I routinely drive both, the SD has a better ride, yet inferior interior. These are different chassis's, front ends are different, there is more room under hood in some areas. The SD is a large vehicle as Forced... mentioned. I get 30 mpg with the SD, I drive to get this mileage, tho. The CD, 27 at best, I think a different rear end gearing.

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83 SD

84 CD