Changed my Rear Continental SP2's to exactly the same tyres/tires so that I now have 2 brand new rear tyres. The fronts still have quite a while to go.

I have noticed my MPG decrease drastically ever since the new rears were fitted - nothing has changedd - same route, same driving style but my MPG has dropped by 7-8MPG!

Any suggestions? I was sitting in a lot of stop start traffic last night and whenever this happens the 535d still returns around 21mpg but last night it was showing an average of 14mpg Any suggestions guys? Is there something to be re-calibrated after putting on new wheels?

I used eTyres.

 

Check the tire pressure.

 

Everything should remain the same.
Check the tyre pressures as mentioned earlier.

If everything else remains equal, your new tyres should be MORE economical than the old ones, as there is actually less rubber on the road on a new tyre.

Even though the circumference (or Rolling Radius, in TyreSpeak) is larger on a new tyre, your iDrive won't know this and will display the same - even though you are actually travelling further. The difference between a new and bald tyre can be around 8% - as those of us with SatNav will testify - check the speedo against the SatNav with bald tyres, then check again with new tyres. Quite a difference.

Even if the incorrect size of tyre has been fitted, the iDrive will still think they are the same size, although this difference would have been picked up by your tyre pressure monitoring system, assuming you didn't reset it before moving off.

Mike

 

If its a like-for-like swap, the only variable could be tyre pressures, unless the rubber compound has changed. Pressures are what affect rolling resistance too; soft (under inflated) tyres drag like hell and take more power to turn. Likewise, hard (correct or even over-inflated) tyres roll much more easily. Were your old ones over-inflated to help them roll much more easily? Correct pressures are essential - and be aware that gauges read differently too....... Always use the same gauge to check.

 

I used eTyres last month to swap out the rear Conti's like for like swap on the 172 rims, my MPG has remained fairly static, if anything slightly better, before the eTyres guy inflated he checked the pressures on the plate by the drivers door to make sure all was correct. It may be worth double checking your pressures.

Good Luck

 

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...I noticed a 6 mpg difference in my MPG when I went from the BMW OEM 71's 18 inch on my 530i, to the new BMW OEM 19 inch 166's staggered set that I had installed a couple of years ago. But I did ck my tired pressure and it was off, BMW reset the tire size on my idrive, and put cold 32 psi in the rears and 30 cold psi in the fronts, and it fixed it for me.

I hope this helps, and good luck!!


R. Eddie Presley

PS,

I bought Bridegestone tires, that I purchased at Kauffman tire, (and saved around $400.00 over Hank Aaron BMW), but had them installed at my local BMW dealer in Atlanta, (Kauffman tires installs and balancing tire they sell for FREE, but their TIRE work is not that hot!! Where I live Kauffman tires pays min. wage and hires and uses HS students to become their "Master tech's", which their Kauffman employee company titles should be "HS Kids" or "Master idiots" (jk LOL), but their tech's are bad about scratching high end rims, when changing or rotating tires on nice cars i.e. our BMWs, I wish BMW dealers here in Georgia would match Kauufman tire stores prices, but they don't, so I buy tires at Kauffman, and pay BMW dealer to install, balance, and alignment, and I still save money and most of all HEADACHES!!)

 

May I suggest the obvious.

A new tire creates significantly more friction and thus more resistence.

As a tire ages, the edges that hold to the road wear down, and fuel economy increases.

Most of us notice a great improvement with fuel economy from the early days. The two reasons are tires and oil (flowing throughout the metals)

 

Thanks guys. The tyres have had the correct pressures [32psi front and 38psi back] since being fitted so its definately not that. ETyres said something very similar to what monacobmw has mentioned...so far no sign of mpg improvement though. Its strange though as it was a like for like replacement.

 

Well, there are a couple of other variables to consider. Has your fuel formulation changed, does it change with the season? Are any of the brakes dragging, including the parking/hand brake? Did you reset your fuel computer? I think average fuel consumption accumulates over time (I do not know the parameters) and it could be that your higher readings were due to a longer history and you had actually been losing mpg over a more recent period, even before changing the tires.

Just some thoughts...