If i have a photo that's 18" x 12", and 300dpi, will it come out clear if i print it on a 16' truck? I am planning on using a stock photo and then blowing it up, but i am worried that it won't come out clearly. I appreciate your help. Thank you!
Do the math.
Your photo is currently 18 inches wide and 300 DPI. You want to enlarge it to 16 feet = 192 inches. That's a factor of 192 / 18 = 11. That means the original 300 DPI will be 28 DPI. Pixels will be 1/28 inch wide, or 0.9 mm. Now you have to decide whether that's good enough.
To decide that, print out something at 28 DPI and see how far back you have to be for it not to look pixellated. That's how far people will need to be from the truck for it to look like a continuous image.
Someone walking up to the truck will definitely see the pixels. 28 DPI is probably good enough for seeing the image from another vehicle on the highway. It's certainly good enough to see the image across a parking lot.