should have clarified: ok yeah obviously you can't adjust the tach, i meant the ratio of RPMs to speed. anyway, to clarify the resistance: i was referring to the amount of RPMs need to sustain a speed with the cruise on. your vehicle is obviously going to work harder if you are driving straight into a strong wind and so you get that slight fluctuation (in speed and RPMs). all i was referring to.

i don't like trusting a GPS for my speed. my OBCD dongle has an app that allowed me to basically drive one mile at a constant speed (again as best i could) with a timer and then i did the math. i always found GPS to be 1-2 MPH slower than actual speed.