speedometer not working after tire install

BGTV8 · 08-13-2012, 10:02 PM

The ECU is confused because you've altered the overall rolling diameter of rear tyres with respect to the O/D of fronts.

OD of OEM on 18" wheels is 225/50R18 and rear is 245/45R18 which is OD on front and rear of ~680mm. The OD of 305/45R18 is going to be 730mm and because the ECU is monitoring the relative numbers of turns of front and rear wheels (expecting them to be very close) but your car is not, it has thrown a hissy-fit because it does not know which end of the car is correct and has been taken out-of-bounds for all sorts of electronic aids - stability control, traction control, electronic brake assist etc.

I'll leave it to others with more (drag) experience to suggest a way aroundyour problem - my simple answer would be to find a tall front tyre with 370mm OD but that might be easier said than done and it would jack the frnt of the car up by at least an inch as well.

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08-13-2012, 10:02 PM	#2 (permalink)
BGTV8 A True Z Fanatic Join Date: Dec 2009 Location: 03350 Australia Posts: 1,516 Drives: 09 Nissan 370Z M6 Rep Power: 39975	The ECU is confused because you've altered the overall rolling diameter of rear tyres with respect to the O/D of fronts. OD of OEM on 18" wheels is 225/50R18 and rear is 245/45R18 which is OD on front and rear of ~680mm. The OD of 305/45R18 is going to be 730mm and because the ECU is monitoring the relative numbers of turns of front and rear wheels (expecting them to be very close) but your car is not, it has thrown a hissy-fit because it does not know which end of the car is correct and has been taken out-of-bounds for all sorts of electronic aids - stability control, traction control, electronic brake assist etc. I'll leave it to others with more (drag) experience to suggest a way aroundyour problem - my simple answer would be to find a tall front tyre with 370mm OD but that might be easier said than done and it would jack the frnt of the car up by at least an inch as well.