G'day. I usually average about 600+ km to a tank. In the space of 2 tanks i'm now getting around 480ish to a tank. Plugs are clean, tyre pressure is correct and performance feels normal. Short of doing a mechanics apprenticeship, what can you suggest I might look at to solve this problem? Are there any known quirks with the 3.5 litre motor? Any ideas would be appreciated. Thanks. David.

A failed oxygen sensor is one thing that might give you a fuel economy drop of about that magnitude that quickly, without seriously affecting performance in many cars. What sort of mileage is on the car? The easiest way to find out if this might be the issue, if you can find someone with the kit for logging from the ECU, is to check the state of the ECU's fuel trims and/or the oxygen sensor input values (no CEL for O2 sensor failure in these ECUs, so no codes to scan for :(). Otherwise you've got to try and get an oscilloscope onto the sensor signal. In your case, considering how recently you've noticed this, pulling the oxygen sensor may not reveal much soot build-up which is the usual physical evidence of a long dead O2 sensor...

Also, what does the trip computer show?
For reference, when driven sedately, my KH will do 10.5 for city/highway driving with best possible around 7.5 on purely long distance highway cycle. Worst case is about 12.5 driven normally in city cycle.
Aggressive driving and it might hit 13, rarely.

Changed the O2 sensor and 'hey, presto', the fuel gauge has slowed down. Magical. Many Thanks.

I was cruising around 13.75 with gentle driving. A new o2 sensor seems to have done the trick. Thanks for your imput.

thats the same problem i have with my tl vrx so i am going to get a new sensor and try that thx

thats the same issue i have with my tl vrx so i am going to get a new indicator and try that again and i do really wish that the never had to deal with that same junk again...