Tag axels are not an option and are based on what the coach chassis will be required to carry. Generally speaking, 40' and under has a single rear axel and over 40' has a dual. I have seen 42' coaches with a single rear axle but that isn't common. If you buy a 45' coach you are going to get duals. Now comes the expensive part, 4 more tires and more maintenance. Bottom line, it will depend on what size coach you purchase.
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