Since fitting 18's to my Octy Vrs I have noticed a reduction in mpg. I was getting around 31mpg (mixed driving) but I now get around 26mpg for the exact same journey. Obviously the new alloys have come with new tyres Federal SS595ZR 225/45/18 so that would account for some reduction in mpg but I wasn't expecting a 5mpg deficit! Is this normal? Oh and should I have the car tracking re-checked?

Thx in advance.

hmmm, think you might have to take into account that with the bigger wheels the computer is reading the speed differently and thus calculating the MPG wrong. Although it will be a little worse because of the size of the wheels its turning, losing more than 10% economy seems a little "unreal" ?!?!?

The ne tyre size is 3.61% bigger, so having a 10% drop is a little odd. Do they have stickier tyres on than what you were running originally?

I know going from 185/60 R14 to 205/40 R17, I got quite a drop in economy, in the region of 10-15%, and I had to pin 3% on the bigger size, and the rest on the Toyo Proxes being stickier than conti eco-contact tyres.

There should be no difference as the rolling radius should be within +/- 2.5% of the original. Why have you gone for 45 section tyres though? I'm running 18's but am using 40 section...

Sorry, they are 40 section not 45 as stated . The 'old' tyres were the standard contis on the octy vrs but had about 2mm left when removed so I guess you could say that the new tyres are stickier! I experimented this morning by 'freewheeling' to see if the speed reduction was greater than normal and sure enough it is therefore I would assume I would have to give it more boot (thereby less mpg) to attain the same acceleration as before?

Bigger cross-section therefore more drag?

Had no problems and no difference when I went to 225/40/18 RS4s over 10,000 miles

I too have suffered this since fitting 8x18 RS6 replicas with the 225/40/18 Federal SS595. I have also noticed a slight degredation in acceleration (which is fair enough)

The difference in rolling radius between the 205/50/17 Conti and the 225/40/18 Federal is 0.1%

At the end of the day a 5/6mpg drop from 30mpg is more like 20% which is not practical when you thing about it, as every journey is costing 20% more with these 18s. Therefore I will be selling mine.

I have read on here that these cheap wheels are heavy, and I noticed this when fitting, however I will be weighing them against the standard wheels soon.

I also think that it is this extra weight that has killed my gearbox...

I have recently fitted a set of 18" OZ vello 5 spokes to my vRS. I have not noticed any reduction in fuel economy - I will see what i get from a full tank next time I fill up.

With regards to acceleration there does seem to be a slight loss, particularly in 1st, but after this things seem to be fine. Maybe a re-map will help? Looking at my speedo now I am able to reach 70mph in 2nd without hitting the rev limiter.

The wheels I have fitted if anything seem lighter than the 17" originals. Was going to get RS4 replicas but was wary about the extra weight.

I have recently fitted a set of 18" OZ vello 5 spokes to my vRS. I have not noticed any reduction in fuel economy - I will see what i get from a full tank next time I fill up.

With regards to acceleration there does seem to be a slight loss, particularly in 1st, but after this things seem to be fine. Maybe a re-map will help? Looking at my speedo now I am able to reach 70mph in 2nd without hitting the rev limiter.

The wheels I have fitted if anything seem lighter than the 17" originals. Was going to get RS4 replicas but was wary about the extra weight.

I was looking through a Mondeo brochure the other day.

There was around a 5-10% difference in economy between the Zetec and Zetec-S.

Only difference is a slight body kit and the change from 16" to 18" alloys.

Cheers

Lee

Is this reduction in ecomony not chiefly due to the fact unless you spend loads of pennies, 18" rims are significantly heavier in weight compared to 17" with the small sidewall profile making limited difference in tyre weight saving? Am I not right in thinking that unsprung weight (e.g. wheel, trye and hub/brakes etc) make double the difference in terms of performance deficit compared to sprung weight?

Lee

Am I not right in thinking that unsprung weight (e.g. wheel' date=' trye and hub/brakes etc) make double the difference in terms of performance deficit compared to sprung weight?

Lee[/quote']

Apparently so, the general concensus is that unsprung has double the effect of sprung weight, and rotational weight is the other killer.

As promised I weighed my wheels when I swopped back to the originals this week:

OEM 'Spider' 7x17 with 205/50 Continental SportContact : 18kgs approx

RS6 Replica 8x18 with 225/40/18 Federal SS595 : 24kgs approx :eek:

Thats a massive 6kgs per wheel extra unsprung and rotational weight. since going back to the 17s, the car is feels much more responsive and agile, and the all important mpg is back upto 30ish rather than the low 20s.

Yes you have to pay around

So if I were to ditch my 18" Team Dymanic Motorsports in favour of OEM 17" rims, my car too would go like pooh off a shovel (which it does already), erm... so, go like the an extra big dollop of pooh off a shovel?

Only if you can get the 17 oems (with contis) to stick as well as aftermarket tyres (proxes etc)....there is also cornering to think of. Larger sidewall = less stability.

Must admit I noticed a little decrease on 0-60 performance, just didn't feel as 'brisk', saying that I dont get as much wheel spin (with esp off) with the 18's as I did with the 17s so that will no doubt counter the effects.

I think it was Continental who ironicly said - "Power is nothing without control"! (maybe it was pirelli).

Had my car dyno'd with 18's on and the loss in tranmission was up 6hp :eek:

Must admit I noticed a little decrease on 0-60 performance' date=' just didn't feel as 'brisk', saying that I dont get as much wheel spin (with esp off) with the 18's as I did with the 17s so that will no doubt counter the effects.

I think it was Continental who ironicly said - "Power is nothing without control"! (maybe it was pirelli).[/quote']

Obviously..18s have a much larger circumference and therefore need more power to fully rotate the wheel than you would need with smaller wheels.