They do work. However those companies are also not spring/shock manufacturers so I wouldn't say that springs are a bad thing I just don't think...
It definitely is not the same as stock. Significantly stiffer. Honestly I don't care for it at all and may go back to stock springs soon, but...
It won't be double the MPG. I would expect about 14-16 MPG though.
Shocks are rebuildable and the intervals will depend on use. 30,000 miles is a good bet if you're using them off road. My guess is the new ones...
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