Speedo reading

C

catboy

Member
Hi all

The Guvnor is now on the road,however the speedo under reads by 5mph. Eg at an indicated 30mph it's 35mph on the sat nav

This is against the sat nav and is the same whether at 30mph or motorway speeds.

The tyres are the correct size so not sure what to do.

Is it a fault or does it need calibrating?
 
If it overreads by 5mph at 30 and at 70 it won't be a gearing fault so recalibrating the speedo won't cure it.
 
catboy said:
Sorry Harvey my mistake it under reads so it's actually going faster than the speedo reading
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It makes no difference, if it undereads by 5mph at 30, and 5mph at 70, it won't be calibration.
 
Don't trust you nav for a accurate speed because it is NOT accurate.
they are usually out by 3-5 mph
 
Satnav speeds are calculated by assuming everything it horizontal and I think it only checks points every few metres so a curved road will affect it too.

How does it read compared with those speed displays in built up areas?
 
Sat Navs are really accurate and the rate of sampling means they are almost totally consistent.

Easy to test this in a car with cruise control which is determined by the distance actually traveled through the wheels. Speed reading doesn't change around a bend at all in my experience.

My old Becker Traffic Pro also used the GAL signal from the car speedo - it therefore worked underground :) It probably needed it as that 1999 piece of technology needed up to an hour to get an initial fix on the satellite.
 
Just follow a friend who has a newer car and compare the readings by phone, hands free of course
 
catboy said:
Hi all

The Guvnor is now on the road,however the speedo under reads by 5mph. Eg at an indicated 30mph it's 35mph on the sat nav

This is against the sat nav and is the same whether at 30mph or motorway speeds.

The tyres are the correct size so not sure what to do.

Is it a fault or does it need calibrating?
Click to expand...

If your distance recorder is correct and you only need to rectify the speed indication, you could take the speedo apart, and turn the needle clockwise on its spindle (you have to hold the drum behind the face steady to be able to do this) to correct the reading.
 
Sat navs are a lot more accurate than speedometers, on old or new cars.

Cars have to allow for tyres beings under-inflated, worn or the wrong size and mustn't under read.

Sat navs do not have to allow for any discrepancies with the vehicle so can be made to be accurate.

Richard
 
Hi All

Checked the car today with another sat nav and it's under reading by 3mph from 30-to very high speeds

Maybe the solution is to move the needle, if I do this will the needle still go to 0mph ?
 
To get an accurate check, look for one of the government sponsored yellow boxes in a 50mph limit, drive past it at speeds increasing in 1mph increments from 50 mph until you see a flash. You will then get an accurate reading sent out in the post, but this incurs a £60 fee.... :LOL:
 
catboy said:
Hi All

Maybe the solution is to move the needle, if I do this will the needle still go to 0mph ?
Click to expand...

Yes, there is a light spring that pulls the needle (and the drum behind the face) to rest against a small post at 0.
Keep in mind that these are delicate stuff though!
 
Try the same nav in a different car first.
Eg. I run 3 cars aside from my P6's.. 05 Toyota amazon, 09 Cls 63 and a 04 Slk 55.
A varied mix I think you would agree. Two different navs used, Garmin and TomTom all read higher than each one of the cars.
Old bill traffic have navs that are highly accurate due to them having several satalites at any one time pin pointing they're position.
Keep it simple and do a quick comparison if I were you.
Good luck
 
Thanks all,

Think I'm going to run her as is for a while then check her later to see if things improve.

She has been standing for 20 years
 
John said:
To get an accurate check, look for one of the government sponsored yellow boxes in a 50mph limit, drive past it at speeds increasing in 1mph increments from 50 mph until you see a flash. You will then get an accurate reading sent out in the post, but this incurs a £60 fee.... :LOL:
Click to expand...
Wonderful!

Removing the instrument fascia you can gingerly lift the speedo needle past its stop pin and see where it rests. From memory there are two tiny marks on the dial's outer rim indicating where it should sit given factor A and factor B (ie. someone told me what they are and I quickly forgot). If it doesn't read right after some use maybe you could play with it thus. Or clean and lube the drive bits.
 
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