Thread: GPS Speedometer?

You can try a prototype I was working on, it's called NMEATest and is here http://www.manwhoring.com/mp3car/ukchris/

I have a delorme GPS and Street atlas 2k4, and I've found the speedometer in 2k4 to be a bit 'iffy'.

From 0 - ~60 mph the GPS speedometer matches my car's almost exactly.

From 60 - 70 mph, the GPS reads about 4 mph slower than my car speedometer
From 70 - 80 mph, the gps reads about 7 mph slower than my car speedometer.
From 80 - 90 mph the gps reads about 12 mph slower than my car speedometerr.

I haven't been much over 90mph.

At first, I thought it was my speedometer, but through extensive ghetto-testing (involving my wife following me and 2 cell phones) it appears that my car speedometer is correct ( +/- 1-2 mph).

Anyone else have any problems? This is over a fairly long sample, with the cruise control set in the car, the gps speed is consistantly off at higher speeds.

Or your wife's speedo is just as bad...

How about a timed mile at 90mph? I'd trust the GPS personally, I had heard in the past that car speedos intentionally read over to try and keep you at a more sensible speed - who knows!

Chris.

I've found the same using a road angel (Speed camera GPS databse device) and the GPS speedo is more accurate than the cars. for example... (roughly what i remember)

My old MR2
Car: 30 GPS: 27
Car: 40 GPS: 36
Car: 80 GPS: 72

New Focus
Car: 30 GPS:29
Car: 40 GPS:39
Car: 80 GPS:79

I've also went through a few speed cameras that are dead on the mark trusting the GPS and although the MR2 said I was doing almost 60 the GPS said 50 and I didn't get flashed, so I would go with the GPS rather than than the speedo

ALso remember the speedo can go out by a big amount just by putting the wrong tyres or larger wheels on the car, I too have heard that car manufacturers make the speedo error read over rather than under, maybe this is to compensate for tyre wear?

Also remember the mechanical speedo with work ona logorithmic scale so a little error at 30 will be a massive error at 120 (not that i'd know)

i use "GPSthing" for my digital speedo....search the forums for the link.

couldn't find it. do u have a link available?

Actually, I read an article some time back about the 'accuracy' of speedometers.


The government has strict laws as to the tolerance a speedometer can have for error. Depending on the country of course. This article also showed which cars had the most accurate and least accurate speedometers. The United States gives a certain amount of give or take (+ or -) on their speedo accuracy for the manufacturers. Germany, however, has only a tolerance for overexagerating speed. There is no tolerance for underexagerating (saying your are going slower then you really are).

So with this in mind they tested Japanese, American and German cars and found actually that American cars were the closest (surprising) while German cars were the furthest off, but overagerating only.

Of course, I don't have the article in front of me and I don't have all the specifics, but that was the gist of the article.

Oh, BTW... gas gauges lie too! By design.

Ok easy fix. Install a program called GPS toolkit, download it from download.com. Run the GPS testpanel, attached to this post, these are both freeware and the testpanel will get speed on any NMEA GPS receiver. I have tested it and it is a couple kph off but close.

Originally Posted by aphesis

couldn't find it. do u have a link available?

http://home.comcast.net/~danielsoh/files/gpsthing.zip