So I guess simply taking a reading of how fast you're going at 3500RPM and doubling it won't get you close enough? lol

Sure it does, as long as you ignore air resistance, making it a purely hypothetical exercise.

With some time logged on the road and being hit by radar and having my detector tell me how fast I'm going, I'm within 2-3kms quicker then what my speedo reads which is so close to equal I'm still laugh'n. Now I'm curious as to how far out my factory summer tires really are.

Probably the same margin of error in the other direction...

I put a little over 1700kms on my new tires over Christmas, averaged about 18mpg doing 60-70mph and the tire conbination worked awesome!!! I traveled through three provinces and conditions varied from prairies with blowing snow to mountains with black ice and the X5 performed better then anything else on the road because I passed everything else on the road and nothing kept up to me. It was amazing and only once I felt a little bit of slip but i was passing people going up hill doing about 80mph also. Two Thumbs up for staying safe on the roads. I hope this helps.

x5dawg. what are those wheels youre running? i love that rim!

BMW style 215 reps I believe...