You should check your speed on grass as it will tend to be slower than on pavement. Naturally it will be a bit slower on tall grass, uphill, or with a full hopper.
I travel at about 2.6 miles per hour (that is 229 feet per minute.)
Of course, my spreader has an electronic speedometer. I can watch it continuously if I want.
With a half-full hopper on grass, 2.5 mph is more likely, (220 feet per minute).
Retired now, but I used to spread at 10 feet measured center to center--which is 7 feet between the nearest wheel tracks.