Rick's b.log - entry 2020/06/15

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FYI! Last read at 22:12 on 2025/04/09.

GPS speedometer C80 review

I used the speedometer on the drive to work, and back, today. Here was it in use this morning.

Note that the indicated speed of 49kph isn't what the needle says, that's showing just a mite over 45.

Time to obtain a location fix

For the first fix, in the morning with the device unpowered all weekend, it took a little under a minute.
Leaving work, the device unpowered for about eight hours, about 40 seconds to get a fix.
Leaving the supermarket after a half hour of shopping, about 10 seconds.
Clearly the device is caching the satellite ephemeris and almanac data to speed up determining satellite visibility in order to obtain a fix.

Accuracy - general

It appears as if the speedometer generally over-reads by 2-3kph (as compared with my phone). I reconfigured the device to drop 2kph from the indicated speeds. It now slightly overreads, but by and large is mostly within 1-2kph of the indication on my phone.
Both are actually notably different to the analogue speedometer. As I said previously, it's more or less bang on at 30kph, but getting progressively worse as speed increases. This is important if I'm "doing 42" and trying to do 45 so as not to overly annoy traffic, and it turns out that I'm actually doing 47. Because, well, that's what both of my GPS speedometers are telling me!

Accuracy - glitching

My indicated speed bounces like a yoyo for the first minute after a fix. This is probably because it starts with a few satellites, and gets more accurate as more satellites are locked in to.
From that point, on clear open roads it seems fairly stable, but it doesn't perform any averaging of readings, so if you're on a borderline between two speeds, expect it to bounce from one to the other depending upon what the receiver determines from one second to the next.
In the forest, well, expect it to get a little weird. Doing a fairly constant 47, it dropped to 44 and then hit 52 (beeping at my speeding) before going back to 47. This probably shouldn't be seen as unusual behaviour as the receiver may have difficulty depending upon what its visibility is. I don't know if it is purely GPS, or if it can also lock in to GLONASS (Russia), BeiDou (China), and Galileo (EU). Nor do I know if it is capable of using the L5 band to 30 centimetre accuracy (the older L1C band has an accuracy of about five metres).

Visibility

Even with the display dimmed (the mobile holder I'm using obscures the light sensor for making the display brighter), it was easily visible in daylight with sunglasses on. There wasn't direct sun on the readout, so I can't tell you if it would still be visible in that case. However I had no problems glancing over to the device to see my speed.

Ease of use

It's wired up to a USB output on a multiway adaptor that splits the car's 12V to a dashcam and a charger for my phone. This gadget has a switch on the side (to turn all the stuff on and off). So once the configuration has been done, it's pretty much as case of "set it and forget it". I flick the dashcam/charger on, the GPS speedometer powers up to, and soon enough it's showing my speed.

Configurability

The configurations are a bit weird. The speed limit (#001) setting is pretty easy, it sets a maximum and the device will beep above that. I drove at 51 (downhill!) and it did indeed beep, though a lower pitched beep than I was expecting.
As it looks as if the device is set to slightly over-read the speed, it is good to have a calibration option (#002) to bring it closer to reality.
The altitude calibration (#003) is utterly bizarre. What is the range 750 to 950 (default 850) supposed to mean? It seems to be giving a mostly-correct reading for me now, but I can't tell you how I managed this. Just trying various different values until one "looked right".
The time correction (#004) is peculiar in that each number is an adjustment of 30 minutes, and the values are back to front. In order to add two hours to UTC to get CEST, I've set the adjustment to -4. That's 4 × 30 minutes.

A nice touch is that the device can have all the extra information turned off to show only the speed. In case the other info is distracting.
Another nice touch is the light sensor for automatically adjusting the display brightness.

If you don't fancy trying to remember all of the button presses to make things happen, fear not. It's all printed on the back.

GPS speedometer setup reminder and light sensor

Other possibilities

It will run off a generic 5V USB supply, so it can be used on... well... pretty much anything that moves. A truck, a speedboat, a bicycle, a lawnmower...
The device appears to take very little power, so it could be powered from a power brick battery.

Measured current consumption of GPS speedometer

Summary

When I'm not certain that I can trust my speedometer, this GPS speedometer, once calibrated, seems to be a useful alternative. It's immediate display of the GPS determined speed does mean that you might need to do the speed averaging and bogus-value rejection mentally, but - let's be honest - you're supposed to be staring at the road and not the speedometer. It's there to indicate your speed when you need to know.

GPS speedometer on my dashboard (this time not moving!)

I can see some ways that it could be improved, however it is entirely functional for a €22 device. I don't have any complaints.

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David Pilling, 25th June 2020, 16:02

Interesting to read about the GPS speedometer. I think you're going to have to find a measured mile on a dead straight road to settle the issue of which is correct.

Rick, 25th June 2020, 17:28

Oh, it's sorted. The speed adjustment is from -20 to +20 with a default setting of 5.

It's probably linear, so barely different at 30, a little off at 45, and likely the indicated off at normal car speeds.

Once the penny dropped, I set the offset to zero and it's now bang on with two local speed sensor gizmos.

Today, being flippin' 30°C (!) the GPS read 48. The needle pointed to about 43. Oh well, at least I know I have something that tells me how fast I'm really going. ;-)

Wookiee, 5th July 2020, 13:10

DP is correct about using a dead straight road to calibrate your GPS speedo, but ideally it needs to be horizontally flat as well, otherwise the distance you travel and the distance the GPS calculates could be different. It all depends if the GPS unit takes account for altitude changes as well.

Rick, 5th July 2020, 17:17

It does track altitude, and it even manages to be less wrong than my phones.
I don't need it to be EXACT. So long as it reads 44-46 when I'm doing 45, it'll be more correct than the moving needle in hot weather (it'll say something closer to 40-42).

Also, I should remember that most speedometers are usually set to read slightly faster than is true, so it's fine if I'm doing 45 and to car behind sees it as 46. ;-)

I just want to know that I'm actually doing 45...

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	Rick's b.log - 2020/06/15
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Rick's b.log - 2020/06/15