19" Tyres = Less MPG / speed?

Googlyamnesiac · 12-06-2009, 12:26 PM

I asked before buying my 19" rims and tyres if I would need anything recalibrated on my e90 and I was told no they would be fine.

After a good drive today I notice that my MPG is about 10MPG less than it normally is (39 rather than 49). I think it's common for this to drop with larger tyres but would it be right to be by so much?

It also makes me wonder if maybe something does need recalibrating and my MPG/speedo is displaying the wrong figures?

Any help appreciated!

gIzzE · 12-06-2009, 01:38 PM

Quote:

Originally Posted by idlerobbo

I asked before buying my 19" rims and tyres if I would need anything recalibrated on my e90 and I was told no they would be fine.

After a good drive today I notice that my MPG is about 10MPG less than it normally is (39 rather than 49). I think it's common for this to drop with larger tyres but would it be right to be by so much?

It also makes me wonder if maybe something does need recalibrating and my MPG/speedo is displaying the wrong figures?

Any help appreciated!

Depends on the car, I went from the 17" M-Sport wheels with the ED run flat tyres on it to a set of BBS CH 19's with Falkens on.
Mine went from 47mpg to 39mpg.

More a case of the tyre width increase more than anything.

They were a 215/245 width to a 245/265 width, big difference in drag.

gIzzE · 12-06-2009, 01:40 PM

Oh, the other thing I noticed was my speedo used to read 84mph while my GPS roadpilot read 80mph, with the new wheels it was reading 82, so more accurate.
So the initial mpg reading was a good 7 odd percent out.

dxb335d · 12-06-2009, 01:49 PM

The bmw sizes all end up the same size. (Inheight) Only difference is width as matey says.

Googlyamnesiac · 12-06-2009, 02:03 PM

So my speedo should be correct but my mpg will probably come down?

I went from 16" RF 205/55 to 19" 235/35 Guess I should expect a 10mpg drop also!?

eek!

dxb335d · 12-06-2009, 02:12 PM

No mate, they should be hardly any noticeable difference. Could be that your driving slightly harder with your fancy new rims on?

gIzzE · 12-06-2009, 02:32 PM

If you went 205/55 to 235/35 then you will have a slight difference, it will be 1mph more accurate, so this will effect you MPG reading slightly, but only 2mpg.

When you change tyre sizes you swap, as Carlos says, for a tyre that gives the same, or as near as you can get it, rolling radius so the speedo stays the same. However, most manufacturers make the speedo over read by about 5-10% for legal reasons.
Better to say you are doing 43mph when you are really doing 40mph so you can't sue the manufacturer when you get pulled.

So I would guess the new tyres are making your mpg read about 46mpg instead of 49mpg and then you will loose around 5% by dropping the run flat tyres, which being hard give excellent rolling resistance properties, so down to 43mpg ish.
Plus you have gone alot wider, that extra contact area will gobble the the other 5 odd mpg easily.

When was the last time you reset the MPG anyway?? The cold weather will also kill mpg on sub 15 mile journeys too.

M3-FAST · 12-06-2009, 03:08 PM

Quote:

Originally Posted by idlerobbo

So my speedo should be correct but my mpg will probably come down?

I went from 16" RF 205/55 to 19" 235/35 Guess I should expect a 10mpg drop also!?

eek!

As Carlos says, the rolling radius is virtually identical - you can use the wheel/trye calculator on the forum to check

http://www.e90post.com/forums/forumdisplay.php?f=12

Its a handy tool that no one seems to use? According to it, your speedo will read 61mph at an actual 60mph (not allowing for the 5% error your speedo wil already have).

The mpg may change a little but 10mpg does not sound right at all. Do as Gizze says and reset the computer and allow for the fact the car is doing more cold journeys at this time of year.

How long have you had the car for? Surely you know how many miles it does on a tank of fuel. 49mpg would give approx 650 miles to a tank whereas 39mpg would drop it to 520. That way you can ignore the OBC and work out the mpg for yourself.

Googlyamnesiac · 12-06-2009, 05:59 PM

Cheers for the input guys as always.

I tend to reset my mpg each time I refuel - my girlfriend calls me a geek but I do 60 motorway miles a day and I like to keep track of how much i'm using :-)

I've only done about 40 miles since the new wheels went on so I guess tomorrow on the way to work will be a better test.

Maybe a remap would help my mpg get back to where they were?! Anyone know what sort of figures it improves mpg by if you were to actually drive the same way you always did? I sense I would be hammering my foot down a lot more though in the first few weeks!

12-06-2009, 12:26 PM	#1
Googlyamnesiac Captain 60 Rep 743 Posts Drives: 2013 F10 520D Join Date: May 2009 Location: Yorkshire, UK iTrader: (3)	19" Tyres = Less MPG / speed? I asked before buying my 19" rims and tyres if I would need anything recalibrated on my e90 and I was told no they would be fine. After a good drive today I notice that my MPG is about 10MPG less than it normally is (39 rather than 49). I think it's common for this to drop with larger tyres but would it be right to be by so much? It also makes me wonder if maybe something does need recalibrating and my MPG/speedo is displaying the wrong figures? Any help appreciated!
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12-06-2009, 01:40 PM	#3
gIzzE Major General 361 Rep 5,985 Posts Drives: F11 + 997 Carrera S Join Date: Apr 2006 Location: Norfolk, UK. iTrader: (2)	Oh, the other thing I noticed was my speedo used to read 84mph while my GPS roadpilot read 80mph, with the new wheels it was reading 82, so more accurate. So the initial mpg reading was a good 7 odd percent out.
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12-06-2009, 01:49 PM	#4
dxb335d The Tarmac Terrorist 1022 Rep 29,344 Posts Drives: 997.2 GT3 Join Date: Mar 2007 Location: ''Fandango Towers'' iTrader: (3) Garage List 2010 Porsche 997.2 GT3 [9.50] 2003 BMW E46 M3 [6.00] 2007 BMW E92 335D [5.50]	The bmw sizes all end up the same size. (Inheight) Only difference is width as matey says. __________________ 997.2 GT3
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12-06-2009, 02:03 PM	#5
Googlyamnesiac Captain 60 Rep 743 Posts Drives: 2013 F10 520D Join Date: May 2009 Location: Yorkshire, UK iTrader: (3)	So my speedo should be correct but my mpg will probably come down? I went from 16" RF 205/55 to 19" 235/35 Guess I should expect a 10mpg drop also!? eek!
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12-06-2009, 02:12 PM	#6
dxb335d The Tarmac Terrorist 1022 Rep 29,344 Posts Drives: 997.2 GT3 Join Date: Mar 2007 Location: ''Fandango Towers'' iTrader: (3) Garage List 2010 Porsche 997.2 GT3 [9.50] 2003 BMW E46 M3 [6.00] 2007 BMW E92 335D [5.50]	No mate, they should be hardly any noticeable difference. Could be that your driving slightly harder with your fancy new rims on? __________________ 997.2 GT3
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12-06-2009, 02:32 PM	#7
gIzzE Major General 361 Rep 5,985 Posts Drives: F11 + 997 Carrera S Join Date: Apr 2006 Location: Norfolk, UK. iTrader: (2)	If you went 205/55 to 235/35 then you will have a slight difference, it will be 1mph more accurate, so this will effect you MPG reading slightly, but only 2mpg. When you change tyre sizes you swap, as Carlos says, for a tyre that gives the same, or as near as you can get it, rolling radius so the speedo stays the same. However, most manufacturers make the speedo over read by about 5-10% for legal reasons. Better to say you are doing 43mph when you are really doing 40mph so you can't sue the manufacturer when you get pulled. So I would guess the new tyres are making your mpg read about 46mpg instead of 49mpg and then you will loose around 5% by dropping the run flat tyres, which being hard give excellent rolling resistance properties, so down to 43mpg ish. Plus you have gone alot wider, that extra contact area will gobble the the other 5 odd mpg easily. When was the last time you reset the MPG anyway?? The cold weather will also kill mpg on sub 15 mile journeys too.
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12-06-2009, 05:59 PM	#9
Googlyamnesiac Captain 60 Rep 743 Posts Drives: 2013 F10 520D Join Date: May 2009 Location: Yorkshire, UK iTrader: (3)	Cheers for the input guys as always. I tend to reset my mpg each time I refuel - my girlfriend calls me a geek but I do 60 motorway miles a day and I like to keep track of how much i'm using :-) I've only done about 40 miles since the new wheels went on so I guess tomorrow on the way to work will be a better test. Maybe a remap would help my mpg get back to where they were?! Anyone know what sort of figures it improves mpg by if you were to actually drive the same way you always did? I sense I would be hammering my foot down a lot more though in the first few weeks!
	Appreciate 0 Quote