Electric pressure washers work best for vehicles. Gas powered washers are a little overkill. If you're considering purchasing a pressure washer, you can find a decent electric washer for under $250. Besides washing your car, they're great from sidewalks, siding, driveways, lawn equipment, and a plethora of other things. Basically if it's outside and needs cleaned, a pressure washer is up to the task.
Washing is just another step in prolonging the life of your vehicle. Keeping the undercarriage and paint clear of dirt and road debris can only increase the life of your vehicle. Salt is extremely rough on a vehicle, so if you live in an area that uses road salt, purchasing a power washer may be a good idea. Let's look at the basic technique for pressure washing a vehicle.
For those who have never used a power or pressure washer before, it is a good idea to practice a little first. You can practice by pointing the wand towards the ground and pulling the trigger. Get a feel for how much pressure is coming out and a safe distance to keep the nozzle from the surface. Its always a good idea to use two hands for better control. And never point the nozzle towards anyone, the water pressure can actually tear or cut the skin. Proper safety gear such as goggles is also recommended. Now that you're used to how the sprayer will handle, let's start on the vehicle!
The first thing I usually do when washing my vehicle, is give it a quick once-over with water only to thoroughly wet it down. Spraying at a distance of around 3 feet usually works best. The best tip angle is around 25 degree's for washing your car. Never put the nozzle directly next to the vehicles paint at a right angle, this can cause serious damage to the paint and even cause it to chip off. I like to spray the under carriage first really good to clear away most of the dirt and mud, then work my way from top to bottom.
Personally, I do not use an integrated detergent or soap solution but apply the soap by hand. I have found that even with a pressure washer, there are water marks and a residual film that simply does not come off unless you hit the car with a wash clothe. I use one of those car wash mittens and a bucket of water with a little dish or laundry soap. After wetting the vehicle down, I run over it quickly with the clothe mitten and really lather it up. Then I work the pressure washer over the entire vehicle again from top to bottom.