On the whole,
driving
inside Guatemala is pretty straightforward and it certainly offers unrivalled freedom as traffic is rarely heavy outside the capital.
Parking and security
are the main problems, and in the larger towns you should always get your car shut away in a guarded car park. Most of the main routes are paved but minor roads are often extremely rough.
Fuel
is extremely cheap by European standards, marginally more expensive than in the US. If you plan to head up into the mountains or along any of the smaller roads in Petén, you'll need high clearance and 4WD.
Renting a car
takes some of the worries out of driving but is expensive: generally at least US$40 a day (around US$230 a week) by the time you've added the extras. If you do rent, make sure to check the details of the insurance, which often won't cover any damage to your vehicle.
If you plan to visit the more remote parts of the country then it's almost inevitable that you will
hitch
a ride with a pick-up or truck from time to time. You'll usually have to pay for your lift - around the same as the bus fare. Rule number one is
safety
: it's not a good idea for women to hitch alone, and if you don't feel comfortable about getting in someone's vehicle, don't do it.