Announced in 1963, the 875 used a four-cylinder 875 cc four-stroke engine from the Rootes group. Crucially for the dynamics of the vehicle, this was rear-mounted, unlike in most other British three-wheelers of the era. It was the same basic engine as used in the Hillman Imp, but thanks to the fact that the 875 had a fibreglass body along with aluminium doors, and weighed less than , the performance was good — better than the Imp. The engine was the low-compression version (compression ratio 8:1) as used in the commercial versions of the Imp – the Hillman Husky and Commer Imp van and was able to run on "2-star" low-octane petrol, which was cheaper than varieties used by larger and more highly tuned engines.
As well as the engine, the car used the transmission, rear suspension and wheels from the Imp.
The car's light weight enabled it to qualify for motorcycle road tax rates, and be driven on a motorcycle licence but in order to keep the weight down the interior trim and fittings were minimal.
Styling changes, revised seats, rectangular headlamps and a larger bonnet opening heralded the "Mark II" of 1968.