what does your m20b25 get mileage wise?

[Kozworth]

New t stat, runs half a notch below the midway.

Havent checked brake drag but the tires are checked regularly and they arent crazy performance tires

[dogsbark26]

The OBC cannot possibly be reading correctly. You have altered the chip (and changed the injectors?).

The OBC makes its calculations based on the duration of signal to the injectors combined with the fuel flow capacity of the original injectors!

[Kozworth]

One of the previous owners chipped the car.

The obc in my car is weird.. itl seem to freeze say on 20.2 and then id havta reset. Not sure if thats normal.. but right now as i type this it reads 18.6mpg. And as i asked before whats then difference between avg mpg and avg m/g when you hit the "push with a pen" button

[coldweatherblue]

filled up my 325i (for the second time, new car) yesterday so I could track the mileage for the first time.. 16 mpg.. ooh.. lol. Of course since the last fill-up the car probably idle'd over 40 minutes while I was messing with stuff on the engine and all the miles were short trips with an often heavy foot. car has a 4.10 diff too so that'll bring it down.

O2 sensor checks out okay, couldn't see any vacuum leaks via carb test.. sometimes throws a lambda 1 CEL at idle.. so currently going through that long list. Hopefully with normal city driving and correct tire pressure, properly running engine, etc should get low 20s.

My old E30 (stock '88 325i) pretty much always got around 21/27.

[stamar]

my bad gas mileage is very likely something suggested here, its the coolant temperature sensor.

seems to hick up when cold. that just dumps fuel runs rich etc. that would make city driving worse than advertised for sure.

[Kozworth]

The issue with my car is i dont believe it runs rich. I do not see any black smoke out the tail pipe when reving, and i do somtimes get a cel when idling. But i cant flash my codes

[nando]

The OBC cannot possibly be reading correctly. You have altered the chip (and changed the injectors?).

The OBC makes its calculations based on the duration of signal to the injectors combined with the fuel flow capacity of the original injectors!

a chip has nothing to do with the OBC correctness. the only thing that would make it's calibration go off is if you changed the injectors to something other than stock.

It's really simple. The OBC needs to know these 3 things to calculate MPG:
-the size of the injectors (basically how much fuel mass will flow at a given PW)
-the duration of the pulse width
-the speed of the car (the OBC gets the real speed, not the "needle" speed you see in the cluster)

The calculation is something like mass x PW / speed

From injector size and PW it can calculate how much fuel you're using at a given instant. then it simply divides your speed by how much fuel you're using.

changing the chip doesn't affect the outcome of the OBC, because it takes the injector PW signal directly. if the injector PW increases because of the tune, it's automatically compensated for because it's multiplying the mass x PW.

The only thing that would throw it off is if your injectors were leaking, stuck/clogged, or if you switched them from stock. You would be changing the "mass" part of the equation which is the only constant.

Honestly, it's probably far more accurate than the fill up/divide by miles method, because you can't account for how much fuel exactly you put in the tank every time. The OBC knows the *exact* mass of fuel you've used.


Oh, MPG = US imperial miles per gallon. M/G = British imperial miles per gallon. The British gallon is 20% larger than the US gallon. If you set it to UK gallons, it will read higher. I'm thinking people that are seeing a "high" reading have their OBC set to UK MPG and just don't know what it means. I guess nobody bothers to read their factory manual. :p

[Earendil]

I used to drive 300 sum miles from college and home. Freeway driving across relatively flat desert at 75mph, 3.73lsd, m20B25 chipped with a MarkD. I once got 28mpg with all the windows down (it was 101 degrees and I didn't have AC). Usually it was 27-29. The OBC usually reads a little bit low for me, meaning my injectors were probably a little bit clogged (i.e. the obc thought I was using more fuel than I really was). Since turning the car into a weekend car, I rarely take the car out of town.

Around town with spirited driving I get more like 15mpg, and rarely see speeds over 40mph, with lots of stop signs and stop lights.

All of my mpg numbers were gathered using entire tanks, or multiple tanks, of gas.

Bonus figure: I know it gets about 10mpg (according to the obc) at PIR ;-)

The British gallon is 20% larger than the US gallon. If you set it to UK gallons, it will read higher. I'm thinking people that are seeing a "high" reading have their OBC set to UK MPG and just don't know what it means.

Without looking to see which gallon is larger, wouldn't a larger British gallon reveal a smaller number on the OBC? If set to British units, you would be rolling through your first US gallon when you were only at 83% of an imperial gallon (using your units).

[nando]

no. Because the gallon is larger, you can travel further on that gallon. it would read higher.

[Earendil]

no. Because the gallon is larger, you can travel further on that gallon. it would read higher.

In my head I was thinking of it as "gallons per mile" which is retarded, but makes me correct. I however submit that you are correct given how it's always read/printed as miles to gallon, and I blame giving up coffee as my excuse.

[dogsbark26]

a chip has nothing to do with the OBC correctness. the only thing that would make it's calibration go off is if you changed the injectors to something other than stock.

It's really simple. The OBC needs to know these 3 things to calculate MPG:
-the size of the injectors (basically how much fuel mass will flow at a given PW)
-the duration of the pulse width
-the speed of the car (the OBC gets the real speed, not the "needle" speed you see in the cluster)

The calculation is something like mass x PW / speed

From injector size and PW it can calculate how much fuel you're using at a given instant. then it simply divides your speed by how much fuel you're using.

changing the chip doesn't affect the outcome of the OBC, because it takes the injector PW signal directly. if the injector PW increases because of the tune, it's automatically compensated for because it's multiplying the mass x PW.

The only thing that would throw it off is if your injectors were leaking, stuck/clogged, or if you switched them from stock. You would be changing the "mass" part of the equation which is the only constant.

Honestly, it's probably far more accurate than the fill up/divide by miles method, because you can't account for how much fuel exactly you put in the tank every time. The OBC knows the *exact* mass of fuel you've used.


Oh, MPG = US imperial miles per gallon. M/G = British imperial miles per gallon. The British gallon is 20% larger than the US gallon. If you set it to UK gallons, it will read higher. I'm thinking people that are seeing a "high" reading have their OBC set to UK MPG and just don't know what it means. I guess nobody bothers to read their factory manual. :p

Well said and I agree except the part about the vehicle's speed being a part of the calculation. I do not believe that vehicle speed is in any way involved in calculating fuel consumption (by the OBC or manual calculation).

[Earendil]

Well said and I agree except the part about the vehicle's speed being a part of the calculation. I do not believe that vehicle speed is in any way involved in calculating fuel consumption (by the OBC or manual calculation).

Technically, no... There is only one sensor that drives both the odometer and the speedo, and all it does is count revolutions of the rear axle. The odem uses that count and the stock wheel size to get a distance, while the speedo uses the same thing and adds time.

However, I believe the sensor is called the speedometer impulse sensor, so it's easy to think of the OBC as using speed in the calculation.

[Earendil]

The only thing that would throw it off is if your injectors were leaking, stuck/clogged, or if you switched them from stock. You would be changing the "mass" part of the equation which is the only constant.

Feeding off the above comments, the other thing that would change it is not having a stock wheel size. If you're running 22" that you are changing the distance portion of the equation, since the OBC/ECU assumes a stock tire size and there is no (easy) way to correct its thinking.

[nando]

you can correct the speedo if you have a motometer cluster, if you have a VDO cluster you need a 100ohm pot and a soldering iron. :)

[dogsbark26]

Yep, yep. I agree with both of you. I am satisfied now that we have accurately described the OBC's calculation.

you can correct the speedo if you have a motometer cluster, if you have a VDO cluster you need a 100ohm pot and a soldering iron. :)

Have you done this nando? Motometer adjustment, that is. According to my GPS, my speedometer reads four to five miles per hour higher than the car is travelling. It seems pretty consistently that same variation if I'm going 35 or 60 mph. I've seen the thread describing the adjusting screw (doohickey?). Just never got around to trying it.

Are we hijacking this fellows thread? Or is this acceptable use?