wiring capacity question

As long as the compressor and the welder are not running at the same time then you will be fine. You might not need to upgrade your service. Just run another 10/3-10/4 line to the garage. Same thing applies, you are not welding/air compressor and your wife is not running the dryer and/or oven at the same time. I connected a friends home to a 100 amp service for the well. He ran 3 years before we actually got the home connected. I back fed the service on the house with an 100 amp breaker. Since my wiring was installed in the pipe for the utility there was no danger of anyone being hurt. He did trip the main a couple of times, usually when both a/c's were running and the wife had the oven and or dryer running. He was surprised that there were so few trips. I am sure if there were more we would have gotten around to the utility a lot faster.

Its not quite per code. They won't grant you an allowance for diversity between these two units. But for your own peace of mind, you could rig up some sort of interlock such that the compressor would be shut off when the welder is drawing power.

You probably don't want the compressor to fire up in the middle of a weld on the off chance that the combination might trip the breaker.

Welders are seldom used to thier full capacity. Usually 70% is a good figure unless you are in a industrial type enviorment. Set your welder to your highest USED rating, turn on the compressor and test weld. The breaker may/may not trip. If you have access to an ampmeter, test the amps of the load at that point. Also, unless the breaker is bolt-in type you want to limit your load to 70% of the rating of the breaker. Manufacturers claim all thier breakers are 100% rated but pactically, run one at 100% continuosly and see how long it lasts!