so i just mixed 87 and 89 gas .
and all of a sudden my speedometer ( in my opinon) is changing slower.
thats the only issue ,
i usually use 89 , but i was being cheap and used 87 today .

Wasting money on 89....

That has nothing to do with the operation of the speedometer.......can check the operation of you speedometer....most smartphones have a lap timer on them....get on the highway.......find a level stretch of highway, set cruise for 60mph.....when you pass a mile marker, start the timer....at the next mile marker, hit stop......

An accurate speedometer will read 60 seconds.....one that is reading slow, will give a reading above 60 seconds, one that is reading fast will be below 60 seconds.....don't go crazy over a one second over/under difference....but if you are getting 3 seconds or more, then there is a problem.....and it'* not gas....you may be confusing mpg with mph......

Are you saying that it seems that you are accelerating slower?