Today I was running some data logs into my X3. While parked an running, the X3 read that I was sitting around 950 RPMs, but the tach in the dash was reading around 1100. What could cause this reading to be different from each other? I would think the X3 is correct though right? My speedo is slow by about 2.5 MPH b/c of the tire change from 245/75 to 265/75. So while it reads 70, I am actually running about 73. But that's not problem to me, just something I gotta watch out for. But that does not answer the question about the different readings of the tach and X3 since the tach measures how many rotations are occurring in the crank per minute. Any help would be greatly appreciated.

 

*bump*

 

Mine dose the same thing on a 2000 4.0L, my tuner will read one number and the gauge will read about 100 to 200 rpms higher depending where in the rpm range i am. I don't know why this happens.

 

Yeah that's exactly what mine does and it is really annoying. It doesn't bother the truck and how it runs, it's just weird.