I would suggest something similar to dilation used in image processing.
Use your vehicle as a kernel to dilate your obstacles. Eg. your kernel could look like this (this would be a good kernel if your vehicle has it's "origin" at top-left):
1,1,0
1,1,0 <-- kernel "origin" is at 1,1
0,0,0
Basically you run through each "pixel" or tile in your case and render the kernel to a target whenever you encounter an obstacle.
To illustrate, here's what will happen with a single obstacle:
[ source ] ----> [ target ]
. . . o o .
. o . o o .
. . . . . .
Then, the obstacles dilated with this kernel would look like this:
. . . o o . . .
. . . o o . . .
. . . o o . . .
. . . o o . . .
. . . o o . . .
. . . . . . . .
. . . o o . . .
. . . o o . . .
Which would be the grid you want to perform your vehicle path-finding on.