You now get a 90 days visa for your vehicle just like a person. But it is not renewable, you have to take the vehicle out of the country for 90 days before you can bring it back for a 2nd 90 day visa or pay the import taxes. You can get details on calculating taxes here: http://www.xelapages.com/howtoimportyou ... temala.htm
Now for real life stuff...I met a woman here in Xela in Aug who got a 2nd 90day vehicle visa renewal at the La Mesilla border crossing after being denied one at El Carmen. I also know a guy who left his car here in Xela for 7 months over his 90 day visa. He did not drive the car it was parked. When he got back we drove it to the border and paid to import it. The fine for overstaying was Q6.00/day. I know another guy who just left Guatemala in Aug and was about 100 days over his 90 day vehicle permit they charged him Q2.00/day for a fine.
So as with everything here in Guatemala it is fluid and every changing with different rules for each border!!
The problem is if you are driving a vehicle that has USA plates with expired paperwork, you are just asking to be hassled. In theory your car can be confiscated and held until you pay the import taxes. The more likely scenario is you are stopped and hit up for some amount of money.
That is what I know.