I'm a Fiesta Metal owner. I've been the owner of this car for around 18 months now and love it. One thing that is very noticeable is the short ratio gearbox, and without a 6th gear the engine does have to work hard to cruise above 80/90mph on the motorway. At 100mph the engine speed is around 5000rpm (1000 below the red line). So, while the punchy little 1.6 engine and close ratio gearing makes this car good fun for driving around town and along country lanes, this car was never intended to be a good motorway cruiser.
My question is, legality and fuel economy issues aside, will keeping the revs at 5,000rpm in 5th gear for extended periods of time damage the engine in any way? The car gets a regular service at Ford, so always has good oil inside, and I always make sure the engine is warmed up before I explore the upper reaches of the rev range (which is only occasionally).