Well, I checked all of the tyres today and the pressures were all spot on except for the rear passenger side which was way down...

Since all of the tyres are new (two weeks old) is this likely to be a) an unfortunate slow puncture b) faulty valve? c) faulty tyre? d) something else?

Cheers guys

Check the leaking tyre again and if it's still going low then it's likely the tyre hasn't sealed properly on the wheel rim, in which case have the original fitters of the tyre refit and seal the tyre again.

It can take a week or so for tyre pressures to settle as air is forced into the new tyre structure & the possibility of a leak as a result of fitted not being 100%. As such each tyre needs inspecting & checking to detect any that may not have settled for whatever reason. After that they should hold their pressure well and all tyres should lose air at around the same rate as each other if they have been properly sealed and none has a small puncture.

At this time of the year your fuel consumption will be most influenced by the colder weather conditions rather than tyre changes due to addition friction demands and cold starting etc.

In addition ensure fuel consumption is measured to brim if not already done. The old tyres that were replaced would have slightly smaller circumferences than the new tyres, the smaller circumferences would slightly lower the car's gearing and the car would actually have a slight reduction in mpg although a trip computer would show slightly higher mpg. When the new tyres were fitted the slightly larger circumferences would slightly increase the gearing and actually mpg would improve whilst the trip would show slightly less mpg when compared with the previous set of results taken whilst the car was ran on the old tyres.

5mm tread wear =10mm loss of diameter=31.4mm (1.24") loss of circumference

~1.75%

Small, but not totally insignificant. I'll have to factor that into my fuel economy logs ;)

One thing i always wonder about when people report their amazing fuel economy's, if they run sat nav they probably realise their car speedo is inaccurate and can read up to 10% slow without taking the valid point of tyre wear* into account.

But aggressively insist their mileometer is accurate and believe the on board fuel display is teardrop accurate too.

It doesn't take the most cynical person to realise that a car maker isn't going to have their cars showing figures that put their car in a bad light when others appear 10% better, most people go by the dash display.

I used to work with a bloke who was convinced his Golf 2.0litre Diesel returned between 70 and 80 mpg in normal use, and went like the wind simultaneously..

*truck tyre wear is significant, on new tyres my regular truck from my last job was travelling at a true 55mph as shown on the calibrated speedo and by satnav, by the time the tyres were recut and getting close to replacement it would be travelling at a true 53mph satnav whilst obviously still showing 55 on the speedo...most drivers would try and time their recalibration date to coincide with worn tyres..:).

Probably find car tyres are similar, they may not have the depth of tread difference quite so much (increased by truck tyres being regroovable) but given the proportionately smaller rolling radius there's more than likely a similar result.

Edited by gordonbennet on 10/12/2011 at 10:57