How inaccurate is GPS? It seems like for his purposes a variance of even +/- 5 mph could be tolerated given one can generally sense approximately what speed they're traveling and can also compare themselves to other cars on the road.

problem is usually line of sight to satellites, it varies a LOT and usually leads to variance in calculations.

I'd expect it to be better than the car speedo. Car speedos typically overread by up to 10%, and if you fit a non-standard tyre, they can be way off.

GPS is susceptible to atmospheric conditions, obstructed views (4 satellites minimum required for positioning) and a number of other factors, most of which are undetectable at the device level. Car speedos are done with over-read deliberately as a safety measure. Better overestimating than under. As a driver you can make conscious decisions about what speed you're driving based on all evidence available, including an understanding of how over/under your speedometer returns results. With a GPS based speedometer you can't realistically do that, you can have no idea what the accuracy percentage is at any particular moment in time. It might be over, it might be under. No one can make good decisions based on junk data.

Soeedometers perform the most precise measurement you can get. When you fit a nonstandard tire you just introduce a constant scaling factor. That is not so bad. GPS on the other hand is atrocious. Don't even try to get an accurate position in places like downtown Seattle. I'm regularly getting misplaces by several blocks because of occlusion and reflections from the buildings. You just can't compute an accurate speed from such a signal.

Besides, GPS can only update once a second. And given the signal jitter you can't get a good speed reading from the last two fixes - when you are going in a straight line. When you are not going straight, the GPS based estimate must underestimate the distance you travelled and thus your speed.

Using this toy speedometer thingy for driving is just dangerous and insane and illegal in so many ways that I wish the article could just be pulled from the web.

I might have this wrong, but I seem to recall reading somewhere that speedos in the EU have to overread by some random amount, with the aim of curbing speeding.

the second generation Prius 2004-2009 has a dashboard instrument cluster flaw where the capacitors go bad after a number of years and it stops working.

this is well known in the prius chat boards.

Toyota also knew about it, but instead of making it a safety recall, they put out a service bulletin: T-SB-0172-09 Combination Meter - Intermittent Display

If you search the web for that you will find a ton of hits.

The solution is to replace the entire instrument cluster circuit board. If you have a replacement board, It's relatively easy, maybe an hour of work if you are handy, and there is no soldering unless you have no replacement and want to replace the bad capacitor yourself, and maybe a transistor. You will probably break your A/C vents in the process because they are made of a type of plastic that becomes extremely brittle with age.

There is a guy in Texas, Texas Hybrid Batteries, who will sell you a refurbished dash circuit board for $150 if you mail him yours. Buying a new one is a bit spendy and may require going through a dealer depending on laws, since the odometer is part of the assembly and has to be 'flashed' to what your vehicle was before the swap. However on cars of this age the odometer is often up to about 200,000 miles and exempt from a lot of the odometer laws.

Is it really that scary? Ok maybe it isn't perfectly accurate but so what? He might be going 33 instead of 30..doesn't really matter.

There's a whole other argument to be made about the quality of a speedo/dash that fails function so soon.

If our drivers were smarter, we wouldn't need speedometers at all. Nobody thinks "I'm going too fast" then looks at the speedometer and decides "Oh, my speed is fine".

Drivers who want to go fast will go fast - for them the gauge is only useful so they can slow down for speed traps.

Not having a speedometer shouldn't make a sensible driver suddenly very dangerous at all.

> Not having a speedometer shouldn't make a sensible driver suddenly very dangerous at all.

of course. Any sensible driver will not drive the car without a speedometer.

Why not? Do you think any sensible driver would overshoot/undershoot their speed by any meaningful amount without noticing? Like without a speedometer everyone would just accidentally go 100mph everywhere or something?

> of course. Any sensible driver will not drive the car without a speedometer.

I think you missed the point of my comment entirely.

Knowing your speed is not required to be safe while driving - being observant, correctly signaling, and driving at a speed appropriate for the conditions are.

And many countries are authoritarian shitholes who care more about revenue generation than protecting people. People don't need a speedometer to drive safely.

> People don't need a speedometer to drive safely.

Yes, they really do. Just ignoring the fact that people tend to drive faster when they don't have a speedometer, people already travel too fast for road conditions as a matter of course. Let alone the fact that at residential road speeds, the difference in life expectancy of someone hit by a vehicle drastically changes with even a slight increase in speed.

Safety checks exist because vehicles are roving machines of death driven by easily distracted people, who have a long track history of ignoring what they can't see.

Your life shouldn't be put in danger because some other arsehole decided that it's perfectly fine to drive a rust bucket with nothing more than a sheet of paper left by way of brakes, or with lights that are so badly adjusted anyone else gets blinded, or with windows in a state that someone sneezing will shatter them, or any other number of things.

But yeah, authoritarian government overreach. Sure.

Chicago is a good example. Nobody follows the speed limit, especially on the express ways (often 30+ over). Going with the flow of traffic is key.