General Poor mpg on cross

[crossman]

Hi all, in the past week or so I have noticed a drop in the MPG on my Cross. I don't hang around in it but always got an average of about 45 MPG. This figure has dropped to between 37-40 MPG even with driving more fuel friendly.I fitted four new Pirelli P3000 about a week ago and to be honest since then I have noticed the drop in MPG, I have checked all the tyre pressures again which are 32psi.
Even though I don't think it is the tyres it just seems strange?. The car is running very well no lack of power, noises smoke etc. I had it serviced with the dealers back in October so wondering if they did not replace the air filter could this affect the MPG?. I was going to check this at the weekend to see what condition it is in. Apart from that not sure what else

 

[Herts Hillhopper]

Changing tyres makes two differences - one of which may be significant

The rubber compound used in tyres affects rolling resistance -- basically friction. Less resistance = better economy, but also less grip. This may affect your figures.

But also, the diameter of the wheels has changed! A new tyre has 8mm of tread, and a worn out tyre (if allowed to go to the legal minimum tread) has just 1.6mm. This means a new tyre has 6.4mm more radius, or 12.8mm more diameter. This difference in rolling diameter may also affect the computer's readings as it simply counts revolutions of the wheel at the gearbox.

 

[Herts Hillhopper]

New tyres now have 'eco labels' on them to show how well they grip in the wet, how they affect fuel consumption, and how much noise they make (outside the car). These let you choose tyres not only by price, but also the compromises between grip and fuel economy too.

The Pirelli P3000 has an 'F' rating (the worst) for fuel efficiency... (also, it is not a winter or all-season tyre, which is what the original spec tyres were, so may reduce your offroad/winter driving grip)

I believe the original tyres on the Cross were Continental Winter Contacts (or CrossContact), which are 'E' rated (so slightly more efficient).

My 1.2 4x4 ran on Vredestein Quatrac 3 tyres for a fair while -- good grip but quick to wear and not so good for economy. Currently on Kleber Quadtraxer (all-season, M&S rated, like the Vredesteins), and as well as lasting better, they've reduced my fuel consumption too - they are 'C' rated for economy compared to the 'E' rated Vredesteins they replaced. (Conversely, the Vredesteins had a better rating for grip, but I've not really noticed any negative effects yet)

(Bear in mind, by the way, that as well as changing your tyres, the weather's been a lot colder in the past week, which will also hit your mpg - two variables have altered at the same time, so its harder to make a true comparison)

 

[crossman]

Thanks for that, having thought about it a bit more it does seem the only explanation that makes sense. As the engine is running and sounding really well and has only just covered 15,000 miles so I was pretty sure it's not a mechanical problem. Looks like another tyre change is on the cards then