Just wondering which one I can rely on more for the actual speed I am doing, whether it is my ECU talk with lcd that shows my speed or my actual oem cluster? I have aftermarket 18'' wheels with 245/40 and I am not sure whether my cluster was re-calibrated. However I noticed that the cluster shows a few km/h more than my readouts on the ECU talk. example doing 100km/h on cluster and it shows only 95 or so km/h on the ECU talk. Thanks in advance.
Get a GPS/Sat Nav and verify your speed with that and compare against your cluster and ECU. The question you are asking could be more accurate either way.
I would say it is correct in relation to each other, car reviews used to sometimes have a comparison of actual speed and what the speedo showed and were frequently a few k's low. BUT Your tyres will cause actual 100kph to read as 96.6kph, compared to stock size. SO 100kph actual is probably reading 101.6 kph on your cluster speedo.
The correlation between ecu and speedo doesn't matter relating to tyre size. They're both getting their speed reading from the speedo sensor in the gearbox. So if your ecu is reading 95kmh then the speedo should also be reading the same. Regarding your ecu speed and actual speed, you can use THIS to compare your current tyre size/s to stock to see how inaccurate it is making your whole car's speed reading. (stock is 225/50/16 btw.)
Thanks everyone for their input. At least I know now that both get their readings from the same area. So I rather stick with the percentage difference (according to the wheel calculator) and subtract it from my actual speed on my cluster Thanks again
I use a free app called speedbox, just once in each car/bike so from then on I know how accurate the speedo is.
Of course the speed reading comes from the same place, you asked why there was a difference between the digital (ECU)and analogue (speedo) reading, as already stated analogue speedometers are frequently designed to read a few k's low. The tyre size difference of approx 3.5% had been taken into account in my first answer. The speedo is reading low compared to actual, factor in the tyre size and its reading only 1-2 k's high at 100kph, very close to being very accurate, the bit I didnt say is that the ECU speed will still be in proportion at about 5 k's low. In summary with stock tyres I would say the ECU is correct but with your tyre size the speedo is correct. Try a GPS app and tell me I'm wrong.
If the outside diameter of the tyres are not consistant with oem specs, then there will be speedo cluster errors proportional to the amount of incorrect tyre diameter.
