[dradernh]

Originally Posted by Mason Hatcher

Curious what RPM the six cylinder with the 6 speed manual turns at 60 and 70 mph...

This graph of gears 2-6 will give you a general idea of the speed and RPM in gears for a manual M240i with a 3.08 diff ratio:

[albertw]

Originally Posted by aerobod

A medium sized air conditioning unit on a car such as an F22 will have a capacity of about 6kW or 20,000BTU/hr. In steady state cruising at 96km/h / 60mph the car will be consuming about 11kW of engine power with the AC off on level ground, using 17kW at maximum AC capacity is equivalent of adding 16km/h of speed to 112km/h / 70mph.

Aerobod, this is a great bit of information, but for me the numbers are not consistent and I hope you will check them.

You say that at 60 mph, with max AC the car uses 55% more gas than with the AC off (11 vs 17 kW or 14.75 vs 22.75 hp). Then you say this is the same as increasing speed from 60 to 70 mph.

The number I've used from the days when the 55 speed limit was introduced is that slowing from 70 to 55 reduces gas consumption by 15%. That number is consistent with my observations in several different vehicles, and is far less that the 55% difference you claim for a smaller speed difference.

[aerobod]

Originally Posted by albertw

Aerobod, this is a great bit of information, but for me the numbers are not consistent and I hope you will check them.

You say that at 60 mph, with max AC the car uses 55% more gas than with the AC off (11 vs 17 kW or 14.75 vs 22.75 hp). Then you say this is the same as increasing speed from 60 to 70 mph.

The number I've used from the days when the 55 speed limit was introduced is that slowing from 70 to 55 reduces gas consumption by 15%. That number is consistent with my observations in several different vehicles, and is far less that the 55% difference you claim for a smaller speed difference.

Hi Albertw, the 55% difference is in the approximate power needs that correspond closely to aerodynamic drag at steady highway speeds, that is proportional to the cube of speed. The specific fuel consumption for a given power output is not very linear, so at low power outputs you won't see anything like a 55% change in fuel consumption.

My experience is similar to your experience, in the case of the F22, I would say more like 10% change in fuel consumption change in going from 89 to 113km/h (55-70 mph), which is approximately a doubling of the power requirement between the two speeds.

[albertw]

Originally Posted by aerobod

I would say more like 10% change in fuel consumption change in going from 89 to 113km/h (55-70 mph), which is approximately a doubling of the power requirement between the two speeds.

OK, I see the problem. My understanding is that power used and fuel consumption are directly proportional. Twice the power should, if I'm right, mean twice the fuel consumption, with a small adjustment for the higher speed, which means that twice the power is used for less time.

[aerobod]

Originally Posted by albertw

OK, I see the problem. My understanding is that power used and fuel consumption are directly proportional. Twice the power should, if I'm right, mean twice the fuel consumption, with a small adjustment for the higher speed, which means that twice the power is used for less time.

When the engine is running at peak efficiency at high power outputs, that is much closer to the case, but the specific fuel consumption varies a lot over the usable power range. I may be able to quantify that better if I look at the detailed fuel injector pulse times for my Caterham for some logged runs I have done, where I can compare detailed engine parameters for specific fixed speeds (I don't have the data for my M240i).

The lowest specific fuel consumption (fuel consumption per unit of power produced) is at or near maximum torque (depending on variation in lambda at particular RPMs and throttle openings), the worst at idle.

[albertw]

Another issue is your assumption that the power required is proportional to the cube of speed. That is true for air drag, but not for most of the other relevant types of friction (tires, bearings, gears, rings, oil and coolant pumps and other accessories) which are either independent of road speed or vary much less than by the cube of road speed. My guess is that these non-cube forces are much more of the explanation for the relatively small difference in fuel consumption between 55 and 70 (vs assuming the power required varies with the cube of road speed) than is the specific fuel consumption.

None of this disagreement alters the fact that your numbers show that the air conditioner can have a very substantial effect of fuel consumption at moderate highway speeds. I have no experience with air conditioning at the temperature reported by the original poster, but I suspect that with a 40 degree difference the compressor works almost constantly. If so, your numbers say it alone could consume up to half of the power it takes to move the car with the AC off.

[aerobod]

Above about 80km/h the aerodynamic drag is predominant on cars with a Cd in the 0.3 range. rolling resistance and other driveline losses are roughly proportional to power delivered and around 12 to 15% for a modern RWD car, measurable as the difference between flywheel and rear wheel horsepower, as measured on a roller style dyno.

The max aircon load is about the biggest variable load, but not necessarily switching to max load with a modern variable displacement system. The alternator on current BMWs is also variable from a load perspective.

Although there can be a fair load change at highway speeds due to the accessories, it doesn’t have a massive change in fuel consumption. The proportional effect of driveline losses also has the effect of scaling the available engine power output due to its linearity and the aerodynamic drag is definitely the predominant force at highway speeds and above, so is responsible for the bulk of power change requirements as the speed varies, but also doesn’t cause a proportional change in power requirements due to specific fuel consumption improving as power requirement increases when below the engine torque peak.

[Mason Hatcher]

Originally Posted by dradernh

This graph of gears 2-6 will give you a general idea of the speed and RPM in gears for a manual M240i with a 3.08 diff ratio:

Interesting. I was just curious. I have the 4 cyl and 8 speed. It tacs right at 2k at 80 mph, so this gives me a relatable comparison.

I realize the two additional ratios makes a huge difference.

[albertw]

Aerobod, this is looking to be a major learning experience for me, so I want to be clear about your statements. You say that 70 mph requires 59% more power than 60 mph, but fuel consumption increases by only 3.4%. That means the specific fuel consumption at 70 must be 65% of what it is at 60.

To me that is an incredible difference, considering the power levels are only 4.5% and 7.1% of maximum hp - the engine is just loafing at either speed.

My understanding is that power losses inside the engine, the accessories and the front wheel friction add a substantial amount to the usual claim of a 15% power loss between the flywheel and the rear wheels. That portion of the loss probably averages out to roughly linearly dependent on road speed. Taking an extreme of half the power change between the two speeds as linearly dependent on speed and half on the cube of speed gives a 37% increase in power needed between 60 and 70. The correct number likely is somewhere between 37% and your claim of 59%. That still indicates a difference in specific fuel consumption that is astonishing to me, but I've only looked at fuel flow vs power in aircraft which seldom run at less than 50% of max power.

The reason this is such a surprise (and disappointment) to me is that the discrepancy between speed change and fuel consumption change should be obvious to anyone who thinks about it and I failed to notice it.

[aerobod]

Originally Posted by albertw

Aerobod, this is looking to be a major learning experience for me, so I want to be clear about your statements. You say that 70 mph requires 59% more power than 60 mph, but fuel consumption increases by only 3.4%. That means the specific fuel consumption at 70 must be 65% of what it is at 60.

To me that is an incredible difference, considering the power levels are only 4.5% and 7.1% of maximum hp - the engine is just loafing at either speed.

My understanding is that power losses inside the engine, the accessories and the front wheel friction add a substantial amount to the usual claim of a 15% power loss between the flywheel and the rear wheels. That portion of the loss probably averages out to roughly linearly dependent on road speed. Taking an extreme of half the power change between the two speeds as linearly dependent on speed and half on the cube of speed gives a 37% increase in power needed between 60 and 70. The correct number likely is somewhere between 37% and your claim of 59%. That still indicates a difference in specific fuel consumption that is astonishing to me, but I've only looked at fuel flow vs power in aircraft which seldom run at less than 50% of max power.

The reason this is such a surprise (and disappointment) to me is that the discrepancy between speed change and fuel consumption change should be obvious to anyone who thinks about it and I failed to notice it.

Normally in calculating the effect of aerodynamics, I would eliminate the linear driveline losses first (in the case of the M240i, 230kW seems to be the rear wheel power from posted dyno runs), then try and estimate the vehicle top speed if it can hit peak power from a gearing perspective, likely about 280km/h plus or minus 5km/h for the M240i (based on it's frontal area, similar cars, etc). At this point the cube of the given speed divided by the cube of the top speed multiplied by the available horsepower after driveline losses will give a close approximation to the power required to overcome aerodynamic drag at a given speed.

At 100km/h with 15% driveline losses, 10kW would be used in overcoming aerodynamic drag, 2kW in overcoming driveline losses for a total of 12kW at the flywheel. At 120km/h, 18kW for aerodynamic drag and 3kW for driveline losses for at total of 21kW at the flywheel.

Looking at the logs for my Caterham for steady state at 3000RPM and 3750RPM in top gear which correlates to 90km/h and 113km/h road speed, I see that I average throttle site 5.4 at 90km/h at 3000RPM and throttle site 6.0 at 3750RPM at 113km/h.

Looking at the fuel map below that shows injector timing relative to throttle position and RPM (and interpolating for throttle site 5.4), I need about 3.89ms of injector timing per RPM at 3000RPM and 4.17ms of injector timing at 3750RPM. Notice how the fuel used per cycle decreases as the RPM increases for a given throttle position. As we are measuring the fuel used per distance travelled, the injector timing per RPM is the value to directly compare fuel consumption at the two speeds, so we see 4.17/3.89 = 1.072 or 7.2% fuel consumption change, with an aerodynamic load change from 90 to 113km/h of 98%.

[aerobod]

In terms of the fuel economy of aircraft engines running at high power outputs vs cars running at low power outputs, you can see a consistency of fuel consumption in the fuel map at high throttle and RPM settings that is more linear at high loads for the car engine, too, but at low loads the engine is much more inefficient, so the decreasing injector pulse width as the RPM increases is an indication of rapidly increasing engine efficiency as the load increases at low throttle settings.

[XutvJet]

Highway mpgs can be influenced by a number of factors.

60mph in 6th might seem ok and fuel efficient at the surface, but the rpms are pretty low (~2,200rpms) and if you're approaching a grade, wind, passing, etc., the motor is going to go into high load situation and use a lot of boost and subsequently fuel. The 8ATs will unlock the torque converter and/or downshift a gear or two to get the necessary torque multiplication so that the engine doesn't have to work as hard.

The OP has not told us how much distance this "trip" covered. That makes a difference. Was this steady state driving like with cruise control or was he stuck in beach traffic? Other factors at play could be lots of throttle inputs to maintain speed such as dealing with hilly conditions, wind, or even crappy fuel like that sold in AZ, CA, NM, NJ, and NY. Wet conditions add drag. The A/C consumes fuel too (not much).

It's been my experience that power modifications have little impact on steady state highway driving as boost isn't being used. Over 5.5 years of ownership, my M235 6MT as routinely seen 25mpg to 29mpg driving 100+mile distances running the cruise at ~75-78mph. If it's windy, hilly, lots of traffic, or winter, MPGs will be are the lower end.

[bcwalks2]

Originally Posted by rainfall

Hey guys!

how much are you guys getting on the N55 manual MPG on a highway?
I went on a trip recently and got 22.5mpg on a highway. Felt a little low, but AC was on the whole trip and it was (literally) 110F/43C outside.

I was quite shocked with so low MPG, esp because I drove most of the time at 60mph and 6th gear... I asked my friend who has a B48 330i 8AT, he said he got more like 30mpg on a similar trip.

More horses drink more?

Also, I tried to use the search function and got overwhelmed w/ tuning results... :P My car is bone stock.

Bought my 2016 M235i 6 speed from GA and drove back to IL… I got 27MPG. Around town I get over 20 and I drive it pretty good.

[Unicorn123]

Interesting thread; good stuff! The engineers are way too smart for me...

My two cents; I consistently get 26 MPG (I have the 8AT) on my ~100 mile RT commute; 10% country road, 90% Highway, at a typical average speed of ~70MPH. My driving varies from early morning ~0545 hours light traffic to a commute home that was really nice during the peak of COVID and is now turning back into the drive from Hades I was used to in the past and hoped would be in the past...

It is not uncommon on the highway to drive 20 over posted to keep up with traffic in the morning and in the evening when there is not too much traffic. This is over ~50K miles. Not flat roads; typical rolling hills in CT. In the last month, my typical evening commute was at 90+ degrees F. Cold tire pressures (PSI) are 33F, 38R.

Car is typically in Comfort mode and occasionally Sport.

Despite the temperature the OP noted, it seems a bit excessive to have seen such a significant drop in MPG. I am wondering how this was calculated in the first place. Actual pump fills? Car readout? Also, as noted, how far? Seems like we are missing some data here!

On a side note, albertw, I see that you do not have your Pitts in your signature anymore. I am hopeful you still have your aircraft. I expect it is a source of great enjoyment!

[albertw]

Originally Posted by Unicorn123

On a side note, albertw, I see that you do not have your Pitts in your signature anymore. I am hopeful you still have your aircraft. I expect it is a source of great enjoyment!

I deleted my non-automotive toys from the signature.

I decided to get rid of the Pitts after 30 years to avoid doing a complete strip and recover to protect the steel parts from the corrosive environment I moved to recently. I replaced it with a mostly carbon fiber monoplane. The roll rate is impressive - 400 degrees / sec - and occasionally useful. Snaps are more challenging to do well. Aside from needing a bit more speed and g because of the higher wing loading, everything else is very similar to the Pitts.

[rainfall]

thanks guys, I did another test, 4 short 20 mile trips this time

the first one I got like 23mpg again, then I changed 4 things: absolutely no AC, no hard pulls, reset the "driving style" on the computer, double checked tire pressure (rear tires were like 2 PSI below spec, so I rectified that).

after these 4 changes, I tried it again. 95%+ highway, no AC, no hard pulls, basically cruising 70% of the time. Trips were about 20mile long. I basically drove like grandma - pretty depressing to see some 8-year old GTS owners honking at me and I was just waiving back but not driving hard

MPG was pretty consistent at 25-26 mpg, but i basically had to switch gears around 1500-2000 rpm. Geez so boring.
I was hoping I could get around 25mpg with some hard pulls and then cruising,... am I missing something? I know someone mentioned air intake filter

[aerobod]

If you can find a flat stretch of road that you can set the cruise control to 70mph in 6th gear when the wind isn't too strong, you should be able to reach within 1 mpg or so of 30mpg (8 litres / 100km). Most other driving conditions will be worse fuel economy than that.

If fuel economy is a major consideration, then a 335d with a Malone tune will give you the same or better performance and 5.5 litres / 100km (43mpg) on the highway at cruising speed, but is only available with an automatic transmission

[TwoTonic]

I suggest calculating your mpg after every fill- it is literally miles (on that tank) divided by gallons added. The read outs provided by the car, especially “instant” MPG are not super reliable. The longer term trends in your mpg will be more meaningful as you’ll have a proper baseline for comparison.

[Bud miller]

lots of variables boys..
temps, speed .tires size and pressures, road cond, elevations, hills, night-day etc etc...

in CA if i drive 60 mph i will get blown off the road��

[XutvJet]

Originally Posted by rainfall

thanks guys, I did another test, 4 short 20 mile trips this time

the first one I got like 23mpg again, then I changed 4 things: absolutely no AC, no hard pulls, reset the "driving style" on the computer, double checked tire pressure (rear tires were like 2 PSI below spec, so I rectified that).

after these 4 changes, I tried it again. 95%+ highway, no AC, no hard pulls, basically cruising 70% of the time. Trips were about 20mile long. I basically drove like grandma - pretty depressing to see some 8-year old GTS owners honking at me and I was just waiving back but not driving hard

MPG was pretty consistent at 25-26 mpg, but i basically had to switch gears around 1500-2000 rpm. Geez so boring.
I was hoping I could get around 25mpg with some hard pulls and then cruising,... am I missing something? I know someone mentioned air intake filter

Maximum estimated factory mpgs are based on VERY conservative driving. You need to run a full tank at a highway cruise (65-75mph) to really gauge the maximum real world mpgs for your car, not just 20 mile stints. I would bet you could achieve 28-30mpg in pure highway driving over around 300 miles.

Overall, you should only expect 19-23 mpg in the city and 21-25 mpg in highway driving. The worse the conditions, heavier your foot, the lower the octane (91 vs 93), and the more power mods you have, the lower you're car will be with those mpg estimates.

Over 5.5 years of ownership since new, I routinely see ~22-23mpg in 50/50 city and highway driving, getting on it fairly often on entrance ramps, and fairly rapid acceleration up to speed. In the winter, that number drops to 20-21mpg due to the extended warm up, oxygenated winter fuel, and denser air.

[rainfall]

Originally Posted by XutvJet

Maximum estimated factory mpgs are based on VERY conservative driving. You need to run a full tank at a highway cruise (65-75mph) to really gauge the maximum real world mpgs for your car, not just 20 mile stints. I would bet you could achieve 28-30mpg in pure highway driving over around 300 miles.

Overall, you should only expect 19-23 mpg in the city and 21-25 mpg in highway driving. The worse the conditions, heavier your foot, the lower the octane (91 vs 93), and the more power mods you have, the lower you're car will be with those mpg estimates.

Over 5.5 years of ownership since new, I routinely see ~22-23mpg in 50/50 city and highway driving, getting on it fairly often on entrance ramps, and fairly rapid acceleration up to speed. In the winter, that number drops to 20-21mpg due to the extended warm up, oxygenated winter fuel, and denser air.

thank you. So basically if someone drives like normal, they should get around 22-23mpg.

Sorry if that wasn't clear, but I wasn't trying to maximize MPG at all costs, I was just trying to see how realistic is/not to get 26mpg+ on a highway while still doing one hard pull every so often. I do let the car warm up, I don't floor until the car has been running for a few miles... etc.

Bottom line is, it sounds like it's not feasible to get 26+ mpg without driving slower than a Corolla

[KennyFSU]

Which I'm fine with, I didn't buy this car for great fuel economy lol.