What are 128i drivers getting for gas economy?

[Qube]

11.7l/100km CALCULATED for 128 vert. This is EXACTLY the same as my wifes 135 coupe, which she DOESN'T want to burn out the tires before 5000km

[Tom K.]

Originally Posted by jbenington86

the manual says that "it can not identify the gradual loss of pressure that will inevitably occur in all four tires over a lenthy period" but somehow i dout that all four tires are leaking air at the exact same rate

Actually, my 128i needs about 2~3 lbs. per tire added every month - as did the run-flats on my Z4. Naturally, there was no dash warning as the relative pressure differentials remained the same.

Tom

[jbenington86]

Hmm sounds like I need to get myself a little air compressor

[backtoreality15]

Thanks guys, that's a good idea. I will check it first thing tomorrow.

[1er junkie]

+1, I check every other week for tire pressure.

[imported_volsfan0911]

TPMS is great but no substitute for a GOOD tire guage and checking every week or every time you fill up for example. IIRC, 10 degrees ambient temp change is almost 1 psi in tire pressure so staying on top of that is a big deal. Especially here in Maryland where "if you don't like the weather, wait 10 minutes" is the norm. It can be 100 degrees last Wednesday, a lovely 82 today. In addition to the safety factor, underinflation can cost you up to 20-25% on mileage.

[Artmasterx]

If you assume the tire volume is fixed and ideal gas behavior, the following is true:

If P = 35 psi (2.4 bar) @ 77 degF (25 C)

8% decrease (2.8 psi) @ 32 degF (0 C)
5% increase (1.8 psi) @ 104 degF (40 C)

The equation is: P2 = P1*(T2/T1)
You have to use absolute temperature units though (Kelvin or Rankine). Rankine is 460 + (temp in degF). Kelvin is 273 + (temp in degC).


So you are looking at a potentially 4-5 psi change over the seasons, if you never check you tire pressure and it holds air perfectly... a bad idea and a bad assumption.

[Tom K.]

As the ambient air temperature went from about 70oF to 85o over the month or so when my tires all lost 3 lbs. pressure, I'm assuming the run-flats all leak air, albeit very slowly. I keep a small 120v compressor in the garage and carry a 12v one in the trunk to ward off punctures. It's worked so far...
Tom

[backtoreality15]

So my tires are set 35/42 front/rear (Which is for maximum load/maximum speed). As I understand it, low tire pressure has an effect on economy and not high... So there has to be something else going on with my car?

[Perfect128]

You are right about the RFT's slowly leaking. I've had my car 2 months and had the TPMS light come on twice. Sure enough, they were down a little. I haven't had the same problem with new standard tires.

[Tom K.]

Originally Posted by backtoreality15

So my tires are set 35/42 front/rear (Which is for maximum load/maximum speed). As I understand it, low tire pressure has an effect on economy and not high... So there has to be something else going on with my car?

With those pressures, you must be carrying a load of around 1,000 pounds in the car - so I'm not surprised at your gas mileage.

Tom

[backtoreality15]

Originally Posted by Tom K.

With those pressures, you must be carrying a load of around 1,000 pounds in the car - so I'm not surprised at your gas mileage.

Tom

I'm not, haha. The dealership gave it to me like that. Do you think lowering the pressure would increase my MPG?

[atr_hugo]

Your tire pressures are way outta whack. I'd go to 36 front/34 rear to start with and work your way from there. These cars are a bit nose heavy (even the 128i) and can benefit from a bit more pressure up front to help mitigate understeer.

[jkp1187]

Originally Posted by backtoreality15

I'm not, haha. The dealership gave it to me like that. Do you think lowering the pressure would increase my MPG?

Probably not -- it's my understanding that higher tire pressure generally leads to lower rolling resistance, and thus higher fuel economy. But I'd make sure that the pressures for front + rear were more in line with each other, I'm not sure what effect 7psi difference between front + rear would have, but it probably isn't helping.

[trabi]

26 mpg, 50/50 city/highway. It helps when you downshift to use the engine break. I'm doing this all the time 6-5-4-3-(2). This saves a lot of gas.
1500 miles on the car

[Tom K.]

Originally Posted by atr_hugo

Your tire pressures are way outta whack. I'd go to 36 front/34 rear to start with and work your way from there. These cars are a bit nose heavy (even the 128i) and can benefit from a bit more pressure up front to help mitigate understeer.

I'd agree for the 135i, but the 128i spec for most tires with normal loads is 32F, 35R, rather than the 36/36 for the 135i SP. Since he has been used to driving with the 7 lb. differential, he might want to try OEM pressures and then gradually increase the fronts if there's an understeer problem.
Tom

[dma550]

Originally Posted by trabi

26 mpg, 50/50 city/highway. It helps when you downshift to use the engine break. I'm doing this all the time 6-5-4-3-(2). This saves a lot of gas.
1500 miles on the car

How does engine braking save gas? :iono:

Though as you put it, engine "breaking" will save tons of gas

[jbenington86]

Originally Posted by dma550

How does engine braking save gas? :iono:

Though as you put it, engine "breaking" will save tons of gas

the entire time you are in gear w/o touching the gas you are using less gas than idleing....i usually dont shift ever single gear though I usually let 6 drop to 1k then shift down to 3rd then let the rpms go down to 1k again then complete my stop

[Phil]

I have 5521 miles logged, and best tank was 33.94 mpg and worst tank was 27.86. It's usually in the 31 to 32 mpg range. I just returned from S.A. to Houston to Corpus Christi to S.A. vacation and averaged 30.40 mpg. Mostly highway and air-conditioned.

[dma550]

Originally Posted by jbenington86

the entire time you are in gear w/o touching the gas you are using less gas than idleing....i usually dont shift ever single gear though I usually let 6 drop to 1k then shift down to 3rd then let the rpms go down to 1k again then complete my stop

Are you saying you use engine braking to slow your ascent? Other than placing a burden on your trans and synchros, I think you'd see the same usage if you just coasted to a stop clutch in, or just planned the stop in advance and braked for it.

I use engine braking riding/racing 4 stroke dirt bikes, it's great for momentum through rough sections; and to keep busy/tired hands from having to grab a big strongly sprung clutch, but I don’t think it's saving you anything on gas, and causing wear you don't want on one of the most expensive components on the car.

[young1er]

Highway: ~30mpg
Around town/ spirited 16-19mpg
Mix 20-22mpg

I have about 7200 miles on mine so far.

[gullyfoyle]

Originally Posted by 1speedbike

Interesting that I'm seeing the 128i owners here getting worse mileage than the majority of those on the recent 135 mileage thread..

could be 'cause 128ers get auto transmission more often. according my trip computer i'm averaging 25.5 mpg. 30%/70% city/highway. i'm too lazy to calculate real gas consumption though.