I've noticed that my Octavia is only getting around 20mpg!

The only thing I changed recently was putting the 18" alloys on. I didn't notice it immediately afterwards and it's the sort of thing I do notice. Now it's a consistent 20-21 mpg (figures from brimming the tank and working it out exactlyso it's not a computer error).

I was previously getting a solid 28mpg (I've got a heavy right foot). The alloys are replicas and are a good bit heavier than the original 16"s but are not ridiculously heavy. Certainly not as much extra as carrying an extra passenger. I managed to fit them myself without to much trouble. It's just had it's 90k mile service at Skoda and that hasn't solved the problem.

Could the alloys alone cause such a huge increase in consumption or should I look elsewhere? I really don't want to have to go to the hassle of removing the alloys, refitting the old ones and running it for a while only to put the new ones back on as it's a right hassle just finding the time!!

Cheers.

The only fault code that appears every so often is 17748

17748 P1340 Crankshaft-/Camshaft Pos.Sens.Signals Out of Sequence

poss boost leak somewhere causing mpg to go down..

poss maf going funny

these will not always throw up a fault....

hows the car pulling compared to normal ???

Seems to be pulling ok. I'll get my liquid on to test that though. The MAF was replaced within the last 5000 miles and that made a big difference but also would suggest that that's not a problem.

The only fault code that appears every so often is 17748

17748 P1340 Crankshaft-/Camshaft Pos.Sens.Signals Out of Sequence

I had a similar error on my furby. Over the past 2 years I suffered from starnge MPG. Most of the time it was good then it'll go crap. Replaced the camshaft sensor and so far it appears to be doing megga MPG.

im wondering if an ecu reset would do much?

You might be surprised by how much extra effort accelerating a heavier wheel/tyre combination takes. The effective change of mass of the heavier wheels is proportional to the change raised to the 4th power, so if the new wheels weigh 1kg each more than stock, this gives [4+((4 wheels * 1kg)^4)/distance to centre of rotating mass, say 33cm)] or 89kg more that you need to accelerate! (centre of rotating mass is assumed, and probably underestimated).