Speedometer and Speedometer Failure

Speedometers in DeLoreans meant for the US market are graduated in MPH in white (and km markings in blue) with an orange highlighted 55mph. The odometer shows total vehicle miles on the top display and trip miles on the lower.  US units show speeds only to 85 mph in accordance with Federal law at the time which aimed to save fuel by convincing drivers that their new car shouldn't go any faster than 85. 
Cars originally intended for the Canadian market are equipped with speedometers calibrated in kilometers that showed speeds to 240km/h and also register the total kilometers on the top display and trip kilometers on the lower. The rest of the gauges were gradated in metric, as well, and used a unique symbol for the brake light.
A 140mph unit was developed for the UK/European market, but only a handful were made and are quite rare today. DMC created and now offers a reproduction speedometer that indicates to 140 mph. It can be distinguished from the rare factory 140 by having fewer blue km markings than the factory unit.
140 speedo
A number of other reproductions have been made and offered over the years within the DeLorean community including 160mph, 170mph, and 200mph units. These are usually readily identifiable by their poor match in lettering/numbering between the other instrument cluster gauges.
Speedometer Failures
The speedometer unit, and indeed, all the factory gauges have proven reliable over the years. The most common failure in the speedometer itself is the trip odometer reset shaft.
The trip odometer is reset by a four-inch-long shaft that sticks through a hole in the plastic lens of the instrument cluster.  Originally, these were made of very fragile plastic and it was common to see even NOS parts already cracked. Always push to reset, and never twist. When detailing the interior of the car be very careful not to snag the reset arm!
Repair of this failure is somewhat complicated and time consuming, requiring careful removal and disassembly of the instrument binnacle, cluster, and speedometer head.  DMC has reproduced the Trip Reset Shaft P/N 105863 in a much stronger material and this upgrade will eliminate the problem. This part comes with illustrated instructions on proper method of replacement.
If your speedometer no longer records mileage as you drive or indicates your speed, the problem more likely lies with the mechanism that leads from the left front wheel to the back of the speedometer head.
As shown in the illustration below, the speedometer is driven by two cables (upper #1 and lower #11), separated by the lambda (service interval) counter in the middle, the angle drive at the inside of the steering hub and dust shield at the outside of the hub.
The most common failure point is the angle drive itself - usually from lack of lubrication (modern replacement units are sealed) in the angle drive or water (turning to ice) in the lower cable. When this unit seizes it usually damages the dust shield. For this reason we strongly encourage that all three items Kit P/N K106130 are replaced at the same time. Our parts come with illustrated instructions for replacement.
The replacement lower cable is slightly shorter than the original, and is better sealed against moisture.
Noisy or bouncy speedos are indicative of a dried out or partial damaged component (angle drive, lower cable, or lambda counter)  - contact your nearest DMC dealer for diagnostic assistance.
Written by Warren Wallingford with edits by James Espey, DeLorean Motor Company (Texas)
Revised 7/5/2016 DAS [edits, details, links]