Speed only registers .5 to .8 mph

what brand ?

The 2006 Dodge Ram 1500 goes from 0-to-60-mph in just over 5 seconds with a top speed of 150 mph. Hope this helps!---------------------------------------------------------
Multi-Diag Access

Press the Stop and the Speed Up keys down while inserting the safety key. This should get you into calibration mode. Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. If you cannot get into calibration or if the EPROM test fails then the upper console will have to be replaced.
If you get pass to this point then proceed with the calibrating the incline and speed.
Press either incline key and the incline will calibrate automatically.
Be careful the treadmill will run at full speed in the next mode DO NOT STAND ON THE WALKING BELT.Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

Press the Stop and the Speed Up keys down while inserting the safety key. This should get you into calibration mode. Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. If you cannot get into calibration or if the EPROM test fails then the upper console will have to be replaced.
If you get pass to this point then proceed with the calibrating the incline and speed.
Press either incline key and the incline will calibrate automatically.
Be careful the treadmill will run at full speed in the next mode DO NOT STAND ON THE WALKING BELT.Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

Unplug unit remove motor shroud then start calibration procedure.
Press the Stop and the Speed Up keys down while inserting the safety key. If you cannot get into calibration then upper console needs to be replaced.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. At this point if it show 2F FF or Fail then upper console needs to be replaced.
Press either incline key and the incline will calibrate automatically
BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON WALKING BELT.
Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled.
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing.
Press the Stop key once and remove the safety key.

Unplug unit remove motor shroud then start calibration procedure.
Press the Stop and the Speed Up keys down while inserting the safety key. If you cannot get into calibration then upper console needs to be replaced.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. At this point if it show 2F FF or Fail then upper console needs to be replaced.
Press either incline key and the incline will calibrate automatically
BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON WALKING BELT.
Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled.
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing.
Press the Stop key once and remove the safety key.

Press the Stop and the Speed Up keys down while inserting the safety key.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test.
Press either incline key and the incline will calibrate automatically.
BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON THE WALKING BELT. Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled. The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments. It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure. THIS IS A VERY IMPORTANT STEP BECAUSE YOU WILL GET AN ERROR MESSAGE IF THIS STEP IS SKIPPED. After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

Here is a copy of a GM bulletin that applies to you concerns of low speed ABS activation.

Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface) #03-05-25-007 - (09/29/2003)
Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface)

1999-2004 GMC Yukon XL

Condition:
Some customers may comment on ABS activation at low speeds, usually below 8 km/h (5 mph). Upon investigation, the technician will find no DTCs set.

Cause
The cause of this condition may be an increased air gap between the wheel speed sensor and the hub reluctor ring due to rust and debris built up on the sensor mounting surface.

Correction
Measure AC voltage and clean the wheel speed sensor mounting surfaces.

Raise and support the vehicle.
Disconnect both the front wheel speed sensor (WSS) connectors at the frame and harness.
Place a Digital Volt Meter (DVM) across the terminals of each WSS connector.
Rotate the wheel clockwise approximately one revolution per second. The minimum reading should be at least 350 ACmV's. If the reading is less than 350 ACmV's, remove the wheel speed sensor. Refer to the applicable Wheel Speed Sensor Replacement procedure in the ABS sub-section of the Service Manual. Plug the WSS bore in order to prevent debris from falling into the hub during service.
Clean the WSS mounting surface on the hub to remove any rust or corrosion using a wire brush, or equivalent.

Important
Make sure that the sensor sits flat on the hub. Check the sensor flange against a straight edge to ensure flatness. If the sensor flange is distorted, replace the sensor.

Apply a thin layer of bearing grease to the hub surface and sensor O-ring prior to sensor installation. Use ONLY Wheel Bearing Lubricant, P/N 01051344.
Install either the original sensor or a new one in the hub. Ensure that the sensor is seated flush against the hub. Refer to the applicable Wheel Speed Sensor Replacement procedure in the ABS sub-section of the Service Manual.
Place the DVM across the sensor terminals and recheck the voltage while rotating the wheel. The voltage should now read at least 350 ACmV's.

Good luck and thank you for using Fixya.

You sould have recieved a "recall" notice in the mail about the cruise control for fire and failure issues. they are adding a fusable link where the cruise control harness connects to the brake master cylinder. if you have not had the recall performed yet you should SERIOUSLY consider getting it done, mainly so it doesnt catch fire, but "could" be the result of the failure you've been experiancing! btw, almost ALL Ford vehicles have been affected with the cruise control recall. hope this helps!