The trick to this one is to choose a task that has 12 cars, not 20. 20 only allows you to get to around 72/73 mph.
I selected >Services >Class 66 >Spring Clear >6V04
Full throttle right from the beginning. Most of the first few miles is slightly uphill. Once it levels off and goes downhill, you'll reach 75 mph.
*Be warned - shortly after you reach 75mph, there will be a red light (approximately 19 miles before you reach the destination. So if you want to successfully complete this service, I suggest saving part way through in case you have to reload.