15" to 16" wheels - drop in mpg??

[Wests Renault]

Since I fitted the 16" 172 wheels to the dci, the mpg has taken a massive hit! Anybody else had this happen? Could easily achieve 70mpg average, now I'm struggling to get 60mpg.

Would never have thought wheels could make such a difference. I know the team dynamics I had before are considerably lighter, so would you say it's the inch difference or the weight of the wheel that has caused this?

Discuss....

 

[TheEvilGiraffe]

Rotating mass has an equivalent of about 5-6x that of non-rotating mass.

If they're 3kg heavier per corner, you're looking at upto 70kg "additional mass" to accelerate. More mass to accelerate = more fuel used.

Not sure on the complexities of larger rolling circumference. I'd have thought that would be better once at steady speed ? Is there a difference in size ? If there is, the computer won't know you're actually going further per revolution...

Have you worked out the MPG yourself or used the computer ?

 

[Jonny1*2]

if you do mainly motorway driving it should make mpg better. it makes the gearing slightly longer due to the extra distance per rotation. 15's should improve acceleration slight, so you might be over compensating with the right foot . But it's not a massive difference, so i'm suprised it's changed that much.

As mentioned, the speedo will be incorrect as the car is programmed to a certain wheel size. The car can be reprogrammed to suit. Dead easy on CLIP.

 

[Wests Renault]

Rotating mass has an equivalent of about 5-6x that of non-rotating mass.

If they're 3kg heavier per corner, you're looking at upto 70kg "additional mass" to accelerate. More mass to accelerate = more fuel used.

Not sure on the complexities of larger rolling circumference. I'd have thought that would be better once at steady speed ? Is there a difference in size ? If there is, the computer won't know you're actually going further per revolution...

Have you worked out the MPG yourself or used the computer ?

Only been using the computer at the moment, but have noticed that £30 usually gets us around 300 miles, now is struggling to get us 250 miles.

if you do mainly motorway driving it should make mpg better. it makes the gearing slightly longer due to the extra distance per rotation. 15's should improve acceleration slight, so you might be over compensating with the right foot . But it's not a massive difference, so i'm suprised it's changed that much.

As mentioned, the speedo will be incorrect as the car is programmed to a certain wheel size. The car can be reprogrammed to suit. Dead easy on CLIP.

The original tyre size was 185/55/15. The current setup is 195/45/16 and this shows a difference of -0.38% in overall circumference.

The team dynamics I was running before used 195/50/15 which was a -1.36% drop from the original size.

So the circumference is there or there abouts for both sets of wheels. I can only presume it is the weight causing the difference :S

If I were to guess I'd say there is probably 5/6kg difference between the 2...

 

[Chip-mk1]

if you do mainly motorway driving it should make mpg better. it makes the gearing slightly longer due to the extra distance per rotation. 15's should improve acceleration slight, so you might be over compensating with the right foot . But it's not a massive difference, so i'm suprised it's changed that much.

Longer gearing doesnt always equal better mpg, there is an optimum value, if you go pass this then it gets worse not better.

As mentioned, the speedo will be incorrect as the car is programmed to a certain wheel size. The car can be reprogrammed to suit. Dead easy on CLIP.

I suspect thats the problem if its a bigger rolling radius now it will take further driven before it registers a mile.
So you'll be getting 60 longer miles instead of your 70 shorter ones.

An actual mile, and a mile on the dash, are not as strongly connected as you would have liked to think!

I very much doubt the wheels are really altering your MPG by more than a couple of percent, certainly not over 15% like you are reporting!

 

[Chip-mk1]

Only been using the computer at the moment, but have noticed that £30 usually gets us around 300 miles, now is struggling to get us 250 miles.



The original tyre size was 185/55/15. The current setup is 195/45/16 and this shows a difference of -0.38% in overall circumference.

The team dynamics I was running before used 195/50/15 which was a -1.36% drop from the original size.

So the circumference is there or there abouts for both sets of wheels. I can only presume it is the weight causing the difference :S

If I were to guess I'd say there is probably 5/6kg difference between the 2...

Did you go from old tyres to new ones? If so thats another couple of percent.

 

[TheEvilGiraffe]

If I were to guess I'd say there is probably 5/6kg difference between the 2...

Each corner !!! Good grief.

6x6 = 36 x4 = a lot more mass to rotate !!

 

[Wests Renault]

I suspect thats the problem if its a bigger rolling radius now it will take further driven before it registers a mile.
So you'll be getting 60 longer miles instead of your 70 shorter ones.

An actual mile, and a mile on the dash, are not as strongly connected as you would have liked to think!

Good point, never thought of it like that!!

Did you go from old tyres to new ones? If so thats another couple of percent.

Yes they were new tyres, the only ones were about shagged.

 

[Chip-mk1]

Good point, never thought of it like that!!

Most people take what their car tells them as correct mate, when it rarely is!

Yes they were new tyres, the only ones were about shagged.

Thats a fair few meters extra off each mile you were doing before then, it was 70 very short "miles" you were getting before, and probably 60 quite long ones now!

 

[Wests Renault]

lol, what is the most acurate way to work out the MPG?

Sat nav and a full tank i suppose?

 

[LIL B]

Brim to brim

Miles.

Bobs.

 

[Chip-mk1]

Yeah you need to take a long known distance journey and see how much fuel you actually use.

Trouble is, you didnt do a before figure of course.

Next time you go on a long journey, get a satnav estimate of distance (they arent totally accurate either of course, but close enough for our purposes, lol) and then compare how your miles on the dash compares, that should give you a good idea of how accurate or not your dash now is.

 

[TheEvilGiraffe]

Sat nav isn't massivly accurate as it doesnt compensate for hills... if your rolling circumference is only 2-3% different it will do just fine off the speedo. Go for a drive, reset both and see how close they are !

Brand new tyres are larger than a set on the wear bars... it's all swings and roundabouts.

Thought about lamba sensors (if diesels have them?! they must have something to check emissions...?) It's also COLD ! Try blanking off some of your radiator.

 

[Jonny1*2]

Tom, as you work for renault, get one of the techs to put it on CLIP, change the setting to 16". Will take him 5 minutes tops

 

[James23]

I doubt the overall circumference difference is much at all (less than 1%) if you went from 195 50 15 to 195 45 16 so I wouldnt bother looking into reprogramming it or anything like that.
Different brands of tyres have different rolling resistances so will naturally cause more/less fuel to be used.
Also - it's very cold at the minute and cold temperatures noticeably harms diesel's fuel economy. A lot of the diesel boys cover half their radiator with a piece of plastic/cardboard when it's very cold to rescue a few mpgs.