My low range speeds can get a bit over 5MPH IIRC, but I usually work slower than that, I do not pay much attention to speed, just comfortable rate. In high range it trams much faster, but I hardly ever use that unless I am on paved road getting to work site. When new, my "Z" would not reach full Kubota stated RPM and I had service guy adjust it to get the full range. I forget what Kubota specs the engine for at max HP, but maybe 3,700 RPM plus or minus 100. I normally operate at about 3,200RPM, but when I want that full power it is there. It makes no difference with speed when my duals are on or off.