Pessimistic speedo

[raceboy]

Right then heres a new one, I think my speedo is reading down, it seems ok up to about 90ish (On private land of course) it then seems to take a lot of extra speed to nudge it upto 110ish although all the other 'runway' traffic is now passing by in a blur
My only real worry is is it reading low at low speeds,
ie 30mph on speedo = 36mph on road
I've not got a GPS with a speed reading on it and it's a nightmare doing car to car comparisons, anyone got any ideas?

[q405mb]

Where are you based? I would be happy to meet with you and put my portable GPS on your dashboard...

H

[raceboy]

Cheers for the offer, but I'm in Nottingham, I can probably get my hands on a mates Origin Blue, I think they have an actual speed display on them, but if it is out, whats likely to have gone pop!

[hut49]

You could try a passenger and a stop watch on a motorway and use the white 100 meter marker posts (those that have the arrow pointing to the nearest phone). Then take the time in seconds to cover 500 meters at constant speedo readings of 50, 60 and 70 (assuming road conditions allow). To reduce the observation error, take the average of 3 readings. Calculate the meters/sec value (i.e. divide 500 by the time in seconds taken to cover the 500 meters) and then multiply this number by 2.23694 to convert to mph. That will give you the actual speeds which you can compare with the speedo readings.

Hutch

[raceboy]

Well latest update, with the aid of a GPS speed unit, the speedo is a bit out,
at 30 mph the speedo reads 32 mph
at 50 mph the speedo reads 60 mph
at 60 mph the speedo reads 75 mph
And then at all other speeds it reads 90 mph including at 132mph on the GPS (All on private land of course )
So I supose it's got to come out and be fixed then

[IPAddis]

If it reads 90mph when your actually doing 132mph, how fast were you actually going when the speedo was reading 110mph (see original post)

>> Edited by IPAddis on Monday 5th August 14:02

[raceboy]

Errrr No idea! At an actual 132 mph the speedo would bounce about a bit, sometimes reading 90, then popping round to visit 110, then back to 90, and by this time we were both laughing at the inacuracy of the speedo, and the velocity at which the other numpties which were using this private piece of tarmac were being overtaken

[rwalker1]

Raceboy, I have the same problem - the other way round. That is when my speedo is readng 70 I seem to be overtaking almost everybody. At a guide ratio 30mph per 1000rpm in 5th it worked out at 90mph actual. This is NOT GOOD and no excuse to Mr Plod when he pulls you over. I know for a fact that speedos are calibrated to read actual speed + 10% and this is a LEGAL requirement. I believe my problem (maybe yours too) is that the transponder pickup (at the diff) requires an air gap of 4 to 8 thou to the receiver on the prop. which is adjustable. I don't do climbing under cars anymore so I'm waiting for the next garage visit and using the rev counter for now.

Roar on.

[raceboy]

So is this a speedo unit problem or something oily underneath, I'm not going to be too happy if after a week off the road, dash out, speedo out, £55 to ETB to 'fix' it, and it still reads wrong! Anyone got a clue?
Ps. Currently driving very slow past all cameras/police cars as I've not got a clue how fast I'm going.

[buzzsaw]

Is 30mph per 1000 rpm in fifth anywhere near accurate? If so my speedo is reading way low as I get a reading of approx 70mph @ 3000 rpm in fifth

[rwalker1]

quote:
Is 30mph per 1000 rpm in fifth anywhere near accurate? If so my speedo is reading way low as I get a reading of approx 70mph @ 3000 rpm in fifth

Do you find that you're overtaking everybody around this speed? This is what I was told in the hallowed pages of TVRCC web. Can anybody confirm?

[JamieBeeston]

I get 80mph at 3000rpm dead on in 5th, and 10mph for every 500rpm then on.. this is on a early 98 4.5

Anyone else can confirm these figures?

[plotloss]

quote:
I get 80mph at 3000rpm dead on in 5th, and 10mph for every 500rpm then on.. this is on a early 98 4.5

Anyone else can confirm these figures?

Yes! Sounds exactly the same as my 4.5

Matt.

[apache]

3000 rpm is bang on 90 mph on my old girl

[rwalker1]

Update(maybe!) on the above. I sent off to Autocar for a copy of the Chim road test June 1993 and this says it does 27.5mph/1000rpm. This would give 82.5mph at 3000rpm. Definitely not 70mph at this rpm anyhow!

Roar on.

[buzzsaw]

Top speed of Chim 4.5 is quoted by TVR as 160 mph. If you are getting 80 mph @ 3000 rpm and 10mph per 500 rpm after that then your top speed will only be just over 140 mph (based on rev limiter cutting in at just over 6000 rpm as my 4.5 does).

[Bob C]

raceboy, the problem may not be your speedo but the sensor unit. My speedo was doing allsorts of strange things from not working at all to bouncing around all over the place (but always reading low). Changed the sensor and it was fixed.

[shpub]

quote:
Update(maybe!) on the above. I sent off to Autocar for a copy of the Chim road test June 1993 and this says it does 27.5mph/1000rpm. This would give 82.5mph at 3000rpm. Definitely not 70mph at this rpm anyhow!

Roar on.

There is a speed/rpm spread sheet that calculates in gear speeds for max revs for various tyre sizes. It will also calculate speedo errors. See www.tvrbooks.co.uk.

Steve

[pbrettle]

quote:
There is a speed/rpm spread sheet that calculates in gear speeds for max revs for various tyre sizes. It will also calculate speedo errors. See www.tvrbooks.co.uk.

Just in case anyone tried to click the link - the "." at the end of the URL means you get a blank page. Internet Explorer again (though strangely not all versions). Simply remove the "." at the end and try again...

www.tvrbooks.co.uk

Cheers,

Paul

[raceboy]

quote:
raceboy, the problem may not be your speedo but the sensor unit. - Changed the sensor and it was fixed.

Right excuse my numptyness, but where are these sensors, and how easy is it going to be to change them?