# Why was the LRV's speed gauge displaying metric units?

In the question Why did NASA use U.S customary units? it was basically stated in the answers that even when metric units were used "internally" (e.g. the AGC), the units presented to the astronauts were in English Engineering Units/U.S. Customary Units, probably because that's what the astronauts – most of them (test) pilots – were most familiar with.

But there is one oddball, as I just found out: the Lunar Roving Vehicle's speed gauge was displaying kilometers per hour instead of miles per hour:

(cropped version of full image of the LRV console)

• Temperatures were indicated in Fahrenheit. But which units were used for distance and range?
– Uwe
Commented Jul 30, 2019 at 14:01
• Weird... you'd expect 20km/h to be the top speed or something... NASA states the LRV's intended top speed was 13km/h... but that could just be the mission limits set by people because someone went 18km/h: "the rovers were designed with a top speed of about 8 mph (13 km/h), although Eugene Cernan recorded a maximum speed of 11.2 mph (18.0 km/h)". It may be a safe bet that the "hard-coded" top speed would be 20km/h-- meaning why display the gauge in mph with a top speed of 12.47mph? (This is obviously conjecture) Commented Jul 30, 2019 at 14:09
• My guess is that it stored its units in metric internally but converting it into U.S customary units was a waste of computing power and as @MagicOctopusUrn said, they're not going that fast and don't really need it to be converted. Commented Jul 30, 2019 at 14:16
• @StarMan No converting to U.S customary units was necessary, just a replacement of the analog scale of the speed gauge.
– Uwe
Commented Jul 30, 2019 at 14:21
• The speed gauge looks like a simple moving coil meter to indicate analog current or voltage. Nothing digital, no semiconductors. Just an electromechanical meter used during the sixities and seventies.
– Uwe
Commented Jul 30, 2019 at 14:30

The LRV was at least partially metric.

But they used a mix of metric and US customary units. Temperatures in Fahrenheit. But when you decide to use km for distances, you have to use km/h for speed.

Snippets from this paper.

The entire Lunar Roving Vehicle Operations Handbook (PDF, 38 Mb) is in meters. The only mention of "foot" is when talking about the astronauts' footrests, and "feet" does not appear at all. Kinda unexpected, considering that Boeing was the rover contractor.

Section 1.5.6 describes the Speed Indicator:

The instrument shows LRV velocity from 0 to 20 km/hr. This display is driven from the odometer pulses from the right rear wheel, through the SPU.

The document confirms that the speed indicator and directional gyro are analog devices. However, the rest of the Signal Processing Unit is digital.

The odometer is even more interesting:

Odometer pulses from all four wheels enter the odometer logic via the SPU line receivers. This logic selects the third fastest wheel for use in the distance computation. This insures that the odometer output pulses will not be based on a wheel which is locked, nor will they be based on a wheel that has excessive slip.

The odometer and the directional gyro are fed into a digital processor which calculates and accumulates the delta-North and delta-East, which is then converted back into range and direction indicators (showing how to get back to the LM).

All distances (even the turning radius) is in meters.

• But temperatures are Fahrenheit, inch is used 16 times, pressures are psi, weights in pounds and lbs. Not fully metric. Not all distances are in meters, inches are used instead of centimeters or millimeters.
– Uwe
Commented Jul 30, 2019 at 20:42
• I didn't say that it was only metric, nor did I imply that the dimensions of the parts were metric. But every measure of the motion of the rover (speedometer, odometer, range) was in meters. Read the manual. Commented Jul 30, 2019 at 20:55
• I still wonder why it was that way; was it a quirk of some influential engineer/manager at the project? I heard that the German engineer Georg von Tiesenhausen conceived and pitched the initial LRV concept to von Braun back in the 50s already. I've seen comments that he was still "involved" in the development but don't know to what extent or how much of his initial work was used by Boeing. Commented Jul 31, 2019 at 6:36