Fuel consumption gone up with new wheels

[AvantSE]

Fitted a set of 18" RS4 alloys to my 2.0 A4 in place of the 16" Trapez rims. Since then I have noticed an increase in fuel consumption and it is quite a dramatic difference. Whereas before I could easily manage anything up to 45mpg (usually around 36-38mpg) now I can't get above about 37mpg on the same roads.

I was aware that the bigger wheels may represent a greater unsprung mass but didn't expect it to impact on fuel consumption so dramatically.

For example, a run to Nottingham and back in light traffic usually sees an average of 37mpg, pretty consistently, and without being too gentle on the throttle. Today I drove like a grandad and struggled to get more than a 34mpg average.

Another long run usually sees me average almost 40mpg, yet with these wheels I can only achieve about 37mpg at most.

Beginning to wish I'd stuck with the original rims; the new ones look great and the steering is much sharper, but the economy is poor.

Anyone else experienced something similar? It is only bothering me because I cover about 35,000 miles a year = more expense.

[Sparky]

Get the 17" Star alloys. Good compromise for economy.

Can't say I noticed a big difference when I took out another A4 with 18's on. Was the same car other than the wheels.

Maybe the paint you've used on the bumpers is too thick and slowing you down

[AvantSE]

LOL re: the paint.

I am not sure whether it's the weight, or whether the *very* slight increase in diameter has had some impact, though I thought the new wheels would've meant better economy, not worse. When I put the new 18s next to the old wheels, the overall diameter with tyres was about 1cm bigger, hardly a major difference.

Bearing in mind I am going off the DIS, I should really calculate it from fuel used but I don't have historic figs from the old wheels.

Ah well, will just have to content myself with the better looks and handling...

[edo]

IIRC you have to tell the VAg brain the wheel size or all the computer stuff is out? Also, if the wheels are wider, this increases drag - but I wouldnt have expected the effect to be so pronounced. Brim it and do a few hundred, and then brim again to see true mpg. Problem is you dont have the old wheels to compare...

[AvantSE]

Is there some way of reprogramming DIS with the true size? It's only a matter of about 1cm or so increase in diameter, but I'd have thought that would make mpg look better as one turn of the driveshaft would actually cover a greater distance as the overall circumference would be slightly greater (or am I barking up the wrong tree?).

There was a noticeable difference in weight between the old and new wheels; I am wondering if genuine Audi 18" wheels would be a lot lighter than the cheap replicas I have? And if there is a big difference in the weight of the tyres themselves?

But I agree that the extra width of the wheels (1") and tyres (20mm) will create increased drag and hamper economy.

[UBM]

Indeed , you need to set up the DIS with the new wheel settings IIRC when changing 2"!

[AvantSE]

Thanks, any idea how I can go about it? I assume it's involves a trip to the stealer? Got to go there tomorrow anyway as it happens so will try to remember to ask. What is involved?

[Sparky]

Yes two things

Reps quite often are much heavier. Not all are, but the vast majority have more weight to them.

Secondly, you need a VAG COM to recode the wheel size. Often when people go from 17's to 18's they just get a smaller profile in tyre to accommodate, but if it's as much as 1cm I think you'll need to adjust.

Many people on TSN have a VAG COM. Just need to ask around and see who is closer. Either have a look at this website or pop into the VAG-COM forum on TSN.

[AvantSE]

Might get hold of VAG-COM myself anyway so now there's an even greater incentive. Just need to figure out which cable etc. I need.

[ribena]

hi all

'scuse the newbie post

my consumption increased with new wheels but put it down to them being wider with more drag

if your new wheels have a larger rolling diameter then surely it will register less miles at the speedo hence higher consumption ???

check it here http://www.tyresave.co.uk/tyresize.html

HTH

[AvantSE]

Had a long run in the car today, and managed to get it up to 42mpg (but was driving like an absolute grandad, hardly touching the throttle).

I called at Audi and they confirmed that the DIS needs recalibrating for the new wheels, which they are going to do tomorrow morning along with activating a few other features. Will let you know what happens.

[UBM]

Cool - let us know what happens!

[AvantSE]

Well, off I went to York Audi and had the car disappear into the workshop for about an hour (probably to justify charging me £32, I bet it was just standing there for 50 mins with no-one working on it...).

They activated the locking an alarm beep, but then told me that "wheel size is irrelevant as the rolling radius is the same". Oh.

So there we have it, although the overall diameter is about 1cm+ bigger, the rolling radius, according to Audi, is the same.

[Chav]

Yep not many people change the rolling radius of their wheels. 17/18/19 should all be within +/- 2% of the OEM size.

Unless you're swapping from really crap wheels, the MPG shouldn't be affected as even the 17" wheels are 235 so no extra drag when switching to 18s or 19s.

Interestingly, my reps are lighter than my previous OEM wheels. All the reps that wheelbase in oldham had in stock looked very good quality to me.

I think assuming reps are crap/heavy is outdated nowadays, as they tend to be made at the same place as the OEMs.

[AvantSE]

Seeing as I achieved decent economy yesterday I am not so concerned now, but still curious as to why some days the car achieves higher MPG than others, on same (sometimes empty) roads and same style of driving.

[Soapsuds]

the weather...... can make a slight difference