You could get a correct speed reading from a Garmin, or a basic trip computer.
If you go the latter route, get a wired one because some of the wireless ones don't work on an ebike.
No need for anything fancy, this one would do the job:
http://www.halfords.com/cycling/cycling-technology/cycle-computers-gps/halfords-essentials-5-function-cycle-computer?cm_mmc=Google+PLA-_-Cycling+Technology-_-Cycle+Computers+&+GPS-_-679286&istCompanyId=b8708c57-7a02-4cf6-b2c0-dc36b54a327e&istItemId=wxqiltatx&istBid=tztx&_$ja=tsid:94971|cid:868641114|agid:42483203263|tid:pla-369695589530|crid:203186067033|nw:g|rnd:2591263816015309636|dvc:c|adp:1o1|mt:|loc:1006886&gclid=EAIaIQobChMI-p-zucX12AIVRyjTCh3MhQaREAQYASABEgKoX_D_BwE
If you go the latter route, get a wired one because some of the wireless ones don't work on an ebike.
No need for anything fancy, this one would do the job:
http://www.halfords.com/cycling/cycling-technology/cycle-computers-gps/halfords-essentials-5-function-cycle-computer?cm_mmc=Google+PLA-_-Cycling+Technology-_-Cycle+Computers+&+GPS-_-679286&istCompanyId=b8708c57-7a02-4cf6-b2c0-dc36b54a327e&istItemId=wxqiltatx&istBid=tztx&_$ja=tsid:94971|cid:868641114|agid:42483203263|tid:pla-369695589530|crid:203186067033|nw:g|rnd:2591263816015309636|dvc:c|adp:1o1|mt:|loc:1006886&gclid=EAIaIQobChMI-p-zucX12AIVRyjTCh3MhQaREAQYASABEgKoX_D_BwE