Garmin GPS keeps taking us off main route

I don't really trust my Garmin (with most up to date maps) even on major highways. I was on I-40 going west and went thru Oklahoma City one night last fall. My Garmin had all the correct dimensions etc. programmed in for my truck with trailer. It warned me there was a bridge up ahead that I could not go under and told me to take an exit. I noticed that the 18 wheelers were not exiting so I stayed behind one that was lots taller than my rig. The Garmin kept warning me to exit and finally told me I had to push an accept button acknowledging I had seen the warnings and choose to ignore them. I stayed behind the 18 wheelers and there was never a low bridge. I stayed on I-40 all the way to NM. Now I use my Trucker's Atlas to check routes and just use the Garmin to show me what the exits look like. I won't buy another Garmin. BTW, this was just one example of bad routing. I have many more examples where it lead me astray.