You fitted a new controller, and probably a new display with it. Everything works, but the speed on the screen is wrong: too high, too low, in kilometres instead of miles, or dropping to almost nothing whenever you stop pedalling. Before blaming a part, check the settings: a replacement display knows nothing about your wheel or motor until told.
Make changes with the bike parked, and test ride in a quiet, open area only after the brake cutoff has passed the checks in our post-install checklist.
Know which menu you are looking at
Confirm the display model first with our guide to identifying your display. KT displays (LCD3, LCD5, LCD8H) have general settings for speed limit, wheel size and units, then P1 to P5 and C1 to C14. "No.2" displays such as the S866, S830 and SW900 use a single numbered P list (P01 to P20 on the S866). Codes look alike, but these are different systems; never copy values between them. On the S866, leave P20 alone: it sets the communication protocol, and a widely circulated S866 manual warns that changing it cuts communication with the controller.
Step 1: Units
If the number looks about 1.6 times too high, the display may simply be showing km/h. On the KT LCD3 and LCD5 the unit is in the general settings; on the LCD8H it is a number, and 3 means mph with Fahrenheit. On the S866 it is P02: 0 for kilometres, 1 for miles. One trap: the S866 speed limit, P08, is always entered in km/h, even when the screen shows miles.
Step 2: Wheel size
The display works out speed from how often the wheel turns and how far it rolls each turn, so a wrong wheel size skews every speed and distance reading. On the S866 this is P06, in 0.1-inch steps. On KT displays it is in the general settings, where the LCD3 and LCD5 offer 13 fixed sizes and the LCD8H offers 15.
Enter the rolling diameter, which is not always the size printed on the tire. Area 13, a US seller of replacement controller-and-display kits, says a 26 x 4.0 fat tire often needs 29 inches entered, on displays that offer it, such as the LCD8H. One forum rider with 20-inch balloon tires had to enter 18 before the display agreed with GPS. The printed size is only a starting point.
Step 3: Does the speed drop when you coast?
If the reading is right while you pedal but falls to around 4 to 6 km/h when you coast, look at the wheel speed signal. On KT displays this is P2. Many geared hub motors have a speed sensor inside, often wired to the white lead of the 9-pin motor plug, though on some motors that wire carries temperature instead. P2 tells the display how many pulses that sensor sends per wheel turn, from 0 to 6. At 0 the display estimates speed from the motor itself, which the KT manual says is inaccurate on clutch-type geared motors once the internal rotor stops while coasting.
Try values from 6 downward until coasting reads correctly. Wattage does not decide it: one owner of a Bafang G062 found it had a single speed magnet and needed P2 set to 1. With no speed wire in the plug, this setting cannot help.
Step 4: The motor value (P1 or P07)
KT displays also ask for P1, which the manual defines as the motor's gear reduction ratio multiplied by its number of rotor magnets. On the S866 the equivalent is P07. Many aftermarket geared hub motors are roughly 5:1 with 20 magnets, so 100 is a common starting point. Sources disagree on whether "magnets" means pieces or pairs, so treat the result as a starting point. P1 mainly decides where the speed limit cuts power, which is why it comes last.
Step 5: Check against GPS, in this order
- Adjust the wheel size until the displayed speed matches a phone GPS at a steady pace.
- Set a speed limit clearly below the bike's top speed, so you can see when the limiter acts.
- Ride and note the GPS speed where the assist tapers off. If it cuts early, raise P1; if late, lower P1. Small changes have a large effect.
Reverse the order and the settings fight each other. GPS drifts under trees and tall buildings; forum users allow about 2 mph of error. Riding a measured distance against the trip meter is a good cross-check. On the S866, P18 also scales the displayed speed from 50% to 150%.
Set the limit to suit where you ride
A wrong P1 moves the real cut-off point: set too high, the assist keeps going past the speed you entered. Raising the limit can also take the bike outside its e-bike class, and staying within local rules is the rider's responsibility.
If no speed appears at all with the wheel turning and your motor has a speed wire, check it is connected and P2 is not 0. If the assist levels will not change either, the controller and display may not be communicating; see why a new display may not work with your controller. Menus vary between versions, so keep your display's manual at hand.