Speedo innacuracy...

[MGZRod]

You all know how the higher your speed, the needle will 'overshoot' a bit. I.E 180 indicated could be about 170 odd.

Does this reflect onto the revs aswell? would 6700 rpm really be around 6400rpm or suchlike?

I don't know how these things work so don't look down on me and I hope it made sense

[arcticgoldfish]

No. The speedo offset is because legally, it cannot underread. There is an offset to allow for tyre wear, tyre growth at speed etc. You don't get those variables with engine speed.

[kambites]

I would expect the rev counter to be pretty accurate and on my Elise the ECU readout certainly matches the needle position, which is not true of the speedometer.

[Turkey]

Rod on my MG, it over reads by about 100rpm at 2000rpm, 200rpm at 3000rpm and 300rpm at 4000rpm, so not a linear % over. The rev limit is 4700rpm, but the rev counter will read over 5000rpm at these revs.

Mine is a Diesel, if yours is a petrol it might be more accurate.

I suspect there will be a lot of variation on different car models and makes.

[thebullettrain]

My speedo and sat nav disagree with each other by 5 mph!

[Petrolhead_Rich]

thebullettrain said:

My speedo and sat nav disagree with each other by 5 mph!

As stated, all speedo's read slightly higher (usually 4-6mph over)

Don't rely on it reading high though!

[Jocal]

arcticgoldfish is right here. The change in rolling circumference for tyres has to be accounted for. The same thing for when you buy new tyres and wheels, it can change the rolling circumference, therefore the seedo. This happened with my Beetle years ago.

[mrmr96]

Turkey said:

Rod on my MG, it over reads by about 100rpm at 2000rpm, 200rpm at 3000rpm and 300rpm at 4000rpm, so not a linear % over.

It's over-reading by 10% AND your needles not on straight, it's too low by 100rpm. Therefore I reckon that at 1,000rpm it should be bang on as the two effects cancel out.

[CraigyMc]

doogz said:

I broke my fancy multimeter on a rather powerful electric fence though, and my new one doesn't have that feature.

Demands further explanation, shirley?
C

[varsas]

Turkey said:

Rod on my MG, it over reads by about 100rpm at 2000rpm, 200rpm at 3000rpm and 300rpm at 4000rpm, so not a linear % over. The rev limit is 4700rpm, but the rev counter will read over 5000rpm at these revs.

Mine is a Diesel, if yours is a petrol it might be more accurate.

I suspect there will be a lot of variation on different car models and makes.

How does the rev counter even work on a diesel? Trying to figure out how to put one on my 2.25 Land Rover, unless it's optical I don't see how it can be done?

[Mastodon2]

My Fiesta does 59mph by GPS at 60mph indicated, and 69mph GPS at indicated 70mph. Not bad for a Fiesta!

[CraigyMc]

doogz said:

The fence was live, and my multimeter is not alive any more.

I'd probably have just pushed a mate into it.

[mrmr96]

varsas said:

Turkey said:

Rod on my MG, it over reads by about 100rpm at 2000rpm, 200rpm at 3000rpm and 300rpm at 4000rpm, so not a linear % over. The rev limit is 4700rpm, but the rev counter will read over 5000rpm at these revs.

Mine is a Diesel, if yours is a petrol it might be more accurate.

I suspect there will be a lot of variation on different car models and makes.

How does the rev counter even work on a diesel? Trying to figure out how to put one on my 2.25 Land Rover, unless it's optical I don't see how it can be done?

Hall effect crank sensor.

[greendiff]

W terminal on alternator.

[varsas]

greendiff said:

W terminal on alternator.

mrmr96 said:

Hall effect crank sensor.

cheers, will look into it.

[Turkey]

varsas said:

greendiff said:

W terminal on alternator.

mrmr96 said:

Hall effect crank sensor.

cheers, will look into it.

It can be both I think. Appareantly the old Vauxhall/Isuzu 1.7TD lumps used a signal from the alternator for the rev counter. On mine it is the crank sensor I believe.

[Turkey]

mrmr96 said:

Turkey said:

Rod on my MG, it over reads by about 100rpm at 2000rpm, 200rpm at 3000rpm and 300rpm at 4000rpm, so not a linear % over.

It's over-reading by 10% AND your needles not on straight, it's too low by 100rpm. Therefore I reckon that at 1,000rpm it should be bang on as the two effects cancel out.

Interesting theory. Maybe they were all like this as my previous Diesel MG, a ZR was exactly the same. The ECU knows the real revs though at least, well the OBD readings tally with the engine spec, with a 800rpm idle and 4700 limit.