It gets a little confusing. At sea level you have 14.7 lbs per square inch of atmospheric pressure. (I'll round it off to 15 lbs) So before you add air to your tire it has 15 lbs of pressure in it. However your air pressure gauge always reads zero before you check your tire. Put 40 lbs of air in it and then go to 10,000 ft. You lose about 3% of atmospheric pressure per 1,000 ft. so now the atmospheric pressure is only about 10.5 lbs, yet your tire gauge will still read zero. You still have the same amount of air in your tire, but your gauge will read 30% less. Tire mfg. give air pressure recommendations at sea level and 68 degrees. They are just somewhat of a guideline because it is unlikely most people will meet those conditions so adjust your tires to fit your needs, less air in poor traction and more air on pavement. I wouldn't change tire pressure at elevation just because your tire gauge says you are low.