137.5 miles in 2.5 hours would be a "rate". To find the "unit rate", you would divide 137.5 by 2.5, to see how many miles you could go every ONE hour.
137.5/2.5 = 55
After dividing 137.5 miles/2.5 hours by 2.5, you end up with 55 miles/1 hour, so your unit rate is 55 miles per hour (mph).
Unit rates are rates that are constant; it is the value of the ratio.