speedo accuracy

Post Mon Sep 02, 2013 9:32 pm

Hi guys, just getting used to driving my e30 around after finally getting it to a road worthy standard (4 years ish, on and off), and loving it!!!

Noticed the other day tho that when i drive past a local speed sign (that actually shows your speed) the car is saying 40mph but the speed sign is only saying 33mph. The car is on standard 325i bbs wheels/tyres and it has a 3.64 325i diff. i wouldn't have thought that changing the diff would cause this. The car was originally a 316i so perhaps is it due to needing a 6 cyl coding plug which would be calibrated to the different wheel/tyre size

Post Sun Sep 08, 2013 10:00 pm

Coding plug and diff. ratio have no bearing on speedo accuracy. Only the tyre diameter affects the speedo, and you say that is as standard.
Your speedo is supposed to read higher than your real speed by design, but not by over 20%, but check it against GPS at a few different speeds to see if you really have a problem, and if you have, whether its closer to a percentage error or just a constant difference.

Post Sun Sep 08, 2013 10:36 pm

that's as i thought. i just did a tyre size comparison calculator. and that accounts for the difference in speed. i.e. my speedo is calibrated for 316i wheels but the car is actually on 325i wheels/tyres

Post Mon Sep 09, 2013 9:19 am

booomer wrote:that's as i thought. i just did a tyre size comparison calculator. and that accounts for the difference in speed. i.e. my speedo is calibrated for 316i wheels but the car is actually on 325i wheels/tyres

All E30 wheels have the same rolling radius. 316i tyres are narrower, but have a higher sidewall height which brings them close to the same overall diameter as 325i wheels.
316i speedos and 325i speedos are identical.