Lately I've noticed that "what size spacers should I get" have been giving the wheel posts a run for their money. This is annoying for three reasons,
1. like wheels, spacers should be a personal choice as they are mainly a way to alter the appearance of your car (if someone wants to argue about performance benefits, go ahead, but you'll probably get more repsponse from Pontiac owners).
2. Inevitably people never supply, and usually don't know the required information so that someone else can tell you what to buy.
3. Calculating the size spacers you need is easier than installing a euroswitch.
That being said, if you are clueless as to offset, et, wheel sizes, tire sizes and coding and the like, you have some reading to do first. (all the below are in the MKIV FAQ/DIY)
Now, you should be able to fit yourself into one of two categories, you already have in you possession the wheels/tires setup you want to space out, or you don't.
If you have the wheel/tire setup you want to space out
This requires that the tires be mounted on the wheels and the wheels be mounted on the car. First level the car side to side. This can be easily accomplished by just measuring from the ground to the highest point of the fender. Theorectically the car should be even side to side, but they usually aren't. Just jack up the low side in small increments until the measurements on both sides are equal.
Now get a plumb bob (you don't need anything fancier than a string with a nut or washer tied on the end.
Tape the plumb bob to the centerline point of the fender (make good tape choices), wait until it stops swinging, then simply measure from the outer edge of the tire sidewall to the string. Convert this measurement to Millimeters and this is the size spacer you need for the tire to be flush with the fender. If you want to poke or tuck, add or subtract to get the desired width.
dimension a is the required spacer size to get the tires flush
(the above picture is not straight on since the original wheels on my gorgeous model are only 5 inches wide and tuck like no ones business. Therefore you can't see the sidewall from straight on behind as shown below)
If you do not have the wheel tire setup in your possession
You can still calculate what size spacers you need, but I recommend waiting until you get the new setup then following the above directions as that is simpler and more accurate, plus you can fool around with your "A" measurement to visualize what going with larger or smaller spacers will look like.
If can't wait because you are impatient, get out your calculator.
First remove the wheel from one side of the car on the desired end (do front and back separatley). You need to level the car. This can be as above, by measuring from the ground to the highest point of the fender lip and making sure both sides are the same. If they are not, just raise or lower your jack so they are equal. *One thing to note, on the front of the car, especially on lowered cars, because you have an independent suspension you may have to literally put the rotor on the ground to level the car, this is because the suspension is not compressed from the weight of the car. If you need to put the rotor on the ground, just put a block of wood under it, it is ok to let the ground compress the suspension. but make sure the majority of the weight is on the jack.
Now make a plumb bob like above and tape it to the centerline of the fender. When the plumb bob has steadied, measure from the wheel mounting point on the hub (not the brake rotor, where the lug bolts thread in) to the string. Note this dimension.
Because I'm not always a pain in the ass I've gone ahead and done this part for you.
On GTI's/GOLFS the "A" mesurement from hub to fender is
71.4375mm on the front (2 13/16")
87.3125mm on the rear (3 7/16")
I am wiling to bet that the front measurement is very close for Jetta's if not identical and the rear is close enough for Jettas if not very close.
*These measurements are only as accurate as the number of beers I drank before doing this, which I can't really remember. It's not my fault if you buy the wrong size spacers.
Now, you want to replicate this distance with your wheel/tire setup in order to get the tire flush with the fender. Again, you can add or subtract to this to achieve a desired poke/tuck look.
To do the calculation you need to know two things. 1, the offset of your wheel in MM (et 42 for example = 42mm) and 2, the width of your wheel OR the width of your tire OR the sidewall width of your tire.
In regards to #2 the best measurement to have is the overall tire width, note this is NOT sidewall width (what is coded in tires sizes). Some people publish tire widths, but most don't. I have seen identically coded tires (205/55/15) vary as much as 3/4 inch in overall width depending on manufacturer and model.
Since overall width is not usually published it is best to use tread width, if you don't even know what size tires you are running you can use wheel width, but usually wheel width will give you low values. *One thing to note, is if you are stretching your tires, you should use rim width NO MATTER WHAT, the whole point of tire stretch is that the wheels are too wide for the tire, so using tire measurements isn't going to give you accurate measurements.
If you plan on using wheel width convert it to millimeters. Now you have your tire/wheel width and the wheel offset. The rest is just math.
Take the tire or wheel width measurement and divide it in half. Subtract the offset from this figure. Subtract the resulting value from the appropriate hub-fender measurement above, and that is the size spacer you need in order to be flush, again, you can vary this to achieve a desired look.
Example. Montreal I rim et42, 205/55/16 tire
(205 / 2) - 42 = 60.5
71 - 60.5 = 10.5
So 10mm Spacers would get me flush in front
87 - 60.5 = 26.5
and 25mm Spacers would get me flush in the back
Things to remember in either case
1. 1 inch = 25.4mm http://www.onlineconversion.com
2. 5mm is only 1/5 of an inch, going down 5mm in spacer size may save you a bunch of money without seriously affecting whether you are "flush" or not
3. If the number you get after calculating your offset - wheel/tire width is LARGER than the hub - fender measurement you are going to poke no matter what. Spacers only extend your wheel and tire out, they can't subtract width. You need to do that with offset.
4. Along with #2, you do not need to hit the values EXACTLY, depending on drop, 15mm inside the finder can look "flush".
If people want to post pictures from straight on (front or rear) with the appropriate values as a reference go ahead, but make sure you include OFFSET, TIRE WIDTH/TREAD WIDTH/WHEEL WIDTH, and any SPACER size if applicable. Alternatively, you could just figure out your hub to sidewall width and give that to us instead (WIDTH/2 - OFFSET + SPACER). The good thing about giving the hub - sidewall value is that is doesn't matter what size wheel tire or spacer you have. A hub -sidewall width of 85MM should look the same no matter what wheels tires or spacers you are using (factoring in of course the error caused by tire/tread width and that it is on the same end of the car).