Speedo accuracy

[Paul_13]

Has anyone successfully adjusted the speedo so it reads more accurate? My speedo over reads by 10%.

Today i modified the tyre rolling radius in the PCM (849 to 934) expecting it to adjust the speedo reading but it hasn't done anything.

Any suggestions?

[warpc0il]

All car speedos are designed to over-read at least 5%, from the factory, to handle tolerances and variables such as tyre wear, and because it's illegal for them to ever under-read.

Also note that "GPS Speed" isn't accurate either, if that's what you're comparing to.
Satnavs and phones have a sample rate that they check their position, and assume that you've travelled in a straight line between those sample points.
Most also act as if the ground is flat, not up and down hills.
If the road is curved and/or up or down hill, then the distance travelled is further than the GPS assumes, therefore GPS will underestimate speed.
GPS should be fairly accurate if road is straight and flat for more than a mile or so.

[Paul_13]

All car speedos are designed to over-read at least 5%, from the factory, to handle tolerances and variables such as tyre wear, and because it's illegal for them to ever under-read.

Also note that "GPS Speed" isn't accurate either, if that's what you're comparing to.
Satnavs and phones have a sample rate that they check their position, and assume that you've travelled in a straight line between those sample points.
Most also act as if the ground is flat, not up and down hills.
If the road is curved and/or up or down hill, then the distance travelled is further than the GPS assumes, therefore GPS will underestimate speed.
GPS should be fairly accurate if road is straight and flat for more than a mile or so.

The reason I ask is because on the BMW Siemens ECU's there is an option to adjust the speedo. Just seeing if anyone had come across similar.

[warpc0il]

I understand that, and my Jeep has a similar feature, as it's not uncommon to fit very different size tyres.

[alisdairm]

https://www.jaycar.co.uk/speedo-correct ... e/p/AA0376

[350matt]

Still keeping the car then Paul ?

[julian.rx8]

These RX-8 speedos seem to be very inaccurate tough. Had mine at about 270kph (177mph) on the german Autobahn back when it was still running ( )
Can't image that these cars will get anywhere close to that kind of speed in reality

[warpc0il]

Where you running undersized (e.g. low profile) tyres?

[julian.rx8]

I was running low profile tires but not with too much difference in rolling circumference (is this the correct english word for it? ) that it would have that much impact

Granted, this was downhill tough

[SeriousSam]

For what it's worth, my car on standard wheels and tyres reads 86mph on a police radar gun when the speedo reads 90. Which, at the time, was right on the threshold between being offered a speeding awareness course and being forced to take the points and fine.

I reckon that's within the expected manufacturing tolerances, and given the circumstances, I'm quite glad that tolerance exists!

[Paul_13]

Still keeping the car then Paul ?

I think so. I started this thread back in September btw