Speed reads correct 0-40 when speedometer is changed to kph. If you can find room just add a 7th battery to the string. Leave charger connected to the original 6 batteries. Or 9.
Use a 12v auto charger to charge the extra one.
You can test performance using a borrowed battery and jumper cables.
Let me know and I will send the needed “spoof” with the mm.
$125 to barbjb@acegroup.cc with a note and address.