100q max speed & tire sizes

[ASh <ASh@cpk.lv>]

I wonder, if the speed, shown on the speedo is very dependant on tire size.
In manual is written, that for 91' 100Q there must be 195/65 or 205/60 or
215/60.
If I have 205/65, could it be, that speedo is showing a little bit less
speed, than it actualy is?
The problem is, that I can't reach more than 205km/h, maybe 210, but no more.
But it should reach 218 (from manual). 
Or can it be that, if I switch to 5th gear already on 100 - 150km/h, although 
in 4th gear ~190km/h can be reached?

And another question:  will the diff lock engage, if accidentally pressed
on diff lock button when the speed is over 25km/h?



WBR,
Armands
K2 & 100Q 2.8V6 '91