| 5th lets me loaf along at around 3000 RPM and 65 mph.
65 mph = 104.6 kph, @ this speed my SM4 ( final & 4th
gear combined ratio = 3.132335, axle height 30 cm via 205/
65R15 tyres, zero toe-in ) needs just 104.6 x 8750 ÷ 33 x
3.132335 ÷ 30 = 2895.8 rpm.
Either your 5th gear's ratio is too high, or axle ( tyre ) height
is low.

| I drop to 4th whle signalling and pulling out.
Sounds like a manual gearbox. Your gearbox is automatic ?