What kind of transmission?
Do you have stock tires?
What rear axle ratio do you have?
For stock tires size and axle ratio look here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
That info is on the driver's door sticker
Most newer transmissions are 5-speeds, manual or automatic
4th gear is a 1:1 ratio with engine RPMs, so if engine is spinning at 2,700RPM then rear drive shaft is spinning at 2,700RPM
5th gear is an "overdrive gear"(OD), usually about .75:1, so engine RPMs are lower than drive shaft RPMs.
So lets say you have stock tires(27" tall), 3.73 rear axle gearing
In 4th gear at 60MPH your RPMs would be 2,785rpms
In 5th gear at 60MPH your RPMs would be 2,089rpms
So if you have an automatic maybe it isn't shifting to 5th gear(OD).
On some automatics you can lock out OD, when driving up hills or pulling a trailer you really don't want trans to shift into OD, you lose engine torque to maintain speed under load, so maybe you just need to "push a button" to get OD back.
This has nothing to do with engine size, it is just the physics of gearing