One problem might be the 4.37 gears. They are factory but only for two years. I suspect that with a 4 speed and 4.37 gears on 28" tires, you're rpms are fairly high on the highway. A little bit taller tire is probably the cheapest way to correct for that and you'd get the benefit of a taller tire. You might try looking for a 30" tire.

I'd fix the carb first though. Then maybe consider a very slightly taller tire.

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