I own each. I believe the 300d non turbo is still a little faster overall. The main difference as well is the rear axel. It is a small amount taller in the 300d. Highway cruise is slightly less revs but the 300d could still use a slightly taller ratio. The 240d needs a taller ratio as well but does not really have the power to utilise it. So that leaves any improvement basically to the five speed transmission. Not the easiest thing to get your hands on but I think a worthwhile thing to do. Both cars running gear are in really good condition. In a 1/4 mile I still feel the 300d would get the 240d as well. It might be close though.To qualify this further certain 300ds are dogs and so are some 240ds. I believe both of mine are high average and fairly strong examples for types. Although one of my 240ds seems to have more steam than the other 240d that is identical. This comparison could be an apples and oranges kind of thing perhaps. I really like my 240ds but the 300d can get them I believe. Might be wrong though as the 300d does feel smoother as well overall. Interesting comparison. Now I guess a 240d automatic is in a class all it's own. Yet some people are very happy with them.