Speedo gauge question..

[dirtydirtyharry]

When I got my 944 and first drove it I felt as if I was at the speed limit but glancing down at the speedo showed that i was doing less than i thought. I put this down to the fact that the 944 is much lower than my old celica and therefore it feels as if you are going quicker.

However I have noticed that when I drove through those road signs which state what speed you are doing that they seem to warn me of my speed even when say, im doing 35 in a 40.

Im aware that modern cars 'over-read' by 10% when it comes to speedo's, ie, the gauge states you are doing 50 but in reality you are only doing 45 (a system which helps prevent car manufacturers being held liable if the speedo drifts slightly and the driver ends up suing them after receiving a speeding ticket).

If you use a sat nav system or speed camera detector, a lot of them will state your true speed (those who use them with modern cars will notice this 10% difference)

HOWEVER, Im beginning to wonder if my 1986 944 speedo is actually set to the true value unlike more modern cars.

Does anyone on here use a gps device which monitors your current speed. If so please would you let me know if it tally's up with the speedo or if the 10% difference still exists.

Many thanks. I dont want to risk driving with the speedo on 77 if it means that im actually doing 77 as opposed to 70! []

 

[mr brightside]

Unusual wheel or tyre size can make a difference but from the look of your car they're about normal.

 

[Riverside]

It varies from car to car on new ones, let alone 20 year old electronics, but mine is pretty optimistic at low speed and correct at 70mph.

From experience with the tom-tom in my car I know that 2000rpm equals 20mph in 2nd, 30mph in 3rd, 40mph in 4th & 50mph in 5th, any faster than that & I trust the speedo. My gearing is taller than yours but once you have done the comparison & mentally 'calibrated' the speedo you'll feel more confident you are at the correct speed.

 

[JamesO]

My 82 944 was bang on with the gps but my 89 has a slightly more optimistic speedo, prob about 3-5% out.

 

[Fred Hindle]

Both my 944 (1984)and my 924S (1986) speedos are different to GPS readings. Not quite ten percent so if the speedo reads seventy five then my GPS speed is seventy.
Call me sad but I have actually tested using two GPS enabled PDAs simultaneously, using different software and both gave the same speeds. If you want to calibrate then best find a road (I use a quiet motorway) so that I can maintain a constant road speed for a period of time to allow for signal transmissions etc.

Cheers,

 

[Alexw]

I know mine is bang on for 30 and 40 as there is those speed checker jobbies between me and work and i use them to calibrate as i don't have a GPS []

 

[zcacogp]

The degree of variation will vary with speed - both the true variation and the percentage variation. I know that my S2 over-reads in some places and reads accurately in others (calibrated from a GPS), but I can't remember where which happens.

Don't trust the speed read-outs on the roadside things. They are notoriously inaccurate (probably more so than your speedometer.)


Oli.

 

[dirtydirtyharry]

ORIGINAL: zcacogp

The degree of variation will vary with speed - both the true variation and the percentage variation. I know that my S2 over-reads in some places and reads accurately in others (calibrated from a GPS), but I can't remember where which happens.

Don't trust the speed read-outs on the roadside things. They are notoriously inaccurate (probably more so than your speedometer.)


Oli.

I second what you say about the roadside gauges - they are very inaccurate. It depends how they have been calibrated (if calibrated against gps then they will be more accurate than if they are calibrated against a speedo for example).

The last 3 modern cars (after 1990) which i have owned have all over-read by 10% (whatever speed you are travelling) ie 33mph when you are doing 30 and 66 when you are doing 60.

I would just love to know if anyone which 2 cars (ie an old 944 and a more modern vehicle) find that their gps system indicates a greater mileage discrepancy for the newer vehicle.

 

[zcacogp]

DDH,

Good Q about the time-variation.

Speedometers tradionally work by spinning a magnet (connected to the wheels) behind a sprung aluminium bar. The aluminium bar had eddy currents generated in it, which dragged it 'round against the spring. The faster the magnet spun, the more the alu bar was dragged and hence you had a speedometer. These under-read more and more as they got older as the magnets lost their strength over time and hence didn't 'pull' as much.

However, I think the speedometers in the oval-dash 944's are electronic, so this won't apply.


Oli.

 

[xenon]

ORIGINAL: zcacogp

However, I think the speedometers in the oval-dash 944's are electronic, so this won't apply.

Yep, Hall-effect sensor on the n/s driveshaft output.

 

[zcacogp]

Which should mean that their accuracy doesn't change with time.


Oli.

 

[ChasR]

Well, I checked mine on the way back from Derby and that is bang on speedo wise!