Here is a link for the gear ratios of the typical 240sx trans KA/SR/CA...
Nissan 240SX Performance Modification/Transmission - Wikibooks, open books for an open world
Plug that info in with the diameter of you tires and you will have a good idea of what speed/rpm you will be turning with based on the ratio of your specific diff.
-Royce
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If it has tires or boobs it's going to cost you money...