A thing that must be noted is, that the 75MHz flavours of the 060 aren't complete versions.

They come in either EC or LC versions, lacking either the FPU or MMU thus actually making the 75MHz versions slower than the 66MHz ones.

Having less circuitry, the processor can be made to go faster without overheating. Then again, the 060 runs very cool.. I don't even have a heatsink on my 060/50.

The 68040 on the other hand, generates heat by the bucketload. A heatsink + fan is necessary in order for the machine to even boot, and even then it's very hot.