linear or exponential?

[Nintendo Fanboy]

I was just driving back from my sisters and it's a 2 hour drive i was puzzled by this the whole way back here we go;

*metric*
when my car does 50 it shows as 60 on my speed-o-meter (i know this because we have signs here that show your actual speed) then i got to the motorway/highway where i'm allowed to do 100 wich on my spedo is about 120-130 (not sure because we dont have the signs there) so on one bit of the motorway is was allowed to do 130 so i went the same speed as other cars and i hit 170 on my spedo.

i figured out that my tyres/tires are smaller then they are supposed to be because the previous owner lowered it and it would otherwise hit the wheelwells.

ive been trying to figure out if this increase in actual speed is linear or exponential because i tryed a top speed run in my car and did 220 (as far as the needle will go) wich should'nt be doable i think in anny "standard" car

please discuss

heres me going top speed
youtube.com/watch?v=QX-CetJYPSo

offtopic, just ordered my ps1 screen with ps1 and controllers for 36 euro including shipping

[eagle5953]

The conversion between mph and kph is linear.

[vskid]

Its linear. Your speedometer is showing about 20% higher than actual speed. To get your actual speed, multiple the speedo speed by 0.83 (according to the numbers you gave, should at least get you in the neighborhood). That means you were really going 183kph (113MPH) at 220kph (137MPH).
220kph, while respectable, isn't that high of a top speed. My car is limited to about 257kph (160MPH), and could probably do quite a bit more without the limiter and better tires.

[Nintendo Fanboy]

The conversion between mph and kph is linear.

my question was is if the diameter of my wheel is smaller, would the spedo give an exponential or linear false reading compared to actual speed.

Its linear. Your speedometer is showing about 20% higher than actual speed. To get your actual speed, multiple the speedo speed by 0.83 (according to the numbers you gave, should at least get you in the neighborhood). That means you were really going 183kph (113MPH) at 220kph (137MPH).
220kph, while respectable, isn't that high of a top speed. My car is limited to about 257kph (160MPH), and could probably do quite a bit more without the limiter and better tires.

thanks for the explanation. i'l see if my gps agrees to your 20% statement (makes sence though)

also, my car only cost 850 euro so speed/price ratio is good enough for me