Speedo accuracy (Not)
Discussion
I have always been a bit worried about the speedo so the other day I went for a run with the sat nav in.
The results were as expected except that it seems to be 8-12 mph fast at all speeds.ie 20mph true reads as 30mph on the speedo & 100mph reads as around 110mph (Private road of course!)
I would have expected the error to increase with speed but this seems not to be the case.
Is there anything I can do in the way of adjustment to sort it out?
It's one of the early black rimmed speedo's if that makes a difference.
The results were as expected except that it seems to be 8-12 mph fast at all speeds.ie 20mph true reads as 30mph on the speedo & 100mph reads as around 110mph (Private road of course!)
I would have expected the error to increase with speed but this seems not to be the case.
Is there anything I can do in the way of adjustment to sort it out?
It's one of the early black rimmed speedo's if that makes a difference.
My speedo was replaced at the TVR factory in 2004 and when checked against my Road Angel reads 20% faster right through. I posted a thread about a year ago about this and one of the answers I got was that my new speedo was geared differently for a later car,(my car being a 94 Griff) It's also worth noting that the mileage will also be 20% more also. Quite important whilst trying to keep miles down within the confines of a limited mileage insurance.
Nobody's mentioned rear tyres/wheels yet, and they can have a direct impact.
If the overall rolling diameter is different to the original size then this will have an impact on the accuracy of the speedo reading.
Though, I guess that to get a difference of 20%, you would need to have considerably larger/smaller diameter - and the difference in readings caused by this would also be proportional to speed.
So I suspect that this may not be the only factor in some of the posts above. Worth mentioning, though...
If the overall rolling diameter is different to the original size then this will have an impact on the accuracy of the speedo reading.
Though, I guess that to get a difference of 20%, you would need to have considerably larger/smaller diameter - and the difference in readings caused by this would also be proportional to speed.
So I suspect that this may not be the only factor in some of the posts above. Worth mentioning, though...
Chim Chim Cheree said:
Nobody's mentioned rear tyres/wheels yet, and they can have a direct impact.
If the overall rolling diameter is different to the original size then this will have an impact on the accuracy of the speedo reading.
Though, I guess that to get a difference of 20%, you would need to have considerably larger/smaller diameter - and the difference in readings caused by this would also be proportional to speed.
So I suspect that this may not be the only factor in some of the posts above. Worth mentioning, though...
If the overall rolling diameter is different to the original size then this will have an impact on the accuracy of the speedo reading.
Though, I guess that to get a difference of 20%, you would need to have considerably larger/smaller diameter - and the difference in readings caused by this would also be proportional to speed.
So I suspect that this may not be the only factor in some of the posts above. Worth mentioning, though...
but to make the speedo read 10% faster the tyre/wheel diameter needs to be a lot smaller. From 18" to 16" is only 7.1% faster reading (+ normal 10% = 20% as said earier). See here
Having been through this a few years back I can understand the confusion. The TVR speed set-up reads speeds with a margin of error. The law states that this should be at least 10% and that the speedo should never read less than the speed of the car.
At slow speed the speedo is very inaccurate, can be as much as 50%, this improves at high speed as the voltage signal from the sensor gets better. but the error should never be less than 10%.
The sensor can be moved closer the the trigger wheel, and the age of the car will dictate where the trigger wheel is, diff or drive shaft.
Unfortunately, in my experience, this did not solve the problem. Mine was over reading by a hell of a lot, at 120 on the motorway I was being over taken by trucks! A bit of an exaggeration, but you get the picture.
The only way round this, having changed the sensor and still no joy, was to send the speedo off to Speedy Cables, and have them recalibrate it. It came back with an adjustable circuit that I could play with when out with my GPS. After some fettling mine now reads 10% error at all speeds down to about 30mph, below that its rubbish!
At slow speed the speedo is very inaccurate, can be as much as 50%, this improves at high speed as the voltage signal from the sensor gets better. but the error should never be less than 10%.
The sensor can be moved closer the the trigger wheel, and the age of the car will dictate where the trigger wheel is, diff or drive shaft.
Unfortunately, in my experience, this did not solve the problem. Mine was over reading by a hell of a lot, at 120 on the motorway I was being over taken by trucks! A bit of an exaggeration, but you get the picture.
The only way round this, having changed the sensor and still no joy, was to send the speedo off to Speedy Cables, and have them recalibrate it. It came back with an adjustable circuit that I could play with when out with my GPS. After some fettling mine now reads 10% error at all speeds down to about 30mph, below that its rubbish!
I was recently suprised how close to a GPS mine was reading 60 on speeedo to 57-58 on GPS, although at the time the sensor gap was too big and below 40 it read nothing (now adjusted). On mine the gap has to be very small for it to pick up anything at low speeds (currently set with feeler guage at 0.25mm), and it reads at speeds above 20 now, if I'm doing less than that I really don't care.
Quinny said:
My locks flick around when the blower fan is on
Does that count??
Really? Does that count??
PP: Maybe degreasing the crown wheel and cleaning the sensor would help, checking the resistance of the wiring from speedo to sensor might also help. Don't forget your car should have the gauges that can be calibrated thru the rear of the speedo. Tyre RR is not exact ~ maybe yours are just way out of range due to size / stiffness / pressure...
Aha seen this kind of post before 
Its also pretty common amongst people with classic cars...
I own an Etype Jaguar which is over 40 years old so the speedo is not as good is it could be although it *has* been recalibrated. It also continually flicks from zero to 25 when travelling below 30 but thats a cable issue.
Sorry, I diverge... this is the Cnimeara forum so enough about classic sportcars. I am posting here because i am in the process of buying a Chim and I am reading EVERYTHING to do with the cars
Back to the point:
What I did in the Etype was use a satnav to hit the various speed limits and then look at the rev counter whilst ensuring I was using an appropriate gear for the speed. Right gear and right revs = correct speed.
So you can pretty much ignore the speedo. Some people also place a small marker on the rev counter glass.
Just an idea to keep the boys in blue at bay.
I look forward to posting pics of my new purchase (dont shoot me but i like the cars with modified lights...)
Cheers
Its also pretty common amongst people with classic cars...I own an Etype Jaguar which is over 40 years old so the speedo is not as good is it could be although it *has* been recalibrated. It also continually flicks from zero to 25 when travelling below 30 but thats a cable issue.
Sorry, I diverge... this is the Cnimeara forum so enough about classic sportcars. I am posting here because i am in the process of buying a Chim and I am reading EVERYTHING to do with the cars
Back to the point:
What I did in the Etype was use a satnav to hit the various speed limits and then look at the rev counter whilst ensuring I was using an appropriate gear for the speed. Right gear and right revs = correct speed.
So you can pretty much ignore the speedo. Some people also place a small marker on the rev counter glass.
Just an idea to keep the boys in blue at bay.
I look forward to posting pics of my new purchase (dont shoot me but i like the cars with modified lights...)
Cheers
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