It would be a transmission issue, not up shifting. If the engine is in "limp mode" it will not accelerate over a set mph.
A car traveling over 45 mph can hydroplane with about 1/10th of an inch of water on the road.
45 mph is the same as 45 mph. How can one of the two be faster?
90seconds
neither
No.
If you are driving for a period of time over 45 mph then use overdrive. If you are driving under 45 mph or in traffic that is speeding up and slowing down over and under 45 mph then do not use.
A car traveling over 45 mph can hydroplane with about 1/10th of an inch of water on the road.
a = (vf - v0)/Δt 10 mph/s = (45 mph - 20 mph)/Δt Δt = 2.5 s
After 1 hour the slower car will be 30 mph x 1 hour = 30 miles ahead of the faster car. The faster car is travelling 45 mph - 30 mph = 15 mph faster than the slower car and so will cover this 30 miles in 30 miles ÷ 15 mph = 2 hours at which point it will have caught the slower car and start to over take it.
problem with your transmission in 5th gear
The first thing I would check is for broken motor mounts. This causes the vibration around 40-45 mph. There is a service / recall bulletin about this at Kia.
If your vehicle has an automatic transmission with overdrive, then at speeds above 45 mph, it should be in overdrive, which reduces engine RPM's to save fuel.