Each rotation of the tires will cause the truck to move forward by the circumference of the tires. So
C = 2*pi*r
C = 2*pi*18
C = 113.0973355 inches
Now the tires is rotating 50 times per minute, so let's multiply the circumference by 50 to get the number of inches per minute.
D = 113.0973355 * 50
D = 5654.866776 in/min
So the linear speed of the truck is 5654.866776 inches per minute. But that unit of measure is rather inconvenient. Let's convert to ft/second.
5654.866776 in/min / 60 sec/min / 12 in/ft = 7.853981634 ft/sec.
So we can also express the linear velocity as 7.853981634 ft/sec.
Since we have a truck, perhaps miles per hour would be a more reasonable unit. So
7.853981634 ft/sec / 5280 ft/mile * 3600 sec/hr = 5.354987478 mph.
And if you desire, you can convert to many other units of measure, such as meters/second, kilometers/hour, furlongs per fortnight, etc.
So in conclusion, the linear velocity can be expressed (to 2 significant figures) as any of:
5600 in/min
7.9 ft/sec
5.4 mph
