I found this review online:

"My machine is a 2004 500 HO. I live at about 4700 ft. elevation. I ride at 5000 ft up to 9000 ft elevation. At 7000 ft at approximately 280 miles, my machine began to cut out and then ultimately stopped running. It was the spark plug fouling out. My machine came stock with a "155" jet. I was getting about 14.5 miles to the gallon with the "155" jet. After putting 50 hours on my machine the local Polaris Trained mechanic said I could change jets. I went down to a "130" jet and got a whopping 31.5 miles to the gallon. Additionally, I did not have a loss on power with the "130" jet. After the tank of gas with the "130" jet I checked the spark plug. The plug was a tan to light brown. I did not see any white whatsoever. When I drove my machine with the "130" jet, I rode long and flat trails at high speed and slow 4WD technical trails and with elevation changes from 5500 ft to 8500 ft elevation. I have now installed a "140" jet and my mileage went down to 23.5 miles to the gallon at 5000 to 6000 ft elevation. During this time with the "140" jet I have went through at least 5 tanks of gas. I pulled the spark plug and it now has a light gray but other wise looks in great shape. I plan on going down to a "135" jet to up my mileage a little. The first time I changed the jet, it took about 30 minutes. Now it takes about 20 minutes. I wish I could carry 10 gallons in my tank. I also believe that any machine will have some specific idiosyncrasies. I purchased the "Service Manual" for my machine. That book has a wealth of information. After a few months and putting a few hours while riding, it appears that I have worked out a few quirks. Riding at or more than 100 miles in one day on a four wheeler will give anybody a good case of monkey butt. I carry an extra five gallons of gas not for me, but for my friends that are riding with me. So what is the best answer? Just dump all the gas you can down your carb or spending a little time and getting to know your machine."

What do you guys think of changing jets to get better gas mileage?

 

You should jet so that your machine runs right. You want your spark plug to be cardboard brown. That color gives your machine enough lubrication for longevity and maximizes your performance. If your plugs are darker in color you can jet up to a smaller number which will give you better mileage. Remember though, if you drop below the elevation which you are jetted for you might have to jet back to a higher number so that it doesn't burn up. I am assuming this is on a non efi.