There should be preference settings like "shortest route" which may be taking you down the back roads. There should also be something like staying to the "fastest route" which is usually the free way, but not always. And there may be one like "no dirt road, paved only" You'll have to look for the preference settings.
On my Garmin 760 for trucks and RVs it does let you put in the weight, height, length, etc. It has kept me off of the real back roads so far.
It gets interesting when the 760 wants me to go one way for the MH settings and the DW's cell phone GPS doesn't care and wants to go another way. And then for laughs, I'll turn on my smaller Garmin if I want to look for a third set of directions.
Bill