There is a fair amount of slightly incorrect info in the preceding posts.
First consideration should be whether or not the garage is attached (or close) to the home or not?
Garage fires are very common, protecting the rest of your house, memories, and family are worth any resulting water damage to your truck.
Heat from a fire, can cause surrounding structures to burn, and you'd be surprised how far they can be. That is WITHOUT considering the wind direction which could seriously threaten the situation.
Also, how far away is the fire station? Is it volunteer or full time? Response time is the single biggest factor.
Whether the garage is heated or not is another consideration. Or does it drop below freezing where you live? This would be the deciding factor between a dry and wet system. A dry system (I'm only assuming) is likely pricier.
Water damage is covered in fire insurance. This is a broad response, I'm sure there are exceptions. I'm a firefighter, not an insurance salesman.
Typical structural damage due to water is actually easily repaired if action is taken quickly. Dry wall is easy to replace and cheaper than lumber etc, and if the builder was smart and left a gap at the floor, might not even be required.
Electrical equipment, not energized while sprayed, will work fine as long as it is fully dry before energizing.
It is accurate that water is not the best extinguishing agent for a fuel (liquid) fire also known as Class B or electrical, Class C fires.
However, in the typical situation that we are discussing, fuel or electricity only acts as the source of ignition while the bulk of the 'real' fire is now a Class A fire (regular flammables such as wood).
That said, the amount of fire associated with a car fire (ie. carb fuel line) is minimal and not worth including in the equation. So you would (and we do) use water.
Sprinklers, rarely extinguish fires, they control them until the fire department gets there. This is due to the mechanics of the sprinklers and the heat required to set them off. As Minimull said, this can make all the difference to the extent of your loss.
And lastly, the cost of residential sprinkler systems has really come down in the last few years, and you can be certain that if you live in an urban setting, it will only be a matter of time before they are mandatory.
I know this was drawn out, but I didn't want any misinformation dissuading someone from any action that could save someone the terrible loss of a fire.
Much better to let the fire go a rebuild... You will NEVER fully repair anything thats been water damaged, and you will most likely have mold/ milwdew/ etc problems...
It's just so hard to watch someone go through a tradgedy like that, and it could have even been worse if someone had been injured or killed.
