MPG Dropped since New Wheels

[Clipboard2008]

Hi

Not sure where to post this question?

I have a T6 T32 204 DSG Highline that was on 17" Devonports. The MPG isn't great at the best of times and have been getting approx 28 mpg.

Last weekend I fitted new wheels (Worx Wraith 20" on 275/40/20 tyres) and the mpg has dropped through the floor, approx 19-21mpg. I'm driving a lot slower than before as my coilovers need raising and my tyres are rubbing when turning.

I can't work out why the mpg has dropped so much? Can anyone help shed some light on this?

TIA

 

[Montecha]

I swapped to 20's from Devonports and then back to 18' Sportlines.

First thing I would say that your 17's and the 20's are different rolling circumferences, so if you are using the MFD for mpg then it's not a fair comparison. Have you checked mpg tank to tank, as speed and distance readings will be different between your two wheel sizes.

That said I always believed that the 20's were heavy, and also the larger tyre gave more rolling resistance. I can't quantify this, but the van felt generally heavier: mpg, handling, acceleration, deceleration, changing direction. My rims were very heavy and the tyres were noise and high resistance. When I went back to 18' I really noticed it.

I would first check mpg tank to tank, and see what that looks like, it might not be as bad as you think!

 

[RattyMcClelland]

Wheels don't make that much difference for experience, oe m 16s to 20s, however if you're reading off the mfd and now have much larger rolling radius wheels ignore the mdf. It will be miles out. Work it out via empty tank to full tank and miles driven between fill ups.
275/40/20 is big. Try 265/40/20 or better 275/35/20.

 

[New2t6]

Maybe a silly question....but where do you get the miles driven figure then ?

 

[mopardave]

Maybe a silly question....but where do you get the miles driven figure then ?

No such thing as a silly question mate.....if you don't know something, just shout out! Right, all you need to do is zero your trip when you fill up. My MFD is a joke where MPG is concerned so I always zero my trip when I fill up and then simply divide the miles travelled (on that last tankful) by fuel quantity (in gallons) used. You can only get an accurate figure if you brim your tank every time. Hope that makes sense??? For info, my MFD is always 7 to 8 mpg over optimistic!!

 

[mopardave]

Also be mindful that there are two MPG readouts. For a more consistent mpg figure, you need to use MPG 2 in your MFD. Don't forget to zero both the MPG figure and the miles figure when you fill up......that way, you'll get an updated MPG figure with every tankful.

 

[mopardave]

I keep meaning to put my 16" claytons back on for winter......it'll be interesting to see if my multi fantasy display will be any more accurate.....can't imagine it'll make much if any difference?

 

[.50]

Once you change the circumference of the standard wheel, you change the distance the vehicle travels per wheel revolution. So not only will the computers MPG display be out, but also the amount of miles travelled, so you cant use the trips milage as its wrong.

In my opinion, you need to times the recorded trip reading by the percentage difference between the standard wheels and any new ones.

There is a big step up from, say, 205/65r16 to 275/40r20.

All that said, I'm finding the mpg on my 150 DSG atrocious compared to VWs figures.
Ironically, my 1.6 biturbo 160ps Renault engined Vivaro fitted with 235/50r18 tyres, half loaded averages 38mpg on mostly short journeys.

Cmon VW !!.

 

[New2t6]

No such thing as a silly question mate.....if you don't know something, just shout out! Right, all you need to do is zero your trip when you fill up. My MFD is a joke where MPG is concerned so I always zero my trip when I fill up and then simply divide the miles travelled (on that last tankful) by fuel quantity (in gallons) used. You can only get an accurate figure if you brim your tank every time. Hope that makes sense??? For info, my MFD is always 7 to 8 mpg over optimistic!!

I’m meaning that if the tyre rolling Size is out the miles will be wrong too surely

 

[mopardave]

I’m meaning that if the tyre rolling Size is out the miles will be wrong too surely

my first response to that would be yes, it would. After all, surely the distance travelled is calculated by wheel rotation.....but there maybe some spooky vw voodoo at work here! Someone far more knowledgeable than me (of which there are many on here!) may have a far more scientific explanation as to how distance travelled is calculated. Personally, I think .50 nailed it.

 

[mopardave]

Once you change the circumference of the standard wheel, you change the distance the vehicle travels per wheel revolution. So not only will the computers MPG display be out, but also the amount of miles travelled, so you cant use the trips milage as its wrong.

In my opinion, you need to times the recorded trip reading by the percentage difference between the standard wheels and any new ones.

There is a big step up from, say, 205/65r16 to 275/40r20.

All that said, I'm finding the mpg on my 150 DSG atrocious compared to VWs figures.
Ironically, my 1.6 biturbo 160ps Renault engined Vivaro fitted with 235/50r18 tyres, half loaded averages 38mpg on mostly short journeys.

Cmon VW !!.

.50....is your vivaro euro6? I suspect my constant regeneration cycles are partly to blame for my mpg.....and mine ain't too bad....but know where near vw's. I'm blaiming euro6 emissions components. You gotta wonder what they'd be like if they stripped it all off.....apart from dirty!

 

[Stephen Cowdell]

No such thing as a silly question mate.....if you don't know something, just shout out! Right, all you need to do is zero your trip when you fill up. My MFD is a joke where MPG is concerned so I always zero my trip when I fill up and then simply divide the miles travelled (on that last tankful) by fuel quantity (in gallons) used. You can only get an accurate figure if you brim your tank every time. Hope that makes sense??? For info, my MFD is always 7 to 8 mpg over optimistic!!

Just a thought on this, if your MFD is crap, where do the Miles get generated from, if its the electronic speed isn't this also inaccurate, so you actually have 2 incorrect reading !!

Or am I missing something

TBH, I really don't care what the MPG figure actually is, however it is consistent when doing a particular journey, at a particular speed, I get a particular MPG, if this alters, I know there may be an issue, either fuel, Load, or right foot, possibly worse.

When people are quoting the VW figures, they are never achievable, as they are achieved in a controlled environment, on a workbench.

 

[mopardave]

Just a thought on this, if your MFD is crap, where do the Miles get generated from, if its the electronic speed isn't this also inaccurate, so you actually have 2 incorrect reading !!

Or am I missing something

TBH, I really don't care what the MPG figure actually is, however it is consistent when doing a particular journey, at a particular speed, I get a particular MPG, if this alters, I know there may be an issue, either fuel, Load, or right foot, possibly worse.

When people are quoting the VW figures, they are never achievable, as they are achieved in a controlled environment, on a workbench.

I don't think you're missing anything mate......apparently there is something that can be altered using vcds. It's not uncommon. I am bothered about fuel consumption.....we all know manufacturers figures are a work of fiction.....and we all know why. That's an argument we could go round in circles about.....but back to this issue. I've mentioned it to vw customer services and they were none the wiser and couldn't offer an explanation. What I don't understand is the variance of accuracy on this forum.....some are good....some are bad.....mine's bad. 8 to 9 mpg over optimistic is I think bad....especially having come from a BMW that was never more than 1 mpg out from my manual calculation. Of course, I'm not taking mileometer discrepancy into account here when I work out mpg......life's too bleedin' short!!

 

[.50]

.50....is your vivaro euro6? I suspect my constant regeneration cycles are partly to blame for my mpg.....and mine ain't too bad....but know where near vw's. I'm blaiming euro6 emissions components. You gotta wonder what they'd be like if they stripped it all off.....apart from dirty!

The Vivaro is a 2015 euro5. It does regen, but without the fuss and frequency of the T6.
In my opinion, its the constant regens that spoil the MPG.

 

[Mooncat]

For that tyre size and assuming the originals were 215/60/17, to get a like for like reading from the MFD you need to multiply the value by 1.055. So if your display is showing 40mpg, taking into consideration the 5.5% circumference increase, the corrected reading would be 42.2mpg.

If you used to get better mpg than the corrected value then the difference is made up by the rolling resistance of the tyre and gearing. Note, you are also probably driving slightly faster now if you haven't adjusted your speed.......so when you would previously be travelling on a dual carriageway at an indicated 60mph, to be travelling the same speed, you need to be travelling at an indicated 57mph now.

 

[Stephen Cowdell]

The Vivaro is a 2015 euro5. It does regen, but without the fuss and frequency of the T6.
In my opinion, its the constant regens that spoil the MPG.

I don't think I have ever had a ReGen

 

[zombie_T6]

I guess a simple way to rule out bigger wheels incorrect mileage would be using a sat nav for a journey and checking vehicle speed against sat nav speed and by re setting the speedo before you leave and seeing if the miles tally up with the sat nav amount at the start of the journey.