and stripes measurement system. I have also made it so that the fuel tank is much larger so it will take longer and cost more to refuel but i think thats a fair balance. You are likely to crash a lot in the beginning until you upgrade the tires to get better traction so try to avoid going top speeds until then yeah? I will mention that you should slow down when approaching tunnels because it may not render in time when you get there and there will be a muddy looking wall that wont allow you to pass and youll ram right into it and die every time lol. You can also drive slightly faster in encampments now i didnt want to jack that up too mcuh because for some reason things wont render if i set it any faster and youll just run into everything and cant see anything

Here are the measurements of speed for each bike and each tier 
Before Mod ,
lowest engine top speed 30mph
best engine top speed 52mph
With nitrous top speed on best bike 78mph

After Mod Tier 1:
Lowest engine top speed 50 mph
Top engine speed 62mph
With nitrous and best engine 93mph

Tier 2
Lowest engine top speed 58
Top engine top speed 69mph
WIth nitrous and best engine 103 mph

Tier 3
Lowest engine top speed 65 mph
Top engine top speed 75 ( capped out) 
With nitrous and best engine 112.5 mph

Tier 4
Lowest engine top speed 70
Top engine top speed 83mph
With nitrous and best engine 124.50mph

Tier 5
Lowest engine top speed 160 mph 
Top engine top speed 240 mph
With nitrous and best engine 360 mph 

Warning Tier 6 will crash your game or make weird shit happen

Tier 6
Lowest engine top speed 240 mph 
Top engine top speed 350mph 
With nitrous and best engine 525mph