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Old 10-01-2019   #1
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Speedometer vs. satellite speed measurement

I thought it was worth expanding what I've been saying about speed of late. I'm assuming that as you can get add-on devices to measure car speed these must be considered an accurate way to measure progress. So with changes to my gearbox I'm now doing approximately 54mph when the speedo reads 50. I covered 86 miles when the odometer read about 80. So I reckon the car reads about 8% under on both counts. (Mathematicians please correct this). I'm now wondering how accurate the speedo was in the original and then the standard 650 gearbox incarnations? Maybe I've done more miles than I thought. I'll need to adjust things in future and I suppose it will change again with slightly wider and different profile tyres? My calculations are important...... we're talking money here.....my company mileage claim. Also worth remembering in respect of speed cameras.
Does anyone else know how closely their speedo compares with reality?
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Old 10-01-2019   #2
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Re: Speedometer vs. satellite speed measurement

I usually apply the philosophy that if I am traveling faster than the pedestrians then I'm moving... that is an achievement...
If I am overtaken by a bus, then I must be within the speed limit...

If my speedo indicated more than 30 mph then either I am going down hill, the wind is behind me and it must be a storm so stop, or I'm being towed by a 2cv.

Speed is never a problem in a Prototypi!
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Old 10-01-2019   #3
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Re: Speedometer vs. satellite speed measurement

Hi Peter

Yes, I've done the speed comparison and my speedo seems to be pretty accurate. However, I do have an Abarth style array rather than original.
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Old 11-01-2019   #4
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Re: Speedometer vs. satellite speed measurement

The local police have placed one of their radar boxes to monitor speed through a road close to our home so, as an exercise, I drove my car past it at a steady speed of 50kph and the screen on the box showed 30mph and a smile . I don’t know how accurate these boxes are but it seemed happy with me and indicated that my speedo reading is reasonably accurate
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Old 11-01-2019   #5
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Re: Speedometer vs. satellite speed measurement

Mine was so far off I just used other traffic as a guide. Faster than them was 'too fast' (and almost impossible)
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Old 11-01-2019   #6
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Re: Speedometer vs. satellite speed measurement

Your speedo is driven from the output to the wheels and is affected by tyre diameter. When built, the regs said it had to be within 10% above or below. Current regs demand +10% to -0, so can read above actual, but never below.
A satellite readout will be very close, but is affected a little by hills, as it measures your progress along the earth's surface, and hills may give a slightly slower reading. Doubtful many satellite receivers will calculate for that.
If you've got a satellite readout, use that and you'll stay out of trouble.

Quote Originally Posted by IanEmery View Post
The local police have placed one of their radar boxes to monitor speed through a road close to our home so, as an exercise, I drove my car past it at a steady speed of 50kph and the screen on the box showed 30mph and a smile . I don’t know how accurate these boxes are but it seemed happy with me and indicated that my speedo reading is reasonably accurate
The roadside radar boxes should give an accurate reading. They should be calibrated regularly, usually at least once a week.

Some may be interested to know that, our local ones at least, are calibrated using a tuning fork. It is struck and held in front of the radar, and vibrates at a rate corresponding with certain speeds. The local guy I spoke to had a box with forks for 20, 30, 40 and 50mph. I'd passed his pair, once in each direction and got different readings. Together we went and calibrated them. An interesting time.
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Old 11-01-2019   #7
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Re: Speedometer vs. satellite speed measurement

Quote Originally Posted by portland_bill View Post
A satellite readout will be very close, but is affected a little by hills, as it measures your progress along the earth's surface, and hills may give a slightly slower reading.
I've always thought that would be the case but not an issue when I'm talking about personal bests.

Interesting anecdote about your interaction with the calibration man; I would never have dared to get involved but it sounds very interesting.
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Old 12-01-2019   #8
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Re: Speedometer vs. satellite speed measurement

If 'fiat500's' speedo had been reading too fast, I might have suggested it was possibly due to wheelspin.

Maybe a slight repositioning of the speedo needle would allow the speed reading to be corrected?

There are companies advertising in the UK car mags who claim to be able to re-calibrate speedos e.g. following a change to final drive ratio. I don't know if they cater for Fiat (Veglia-Borletti?) speedos or if the cost might be prohibitive (sometimes they have to make new drive gears).

If the OP plans on changing tyres, he might, by careful choice of size, correct his current speedo inaccuracy by comparing rolling radius/turns per mile of possible tyres.

As a point of possible interest (to some ), Smiths speedos have a number in small print at the bottom of the dial which indicates the number of turns of the speedo cable per mile. Iirc, if you drive the speedo at this speed, it should show 60 mph.(1 mile per minute). (you could use a variable speed drill and a digital shaft speed indicator for this).Fiat speedos have this turns per mile number inked? on the rear of the speedo casing.

AL.
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Old 12-01-2019   #9
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Re: Speedometer vs. satellite speed measurement

Quote Originally Posted by F123C View Post
If 'fiat500's' speedo had been reading too fast, I might have suggested it was possibly due to wheelspin.
.

My car is very capable but wheelspin is well out of its range of talents
I'm not looking to correct the speedo as it does still function as a gearchange indicator because the red markings are still applicable. If I corrected it using appropriate, smaller tyres the car would look even sillier than it does now and would remove the benefit of the higher c/p ratio which has given me the benefits but caused the issue.
It looks like I've just used up the built-in 10% optimism the speedo had from new.
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Old 12-01-2019   #10
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Re: Speedometer vs. satellite speed measurement

GPS will give a very accurate speed reading if you have been driving in a straight line on level ground for at least 3 seconds. It will be within the 1MPH resolution of a car GPS main display if you have been at steady speed for 2 seconds under most road conditions.

The reason for the time requirement is that normal car GPS receivers only update once per second. Most car GPS receivers have a satelite data screen that displays speed to 0.1MPH and this
is accurate given the straight, level and 3 second rule above.
Specialist GPS units for measuring car performance have receivers with 5, 10 or 20 updates per second giving much better dynamic performance.
We considered fitting a GPS speed indicator to Thrust SSC but there were two problems. First was "Selective Availability" that degraded accuracy. This was switched off years ago. The second, bigger problem was that the units had maximum speed limits to meet US export regulations so they could not be used in weapons. At the time the limit was lower than the cars speed. Another bother was that most lower cost units (<£500 back then) had an even lower speed limit of around 105 MPH so they could not be used in aircraft in plac of the much more expensive aviation units. I still have my Garmin GPS II that I bought in 1996 (£249 IIRC, a lot for me at the time).

Robert G8RPI.
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