that website has some great information, but their way of measuring a 5 lug wheel is wrong and can easily lead to mis-measured bolt patterns. to measure their way,... first: you have to "eye-ball" the center of one of the lugnut holes. that can become problem/variable right from the start, especially if you are measuring millimeters. second: they say you are supposed to measure to the far side of the other hole. 2 wheels can have the same bolt circle, but have different sized holes for different lugnut applications, and you can end up with 2 different answers and either one or both can be wrong. look at the picture i have attached for an example: if a wheel has enlarged holes for shanked lugnuts, or came from a vehicle that had larger/smaller wheels studs, the measurement can vary since you are not dealing with the center of one of the stud holes. look at the upper hole, the center is always the center ... but on the lower hole, the outer edge of the hole can be many different places and so the measurements will differ. i used a digital caliper with their method to measure the pattern of a stock steel chevy corsica wheel. the measurement i got was 103.13 mm. a corsica is a 100 mm pattern. when you get into measuring smaller patterns and start measuring in millimeters, more inaccuracy is the norm.
lets say you are at a swap meet and you see a set of wheels that dont have the pattern size stamped on them. you can follow the formula on this picture and you will get the correct number every time. this of course is not the only way to get the right measurement i have used about 10 variations of this way to get the correct measurement when working with 5 lug wheels. if the set of wheels are of a "uni-lug" style, more problems arise and you will have to take a uni-hole and subtract its width from its length to work out a new formula(which i'm not going to get into at this time).
Since the bolt patterns are in 0.25" increments the center of one hole to the far side of another is a viable method. At least I used it for 15 years while working in the tire business myself. After a couple years though I could (and still can) eyeball a wheel and tell you the bolt circle on the old 5x(4.5/4.75/5.0/5.5) bolt circles on the older cars and trucks.
