16-17 feet wide

I would dig the holes deep enough to accommodate layer of crushed stone (for drainage) under the posts. Mount the gates to the posts and lash everything together using clamps and scrap lumber. Once you get it like you want it brace the entire mess really well. Only then pour concrete around the posts. You need enough mass around the post to resist the significant force that the 8 foot wide gate will put on the post.

Thanks blindstar!

Your posts need to be very rigid. I used 4 X 6s on a similar project, and they have way too much "spring" when loaded with the weight of the gates.

Thanks Hook!
Did the "spring" cause problems? If so , what did you do?

If they sag anyway, TSC stores sell a wheel that you can attach on the moving end of gate to prevent catching on the ground. Perhaps you could spray it black, to be less noticable.

I have the wheels on some large farm gates. Gates are hung well, but supporting the moving end, prevents drooping, any changes in gate itself from the weight pulling down. I HATE gates that drag in the dirt. Without support, you need a HUGE frame and post to support that weight. Look at the old fancy gates, REALLY heavy frames to hold up the scrollwork, monster posts to hang from.

And some *#* person will come along, STAND ON the gate to look around, to add to the drag problems or create a new problem.