Pretty inaccurate speedo/mileometer

[.Mark]

Had the speed sensor replace probably 18 months ago or so but only really used the sat nav in the car to check it today. I know they are all out, and I knew mine was over reading, but an indicated 80mph was actually 67mph by the sat nav, and an indicated 120mph was 90mph in reality. Also I covered 38 miles according to sat nav but trip meter said 49 - so I think it is really way out.

Neither worry me too much, if it's over reading on speed then at least I am within the limits, and the 'extra' miles when the car has done 90k don't matter too much, nor am I ever going to sell so it's largely irrelevant.

I may get round to sorting both though once other things are done, so where to look? Road speed sensor - or is it just way out and needs recalibrating - if the latter - where?

[finestjammy]

Mine over reads by a similar amount. I know that my rear tyres are lower profile than the originals, so that is one factor. I belive that there is an adjustment screw at the back of the speedo on some early models.

[SMB]

.Mark said:

Had the speed sensor replace probably 18 months ago or so but only really used the sat nav in the car to check it today. I know they are all out, and I knew mine was over reading, but an indicated 80mph was actually 67mph by the sat nav, and an indicated 120mph was 90mph in reality. Also I covered 38 miles according to sat nav but trip meter said 49 - so I think it is really way out.

Neither worry me too much, if it's over reading on speed then at least I am within the limits, and the 'extra' miles when the car has done 90k don't matter too much, nor am I ever going to sell so it's largely irrelevant.

I may get round to sorting both though once other things are done, so where to look? Road speed sensor - or is it just way out and needs recalibrating - if the latter - where?

I see you have non standard wheels in the photo's, have you confirmed they are the same circumference as the originals?

[.Mark]

SMB said:

I see you have non standard wheels in the photo's, have you confirmed they are the same circumference as the originals?

Completely Bog Standard wheels on the Chimaera - I think you are looking at my Cerbera

Must change the pics, they are not that clear.

[SMB]

.Mark said:

Completely Bog Standard wheels on the Chimaera - I think you are looking at my Cerbera

Must change the pics, they are not that clear.

ok sorry didn't look that closely to see 2 cars!, that said what tyre sizes are you running

early cars had 225/55 rears, later ones have 225/50, makes some difference, but not as much as you are seeing

[.Mark]

I'd need to check. Mines a 1996, but it's as she left the factory.

[finestjammy]

SMB said:

ok sorry didn't look that closely to see 2 cars!, that said what tyre sizes are you running

early cars had 225/55 rears, later ones have 225/50, makes some difference, but not as much as you are seeing

This is part of the problem with mine, I think I'm running 245/40 at the moment compared to the standard 225/55, when they are worn I'll go back to the standard size.

[jhfozzy]

To the OP:

Strictly speaking, you're outside the limits as C&U say that speedos must overread by a maximum of 10% and underread by 0% i.e. at 80 on the speedo, the minimum your GPS should be reading is 72.

There are a few reasons why speedos overread these include:

The pickup [if it's a pickup type] being too close to the rotating ring and picking up spurious magnetic fields.

Tyres overall circumference being reduced [lower profile percentage / thinner tyres at the same profile percentage / using smaller wheels without increasing the profile.

Speedo itself going out of calibration.


If you're fussed about it and want to come back within limits, usually the easiest [but not correct] method is to increase the tyre profile if your bodywork will allow it by putting tyres with a larger profile %. for example, if you have 225/50, go to 225/55 or even 225/60.

The correct method would be to check to make sure you've got exactly the right tyre size on as it left the factory. If they are correct, then I would change parts around to see which one was faulty, speedo pod, speedo cable, pickup or drive in the box.

Edited by jhfozzy on Monday 23 May 15:34

[.Mark]

jhfozzy said:

Strictly speaking, you're outside the limits as C&U say that speedos must overread by a maximum of 10% and underread by 0% i.e. at 80 on the speedo, the minimum your GPS should be reading is 72.

There are a few reasons why speedos overread these include:

The pickup [if it's a pickup type] being too close to the rotating ring and picking up spurious magnetic fields.

Tyres overall circumference being reduced [lower profile percentage / thinner tyres at the same profile percentage / using smaller wheels without increasing the profile.

Speedo itself going out of calibration.


If you're fussed about it and want to come back within limits, usually the easiest [but not correct] method is to increase the tyre profile if your bodywork will allow it by putting tyres with a larger profile %. for example, if you have 225/50, go to 225/55 or even 225/60.

The correct method would be to check to make sure you've got exactly the right tyre size on as it left the factory. If they are correct, then I would change parts around to see which one was faulty, speedo pod, speedo cable, pickup or drive in the box.

Thanks John,
I know everything is exactly as it came from the factory so the problem is 'mechanical', I'm not about to stick new tyres on to correct it so really ought to take the other route. My favourite option is the pickup at the diff, I had the sensor replaced recently, and suspect that it's not quite within tolerance.

[jhfozzy]

.Mark said:

Thanks John,
I know everything is exactly as it came from the factory so the problem is 'mechanical', I'm not about to stick new tyres on to correct it so really ought to take the other route. My favourite option is the pickup at the diff, I had the sensor replaced recently, and suspect that it's not quite within tolerance.

There is currently a problem with sensor tolerance in the 6 speed ford transit.

If you get one fitted with a tachograph and rev the engine whilst stationary, the speedo will go up to about 10km/h. The only way [we've found so far] to correct this is to slide a 1mm washer over the end of the sensor and screw it back in therefore spacing it out to the correct amount.

That's only a solution obviously if the chimaera sensor is mag pickup / hall effect [I'm not sure which type is fitted]. If it's rotary you could possibly have had a sensor with the wrong amount of pickups. You can get sensors with 1 pulse per rotation up to ones with many pulses per rotation like Tachograph rotary sensors which are 8 pulses per rotation.

[.Mark]

The sensor works off the diff, a kind of castellated affair that the sensor - well - senses. I know the gap is quite key so I need to get it checked.

Anyone know what the gap should be?