How accurate are your mpg estimates?

[J.C.]

The car estimated 37.9 mpg over 384 miles (49% EV) but when I filled up my actual mpg was 33.5.

I could live with a 5% margin of error, but this is so bad I just can't trust it at all now.

 

The fuel gauge was above F when I got it and it showed as being above the F after this tank.

I don't top off the tank so there's no chance that I mistakenly added 1.3 too many gallons. 

 

At any rate, does anyone else have this issue?

I'm not necessarily shocked at 33.5 because that it's been 0 to 32 degrees here (32 is a great day) with most of the driving occurring around 0-10 degrees and if I don't run the heat on low, my feet will go numb. 

Don't get me wrong, I'm not thrilled, this is 97% city driving at around 35 mph with lots of braking. 
But I can withhold judgment until the weather is at least in the 40's. It's not usually this cold for this long.

I should mention most of my driving is part of doing deliveries so once the car gets warm it stays warm (more or less); this isn't a case of cold starting it and driving to work, then cold starting and driving home.

As a secondary question, how is the C-MAX fuel gauge? It maybe seemed like the kind that runs a little slow at the top (F) of the gauge and then runs a little faster at the bottom (E).

In my experience every brand or model is different and this is my first Ford.

 

[plus 3 golfer]

There are two components to computing FE: distance traveled and fuel used.

 

Distance traveled as determined by the car is generally less than actual distance traveled by between by 1% to 2%. Fuel used as computed by the car is generally less than the actual fuel put in the tank and a greater % less than the distance variance.  So, your actual FE in mpg will likely be somewhere between 37.9 and 33.5 mpg.

 

Your car is likely operating a good bit of the time in open loop operation which means that fuel / air mixture is not optimized and it is more difficult for the car to estimate fuel being used.  IIRC, closed loop operation begins when the coolant temperature reaches around 140- 145 F.

 

Don't worry about how linear the fuel gauge is.  I look at the Miles To Empty (MTE) display and gallons used on the FE displays.

[SnowStorm]

Wait for more fill-ups before getting too worried.  The gauge is a bit 'dead' at the top as many of them are.  I checked the last 15 fill-ups that were over 7 gallons and my car showed 2.4% low to 6.2% high with an average of 3% high.  I calculate actual MPG with an odometer correction (as mentioned above) of 1.7% which would give you 34 MPG.

[cr08]

Your car is likely operating a good bit of the time in open loop operation which means that fuel / air mixture is not optimized and it is more difficult for the car to estimate fuel being used.  IIRC, closed loop operation begins when the coolant temperature reaches around 140- 145 F.

 

I'll have to check mine and see what it reports but I recall on my old '07 Focus even during the frigid winter months Torque would report it going closed loop pretty quickly around 15-20 seconds in from initial start. Seemed based on that that it only cared about the O2 sensors getting up to usable temp.

 

Hopefully I'll be able to take mine out this weekend and verify after all our snow starts melting. Sadly haven't driven mine all week due to shitty snow management by our city. Our cul-de-sac has been untouched since our major snowfall and freezing rain last Friday. Tried going out Monday and got stuck 2 feet from the edge of my driveway. Have had to commute with my brother only because he has an AWD SUV with brand new tires and even then it is sketchy getting into our neighborhood.

[J.C.]

Yeah this car has been terrible in snow.  I can see why the single owner before me from Michigan sold it.
I'm 100% okay with that, we don't get that much snow here [or 0 degrees]. I can live with 10 good months out of my C-MAX and 2 months of 34 mpg, which is still the best fuel efficiency I've ever gotten in my life.  :) 
 

[cr08]

Yeah. I was kinda hoping it'd be better than rumored but the unfortunate side effect of EV propulsion. Lots of torque to break traction with. And the traction control unfortunately seems to be pretty lackluster. For me it seemed to let it spin unmitigated for a few moments then cut back all power and then try again. No sweet spot in between.

Like you though I'm not too miffed since we hardly get this much snow and ice here. At most we get a very light dusting once or twice a season and it remains pretty driveable. This last blast was just a fluke.

 

Now if I lived up in the snow belt where I grew up I likely would have reconsidered. That or kept my old Focus as a winter beater.

[ptjones]

Your gas mileage doesn't have to be that bad, I'm still getting 51.5 mpg actual with temps 17*F to 40*F, are us using Winter Tips: http://fordcmaxhybridforum.com/topic/5511-how-to-improve-winter-mpgs/:)

 

Paul

[plus 3 golfer]

I'll have to check mine and see what it reports but I recall on my old '07 Focus even during the frigid winter months Torque would report it going closed loop pretty quickly around 15-20 seconds in from initial start. Seemed based on that that it only cared about the O2 sensors getting up to usable temp.

 

Hopefully I'll be able to take mine out this weekend and verify after all our snow starts melting. Sadly haven't driven mine all week due to shitty snow management by our city. Our cul-de-sac has been untouched since our major snowfall and freezing rain last Friday. Tried going out Monday and got stuck 2 feet from the edge of my driveway. Have had to commute with my brother only because he has an AWD SUV with brand new tires and even then it is sketchy getting into our neighborhood.

Here's what I believe happens.  The O2 sensor heaters ramp up quickly to a temperature above the closed loop heater control temperature.  The O2 sensors temperature is then allowed to fall to the closed loop heater control temperature (based on measured impedance) and the O2 sensor temperature (impedance) is now controled around that temperature.  The heater is now in closed loop operation and the PCM will enter closed loop fuel operations if other entry conditions are met.  The closed loop heater control happens fairly quickly which is likely the 15 seconds or so.

 

But the fuel trim and other monitors do not start operation until closed loop O2 heater operation, specific coolant temperature and certain other paramenters are met.   "If the heated oxygen sensors (HO2S) are warmed up and the PCM determines the engine can operate near stoichiometric air to fuel ratio (14.7:1 for gasoline), the PCM goes into closed loop fuel control mode." _{-service manual.}

 

From the Ford OBD System Operation Summary for Plug In and Hybrid Electric Vehicles document:

 

"During closed loop operation, short term fuel trim values are calculated by the PCM using oxygen

sensor inputs in order to maintain a stoichiometric air/fuel ratio. The PCM is constantly making adjustments to the

short term fuel trim, which causes the oxygen sensor voltage to switch from rich to lean around the stoichiometric

point. As long as the short term fuel trim is able to cause the oxygen sensor voltage to switch, a stoichiometric

air/fuel ratio is maintained.

 

When initially entering closed loop fuel, SHRTFT starts 1.0 and begins adding or subtracting fuel in order to

make the oxygen sensor switch from its current state. If the oxygen sensor signal sent to the PCM is greater than

0.45 volts, the PCM considers the mixture rich and SHRTFT shortens the injector pulse width. When the cylinder

fires using the new injector pulse width, the exhaust contains more oxygen. Now when the exhaust passes the

oxygen sensor, it causes the voltage to switch below 0.45 volts, the PCM considers the mixture lean, and

SHRTFT lengthens the injector pulse width. This cycle continues as long as the fuel system is in closed loop

operation." 

 

But, the fuel monitor (and other monitor like the Air/Fuel ratio monitor) has the entry conditions below.  When the engine is "too" cold, the PCM will run the air / fuel mixture rich. So, the monitors won't run under 155F as the engine may not be running near the stoichiometric A/F ratio and "incorrectly" trigger a fault condition.

 

"Entry condition

...................................Minimum Maximum

Engine Coolant Temp ...155F .....230F

Intake Air Temp ............-40F ......150F

Engine Load .................30%

Purge Duty Cycle.......... 0% ........0%

Fuel Level...................... 15%"

 

IMO, the computed fuel used would be more difficult to quantify when ECT is less than 155F.   Since the OP indicates city driving with stops there's a good chance ICE coolant temperture is under 155 F a lot of the time in very cold weather.

 

OP, you need grille covers to help raise coolant temperatures.

Edited January 21, 2018 by Plus 3 Golfer

[J.C.]

I will add some tape or grille covers.
I agree that it was the temps. It was pretty close to 0 here for many of those days. 

I just filled my 2nd tank (temps 32+) and got 44.2 mpg with estimates of 46.4. So not only was the mpg good, the estimate variance went from over 11% to under 5%.

[fbov]

First off, welcome!

 

Second, get used to pump-based MPG short-falling the car's estimate. My 100 fill-ups, in-car fuel use averages 5% lower than the pump. The car misses some of the fuel use... Plus3 has some interesting insight as to the cause, but it's in my "get used to it" bin. 

 

Yeah this car has been terrible in snow.  ...

 

ALL CARS are terrible in snow... those that do work owe it to their tires.

 

Yeah. I was kinda hoping it'd be better than rumored but the unfortunate side effect of EV propulsion. ...

Now if I lived up in the snow belt where I grew up I likely would have reconsidered. That or kept my old Focus as a winter beater.

It's much more a side effect of using tires not designed for the purpose. 

 

OEM Michelin Energy Saver A/S and an All-Season tire that works well in snow

1)  2) 

 

And now, a real snow tire, and a new, all-season that also carries the "3-peak mountain snowflake" symbol identifying a snow tire.

4)  3) 

 

Clockwise from OEM, we have...

1) a low-rolling resistant tire with hard tread rubber, large monolithic tread blocks and straight circumferential tread grooves  

2) a compromise tire with hard tread rubber, fully siped tread blocks and zig-zag grooves around the tire plus a center rail. 

3) a compromise tire with hard tread rubber, long, narrow, siped tread blocks with no circumferential groove

4) a studless snow-and-ice tire with hydrophilic tread rubber, small tread blocks divided by many sinusoidal sipes, and a center groove. 

 

In snow, a new set of OEM's is not comparable to the other three, and a worn set are dangerous. 

 

In deep snow the top two are not comparable to the bottom two. We have these on our cars this winter... 

 

So don't blame the car if you're not using tires suited to the task. 

 

Have fun,

Frank

 

[GordieT]

You don't say if you garage your C-Max Hybrid...?

 

If you are leaving it outside in 0° to 32° weather, it WILL immediately start the gas engine when you turn they key on.

 

A layman will know this is happening because you will audibly hear the engine kick in immediately.

 

As you know, the Ford C-Max Hybrid starts silently under normal (not severe) conditions.

 

Remember, without all the fancy terms, this car is always seeking to find the best fuel mix between high voltage batteries and gasoline.

 

Also, the real time MPG rating is automatically averaging between the use of gas, HV batteries and power returned through braking.

 

A good way to see this is to choose a trip odometer and zero it out when you start up. In that weather, for a while you will be lucky if it climbs higher than 10 MPG. I've seen it as low as 4 MPG in sub freezing weather.

 

At that moment of startup, it's drawing 100% gas.

 

The positive news is that it also immediately starts sending a charge to the high voltage batteries.

 

I tested my 2014 Ford C-Max Hybrid SE during freezing weather one morning, on a five mile run to the grocery store, over roads that are slightly rolling, with both my house and the store at about the same altitude (so one is not "uphill" from the other).

 

On the trip TO the store, I hit 14 MPG. On the trip home a few minutes later, I hit 28 MPG.

 

Here is the other factor: I was running my heater full-force all the time.

 

I also avoided rapid acceleration while braking conservatively (no short, hard stopping).

 

Your personal driving habits will greatly affect your MPG. Of course, most of us love the way these babies accelerate so quickly, and if you've learned to brake without caution, you will likely continue that habit unless you consciously work to change it.

 

For that reason, I keep my display set at "engage", to see braking, gas and HV batteries usage.

 

On a road trip from Nashville to the Jacksonville, Florida area (during warm weather) and returning to Nashville, I averaged 42 MPG for the trip, using cruise control whenever possible and still also running the A.C. during the whole trip.

 

On a warm day, my round trip to my local grocery store averages 56 MPG, as opposed to those numbers I quoted in freezing weather.

 

This Fall, I was doing a lot of moving, using very poor driving habits, and I had 34 MPG (leadfoot numbers).

 

Cheers!

[ptjones]

GordieT you need to make Grill Covers.  :)   For the last 50k mi. I have averaged 53.3 mpg on my Smart Trip Gauge so that means 52.3 mpg actual (fuelly.com mileage). On my Life Time mileage is 1 mpg better than actual fuelly mileage for 185k mi. 

 

Paul