Decathlon's cheapest wireless computer has seven functions: speed, maximum speed, average speed, trip distance, total distance, stopwatch, and clock. All the speed measurements are to 0.1mph or 0.1 km/h.
Time triallists will like the average speed function best: it's invaluable when you're riding against the clock. The display is large and easy to read and it seemed to be showerproof. Set up is straightforward, although the stiff reusable zip tie that attaches the transmitter wasn't a perfect fit on the test bike's elliptical fork. At least the transmitter distance of 45cm allows some leeway in positioning. Two buttons cycle through the functions, with speed and one other displayed. You can reset average speed, max speed, trip distance and stopwatch - but only all at once, not independently - by pressing both buttons. It's programmable for one wheel size at a time so isn't ideal for swapping between bikes.
Decathlon's own DC9W wired unit offers the same functions (and more) for less, so this is now one of the cheapest wireless computers available; Halfords' 6-function wireless is £16.99 and doesn't do average speed.