I would like to run this scenario by you all....

Yesterday I took my car into Lexus to have my iPod integration fixed and a rattle on the passenger seat belt housing sorted.

The day before I filled up at a local Tesco and as is normal I reset the Trip B counter.

I took my car in and it had 17 miles on the Trip B counter and when I picked it up afterwards it had 25 miles on it. So Lexus has done 8 miles in it (presumably to drive around and check the rattle - fair enough no problems there).

Now for the problem. I reset the computer on the morning I dropped it off at Lexus which is 7 miles from home.
I remember the mpg was about 31mpg - it's nearly all motorway door to door.

However when I picked the car up it was showing 19mpg (or thereabouts).

Remember Lexus had only driven it for 8 miles.

At the time it didn't register but today when I drove to work I noticed the tank average was 25.2 mpg.

No way do I ever get that low a tank average so I did some calculations.....

I have done 52 miles since the fill-up.

I dropped it off at 17 miles and picked it up at 25 miles - so Lexus have driven 8 miles in it.

My mpg had dropped from about 30mpg to 18mpg in that 8 miles.

I worked that out by calculating petrol amounts as approx 14 mpg.

I then worked out the mpg drop over the 52 miles on the trip meter.

Conservatively I would get about 28.5 mpg over the 44 miles I did.

For the fuel consumption to drop to 25.2 mpg over the 52 miles then they would have had to do 14 mpg over the 8 miles. The figures approximately match.

Before I speak to them have I got something wrong here ? Is it feasible to get 14 mpg driving sensibly around the locality.

It would have been in middle of day so no rush hour traffic etc.

I would be mortified if I thought some cocky mechanic had caned my car around town and if I am right I will be getting in touch with head office.

I'm not accusing them of anything but the figures do look worrying.

Advice please folks...

I have had very similar issues when mine has been in on a couple of occasions....fair enough I'm in a 220d but have seen exactly what you are seeing....the mpg is dire on the tank of fuel that was in when the car went in until I refuel (full or part) and then it is back to the levels I am used to.

I've mentioned it on here before but I reckon they quite often do something with the ECU or similar that cocks up the internal calculations on mpg

Of course, I may be barking completely up the wrong tree but it's my theory anyway!

It could just be that they run the eingine whilst at a stand still to check things out.

Wouldnt take long for a big engined car to get down to those levels of mpg when doing nothing more than ticking over

I think they leave the engine running when they do stuff to it, so that would probably kill the mpg reading if it was left idling for 30mins for example. My dad's Prius was serviced today and when it we picked it up the batteries were completely dead - for exactly the same reason in that they leave the cars on but as this is a hybrid the engine will switch off and just run off its batteries.


*EDIT* Doh too slow, as Stuie said above

Cheers folks. I'm dropping this now.

I think i'm just being a bit cranky this week as I've got a bad back.

Thanks for your help everyone.

Next time you drop the car in, fit secret cameras to the interior, and playback footage to see if a technician is grinning as he canes your car.

If you drive it only around a town for a week you will see only 24 ish mpg, so nothing unexpected if they left it running with the music up loud so they didn't hear the rattles.

Check the rear tyres to see if any rubber has been burned off them.

Might be wise to do a search on youtube for IS250 burnout videos. See if your car is in there.

Good Luck!

its more than likely that the battery was disconnected which would have reset the ECU, over a few days it should relearn to a truer reading

I thought that at first as they may have done this when fitting the iPod kit, but wouldn't that have reset the trip mileage as well?

If it helps your state of mind - I left my boot light on for 2 weeks while away which resulted in a flat battery - once RAC re-started it I noticed both the MPG readouts had reset to zero - trip mileage was not changed - when they fitted your ipod kit I'd presume the battery disconnect was long enought to reset the mpg to zero - relax :-)

Maybe it did and Lexus drove 25 miles and not 8????