are you working in a studio with a team of vinyl cutting/posing ?
some guidelines that are quite important:

-always give at least 2inches of bleed up and down of your design so the guy who will pose the vinyl will have room to play with. More safety for the job, and you will have far less mistakes in the end. scrapping 12 foots by 8 foot of printed vinyl is bad, real bad. Try to know the printer limitations you will have to send your files to.

- here we shop istock for high quality photo. Relatively cheap and can be charged easily in a global price.
- an important note; a truck, a car or whatever rides moves fast. your design must be easy to look at for the first 2 seconds.
- informations not to miss: logo, phone number on each sides, license when applicable (RBQ). But the real area of importance is the back. this is the area that is the most seen. at red lights, stop, every car that follows it... The back is where you squish all the informations.
the sides have a nice visual and a minimum of stuff.

as for measurements, nothing's better than taking them yourself, with photos. I usually take pictures as high quality as possible with my digital camera. gotta take them as straight as possible, to avoid curves and wrong perspective. A quick way to make your measurements as perfect as possible:
my formulae: ((original measure)x100) divided by your photo's measurement. it will give you the ratio in % you should scale your picture.
example: your truck has a door of 50.5 inch, but your photo is much smaller. the door on it has 3 inches !
formulae: (50.5 x 100) 5050 divided by 3 = 1683.3333
meaning you gotta scale your photo by 1683.3333%. if you take your measure after this scaling, i garantee it will be tightly close to your original size.
the more quality and details your photo have, the more accurate your scaling will be.

these are my personal guidelines, and it depends on your customer too. sometimes, they got bad taste and a buckethead. They will stick to their idea of what is good, and you gotta do with it. after all, your customer pays, so he has the final word...

elseway, you must, and its obligatory, you must have fun !

one thing to mention, its to study your vehicle. some of them got pretty rounded curves, windshield (a new beetle or the HHR as an example). check these carefully and adapt your design. sliding doors are a pain. you will lose 2 inches of vinyl. be careful, cause if you place text, or a photo, you will have a bad surprise in the end. parts will be missing. other areas line gas trap, logos, door handles,etc... mind that vinyl cant be applied on a plastic or rubber surface, like a lot of cars have (bumpers as an example)

i hope these tips will help ! I make a couple of trucks and car every week ! This is what I gathered of tricks after a lot of practice. there might some i forget, but the general ones are there
good luck with your project, and work hard ! keep me in touch, im curious!

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'' I want to draw better ''
Efforts constants, étude quotidienne et toujours essayer d'évoluer.