Specifying a vehicle for a particular route and application can be done by gearing a suitable engine.

Those intended for 'fast' interurban routes require adequate power and gearing to keep the engine speed below an inefficient value, whereas a 'city bus' can operate adequately at lower maximum roadspeeds.
Whether the Streetlites can be adjusted to cope with interurban use depends on whether the gearing will allow increased road-speed without compromising fuel economy.

I have remarked before that there are many options available when specifying vehicles other than the engine size, and, with suitable gearing a large capacity engine might well operate more efficiently than an overstressed lower-powered engine.

I quoted the example of Jaguar's back-to-back economy tests of 2.4 litre and 3.4 litre cars where the 3.4 litre one was more economical - partly due to the gearing and partly due to the smaller-engined car having to 'strain' to keep up with the larger one.

The same applies to buses, a 9 litre engine might be more fuel-efficient than a 5 litre one according to gearing and route characteristics.
Most manufacturers offer a choice of gearing (number of ratios and final-drive ratio) - as well as some transmission types having different 'economies', and a suitable initial choice of specification is essential if fuel economy (and performance) is paramount.

Whether simply increasing the governed speed from 50 mph to 58 mph is possible (and desirable) depends on the vehicle specification. It might be a simple task of adjusting the software that limits the maximum speed, or it might mean running the engine at an inefficient speed that negates the purpose of choosing that vehicle type.