whatever your voltage is, you need to run the wire slow enough to move your puddle, (oscillate), to tie in good and keep the heat consistent.
on most machines, your digital readout should be around 19 on your heat, and 18 on your speed......for 18 guage.
that's how I would run it, but I do not use a mig the way most do. I don't trust the weld going in unless I move fast and fill to the tie in points, creating a convex bead with the higer heat, and slower wire speed. most migs are gonna be using a hard wire, which I don't trust much on anything that is going to be tested by abuse, or weight/pressure.
If you can picture a tom and jerry cartoon, where jerry runs so fast on the rug, it bunches up against the wall. thats how I mig weld. or the way ribbon candy is made....