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Computer-calculated fuel mileage vs. actual measurement at the pump

5K views 35 replies 9 participants last post by  Dave2302 
#1 · (Edited)
Been reading of some discrepancies over on the W211 forum, especially the CDI's, about the MPG dashboard readouts being, shall we say, optimistic.

http://www.benzworld.org/forums/w211-e-class/1725554-mpg-reading-overstated.html

So, I wondered if this applied to the W220's as well.

Given that I've been driving the S600 TT as my daily driver, I figured it should be the first candidate for examination. Here is my test methodology.

1.) Ensure the tires are properly filled up.
2.) Fill up the gas tank.
3.) Start the engine.4
4.) Reset the MPG counter in the menus (on my car, that's three menus down).
5.) Reset the trip odometer.
6.) Go driving.
7.) At next fill-up, take down what the computer says, "after reset". Compare them to calculation from actual miles traveled from the trip odometer vs. gallons of gasoline it takes to fill the car back up again.
8.) Rinse and repeat.

So, here we go with some examples from some receipts that I had kept.

Late December 2016, after 400+ miles of pure highway driving, averaging 78 MPH:

Computer readout: 19.3 mpg
With calculator: 19.53 mpg

Again, late December 2016, 350 miles of pure highway driving, averaging about 68 mph:

Computer readout: 20.5 mpg
With calculator: 20.67 mpg

March 2017, combined driving, 320 miles:

Computer readout: 13.6 mpg
With calculator: 14.3 mpg

Early April 2017, mostly highway driving, with some city driving, 360 miles:

Computer readout: 17.0 mpg
With calculator: 18.2 mpg (hmm...big difference here....)

Mid April 2017, mostly highway driving, with some city driving, 370 miles:

Computer readout: 18.7 mpg
With calculator: 18.2 mpg (hmm...this time it's slightly lower)

Last week, pretty much all city driving, 280 miles:

Computer readout: 11.3 mpg
With calculator: 11.7 mpg


The car is a 2003 S600 twin-turbo with the following relevant characteristics:

1.) 19" made-in-China replica AMG-style staggered rims, 8.5" wide in front and 9.5" wide in back,

2.) Michelin Pilot Sport AS3+ staggered tires, brand new, 245/40R19's in front and 265/35R19's in back,

3.) Strutmasters coil-over spring conversion kit, i. e. no automatic lowering of the car at speed,

4.) Costco 93-octane gasoline,

5.) Mobil-1 0W-40 European-spec motor oil from Walmart (with proper M-B brand oil filter), and

6.) Meyle 236.10-spec transmission fluid (it's a 5-speed 722.6).


These examples aren't the only time I've done these comparisons; I do it at every fill-up. These are just a few representative examples.

Initial conclusion: it appears that, most of the time, the computer on this car displays slightly lower fuel efficiency numbers than the calculation at fill-up. So, it's generally slightly conservative, but pretty close to actual. This, I don't mind.

A little side note: just this week, I got those made-in-China junk wheels replaced with some refurbished, genuine-MB, made-in-Germany wheels at all four corners. Turns out the previous owner had gone cheap-o in this area. All four of them had multiple weld repairs for small cracks in the bead area, and two of them had re-cracked and started leaking air. Hmpf....ya just don't do that to a Benz. Anyway, that should not be a problem anymore, and the car looks considerably better now, too.

Back to regularly scheduled program....

The next car to get this fuel-measurement treatment will be the gray S430, also a 2003 model.
 
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#3 ·
Hi,

Also consider that if the Tyre Sizes are not calibrated correctly in COMAND / Dash Cluster, or are over / under inflated, new / worn this will have an effect :wink

Cheers Dave
 
#4 ·
Good points. The average temperatures when I've been doing these comparisons has been between 10 and 20 deg. C, so the energy density should be pretty close.

Agreed on the tire sizes, too. Turns out that 19" wheels were apparently an (expensive) option on the 2003 S600's here in the USA, so the previous owner apparently did stay with a stock size, even if he had gone cheap-o on the brand and condition. Like I mentioned, that problem is now fixed, as of earlier this week.

The dash readings are close enough for me to be fine with them, at least on this car. Generally, they're actually pretty close.

Speaking of wheels, the grey S430 is getting its wheels refinished (bye-bye, curb rash), and they should be done today, so I'll be putting those on this evening and begin with the tests on this car.
 
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#6 ·
Some comments...

The computer calculates mileage by (in effect) adding up the amount of time that each of the injectors are open. That will give "reasonably accurate" mileage readings, as long as none of the affecting parameters (fuel pressure, injector size, etc.) have been changed.

The amount of fuel used and the measured distance traveled are used to calculate the fuel economy. Obviously, if you have not entered the proper tire size and wear in the car's computer, the actual mileage will be off. Of course, you are going to use the same inaccurate distance to calculate your mileage.

Virtually all late-model light vehicles have a built-in "air bubble" in the fuel tank, usually just a small bulge or dome in the top of the tank, or an interior partition that creates a similar area. This gives a small air cushion during those times where you fill the tank with cool fuel or on a cool morning, and then the tank and fuel get hotter. The air cushion gives a bit of room to allow the expansion, preventing fuel venting or flooding of the carbon canister. This air cushion makes it very difficult to fill the tank to exactly the same level each time, and you have no way of knowing how much more or how little more fuel you are adding.
 
#8 ·
Hi,

I got my S500 down to 9 when (ahem cough) driving it 60 miles odd on a race track (cough ahem) :rolleyes:

My G/F got 27 on the return journey :grin

Generally it was averaging around 20 with me driving it reasonably sensibly :wink

Cheers Dave
 
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#9 ·
Been doing some leather refinishing (the Leatherique kit) on the silver S430's's Alpaca Grey leather, since I had the interior apart anyway due to the trunk wiring harness experiment. The leather really wasn't in bad shape before--no rips or tears, just a few blemishes that I wanted gone--and now, it's looking pretty darn sweet. Since it's hard to get exact matches on colors like Alpaca Grey, even with a color sample, I went ahead and did the entire seats while I had the interior apart anyway. It got three coats over the last week and a half, for durability. That leather has now cured/baked in Virginia's sunny weather for the last three days and should be good to go by now; been driving another car in the meantime.

And now that this is done....

Time to continue the fuel-mileage experiment! This particular S430 is now my daily driver again, and we'll see how it does. Ambient temperatures during the day are now between 27 and 30 degrees Celsius, dipping to between 20 and 22 at night. Welcome to June in the State of Virginia.

Will update with what I find and compare it to the measured fuel economy of Dad's S430 (didn't know about the computer measurements back then, so I did only paper/pen). In the case of Dad's car, it got 26.67 mpg over 2,100-and-change miles, at an average speed of about 75 mph on the Interstates. Tires on that car are Vogue Custom Radial Wide-Track II Whitewalls, 225/60-R16. I've got a road trip coming up with the silver S430, and I expect to see similar results.
 
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#10 ·
With the Bosch-type injection, you can improve the mileage noticeably with driving technique.

With fully-closed (emphasis on "fully") throttle above approx 1450 RPM, fuel flow is totally shut off. This means that if you manage to make yourself totally lift off the throttle and coast, you are getting infinite mileage for that distance. You are making distance, but using no fuel at all.

Since we have been trained as long as we have been driving to maintain the speed of the vehicle, it is very difficult to make yourself do this "squirt and coast" technique, but this is what the economy run drivers have always done, even in the days of carburetors. The technique works even better with injection.
 
#11 · (Edited)
OK, just got back from a 1,600 mile road trip to/from Macon, GA this Saturday. Time to publish some results.

The car, with relevant info:
--------------------------
  • 2003 Mercedes-Benz S430 with AMG Sport Package
  • Tires: Bridgestone Potenza RE970AS Pole Position, 245/45-R18 in front, 265/40-R18 in rear, now with about 6,000 miles on them
  • All new front suspension, with Arnott reman AIRmatic springs (in other words, bone-stock)
  • Mixture of mostly Fuchs Titan 236.12 trans fluid, with 4L of Meyle 236.10 fluid
  • New conductor plate, trans filter, and transmission electrical connector, all M-B brand (hence the mix of trans fluids--had to drop the pan to do the conductor plate and filter)
  • Mobil-1 engine oil, 0W-40 European formula, with about 3,500 miles on it at start
  • Costco 93-octane gasoline on the way down, a 40/60 mixture of Kroger and Costco on the way back (again, 93 octane)
  • Ambient temperatures: highs of 35 deg. Celsius (95 Fahrenheit), lows of 27C (80.6F). It's summertime, summertime, sum-sum-summertime!
--------------------------

I did the conductor plate the afternoon before the trip, to make sure the transmission would be OK. That was either confidence, foolishness, or a combination of both. I'm leaning toward the latter, with a tilt toward the middle option. Yes, I checked with STAR for any codes after the work was done, and a test drive was also performed (still no codes). Then, I went to Costco and filled up the car with some 93-octane, drove straight back home (it's only 3 miles), and packed essentially a backpack for the week's trip. The car didn't get driven again until the next morning.

Stats to come soon as I get home from work. Just realized the gasoline receipts are in the car and not with me! Will update this post in a couple of hours.
 
#13 · (Edited)
Ha! I can laugh because that's about what my S600 gets, too, after 12 miles of driving! We don't buy S600's for the Mileage; we buy them for the Smileage. :-D

The morning that I left, I topped off the S430 at my local Costco. Note: I had just replaced the transmission conductor plate the afternoon before. This was 18 June 2017, after a tankful of mostly city driving.

Miles: 358.6
Gallons: 22.108
Calculated efficiency: 16.22 mpg
Computer said: 17.3 mpg

Got on the highway and made it to Lexington, South Carolina (SC) the same day (18 June). Filled up there.

Miles: 485.0
Gallons: 20.000 (yep, right on the button)
Calculated efficiency: 24.25 mpg
Computer said: 29.5 mpg
Average speed: 72 mph

Stark difference, isn't it? The computer's now reading pretty darn optimistically, about 21% high. Let's see how things continue to go.

Got to Macon, GA. This was about 60% to 70% highway driving, with the rest being city driving. Filled up at the local Kroger's on 23 June 2017.

Miles: 405.8
Gallons: 19.528
Calculated efficiency: 20.78 mpg
Computer said: 26.1 mpg

Wow...the computer was even more optimistic this time, by about 25%! What the heck is goin' on here??

Decided to do a fair amount of highway driving around these country "freeways" and explore the countryside. This was the evening before coming back. Speed limit ranges between 55 and 65 mph. Then I left the next day for home. This was virtually all highway driving. Stopped at the Costco in Augusta, GA to top off and get a fuel measurement; I had just over 1/2 a tank remaining. This was 24 June 2017.

Miles: 249.2
Gallons: 10.359
Calculated efficiency: 24.05 mpg
Computer said: 30.1 mpg
Average speed: 55 mph

Again, the computer was about 25% optimistic.

Got close to home and realized I was running low, so I stopped to put in 5 gallons so I didn't run out of gas. This is the one and only time I've ever used regular (87 octane) in this car; all other times, I've been using premium (93 octane). The price differential at this gas station was horrendous. When I got close to the Empty line, I filled back up with 93-octane at my local Costco. Good to be home.

So, here we go.

Miles: 616.7
Gallons: 5.004 (Sunoco 87 oct) + 22.051 (Costco 93 oct) = 27.055 gal
Calculated efficiency: 22.79 mpg
Computer said: 27.6 mpg
Average speed: 49 mph

Note that this last fillup also did contain some city driving, hence the slightly lower numbers. But again, the computer was optimistic by about 21%.

It appears that, generally, the computer is optimistic by between 5 and 6 mpg after replacing the conductor plate. The computer used to be considerably more in line with hand calculations. I have verified that the speedometer is accurate; there's a place nearby that does speedo calibrations.

I have also noted that the highway fuel efficiency of this 2003 S430 is a bit less, by a couple mpg, than my Dad's 2000 S430. Dad's got 26.6 mpg on the 2,450-mile trip out here to Virginia. Both have the factory AIRmatic suspension. Both cars are bone-stock. So, how come the '03 gets a bit less?

I believe that to be due primarily to the tires. Both cars have all-season radial tires on them, but Dad's car uses 225/60-R16 tires, and these tires are rather fuel-efficient. This car, by contrast, has wider, ultra-high-performance tires on it. Rolling resistance will thus be higher. I suspect the sports-car Bridgestones on this car are also a little bit stickier, per square centimeter, than the Vogues on Dad's car. Remember, this '03 has the AMG Sport Appearance Package from the factory, which means staggered (and wider) tires. In city driving, I don't see a whole lot of difference, but on the highway, it's more visible.

So, a few things I learned from this road trip:

1.) Verify that your car's computer is properly calibrated. Sometimes, the computer might lie.
2.) Wider tires look great, and they can help in cornering. But, within reason (safety first, after all), skinnier is better for fuel efficiency.
3.) S-class cars are GREAT for cruising!
4.) For a highway cruiser, the S430 is more than powerful enough. Plenty of passing power. This is coming from someone who's done long-range drives with a Diesel pickup truck and a Honda Civic.

Despite these overly optimistic computer readings, I'm actually going to miss this car when it sells...I like it a lot. And I wouldn't even be doing so if I didn't already have Dad's.
 
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#15 ·
The s500 is pretty good on gas and my s55 was not horrible either. Too be honest if i dont put my foot down on the s65 it wont be this bad lol. I have a friend that has s350 w220 which was the 3.5 v6 model and i had better mpgs in my older 01 s500. Same thing when i use to have a s320 w140 and my current 97 s500 the v8 did way better on gas.
 
#16 ·
Hi,

When I was using my S 500 I got it down to 9mpg on one very quick 60 mile trip :laugh, but my average over a couple of months was a respectable 23mpg, Mel got 27 mpg on the return 60 mile trip :laugh

My C Class, (2.2CDI) is currently averaging 48.4mpg with my "Lead Right Foot" Driving it hard, when Mel used it a few days last week it was up to 55mpg.

Cheers Dave
 
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#17 ·
Yeah, well, that's a 2.2L CDI...duh! Of course it's going to get numbers like that!

Also, do you all still use Imperial gallons over there, or have you been cursed with the smaller "American gallons" (3.785L) like we have? Personally, I think the USA should've finished the liter conversion that we started back around 1980....

At some point, I'll do the same computer-to-hand-calculated comparo with my best friend's 2000 S500 (225/60R16 tires, just like Dad's S430), and we'll see how that goes. But the next comparo will be my Dad's S430, which I should start driving daily pretty soon now.
 
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#18 ·
Hi,

We still have "Imperial Gallons", 4.54 Litres :wink

I hadn't remembered that USA Gallons were less, so that makes your figures seem very good, could this be the reason that your Trip Computer is ambitious ??

Is it programmed for "small" Gallons ??

I went on a 120 mile round trip yesterday afternoon with the CDI, tourists in Camper Vans holding everyone up, and I wanted to do the trip as quick as poss, get back and do some more on my Workshop Extension :laugh

So I was pushing it real hard, overtaking everything.............
Got it down to 41 mpg by the time I got home :devil

Cheers Dave
 
#19 ·
You make me sick.

But...soon my E320 CDI will be done, road-worthy, and licensed...and my Diesel envy will come to an END! MUAHAHAHAHAAAA!!!!!!!

Before the Conductor Plate swap, the computer's readings were pretty close to actual. So, I'm fairly convinced that it's something with the Conductor Plate. I've read that there's an adjustment in STAR to fix this sort of thing, so I'm going to look for it.
 
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#20 ·
Haven't found the STAR setting for this yet, so here's another one from the S430. Seems to be slowly coming down to something closer to reality.

Miles: 348.0
Gallons: 21.935 (Costco 93 oct)
Calculated efficiency: 15.86 mpg
Computer said: 20.0 mpg
Driving type: City

So, this time, it's optimistic by 4.14 mpg, which is less than the 5-6 mpg error previously. Perhaps the computer will continue to correct itself over time; if so, that would preclude the need for the STAR setting.
 
#21 ·
Hi,

In relation to Conductor Plate swap, the SDS procedure would have been to reset the Adaptives in TCU, however, in reality they will probably sort themselves out over some Mileage :wink

Make Sure that Tyre Sizes are set correctly in COMAND and Dash Cluster and that about covers it :wink

HTH,

Cheers Dave
 
#22 ·
Funny you mention tire size. I actually found that setting in STAR while I was looking for the speed limiter. Everything's still good there, and I do have the stock size tires. Will check COMAND in the Dash Cluster as well.

Let's see what happens with the next tank.
 
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#23 ·
Hi,

Check Tyre Size setting in COMAND and in Dash Cluster on SDS Coding, I think it might be something like do a "first time start on the COMAND" Unit :wink

Speed Limiter is in Developer Mode, all in German, "Implizite Kodierung" look for "V-MAX" or something similair, this is all from a very tired and fried ageing brain atm :wink

HTH,

Cheers Dave
 
#24 ·
Hi,

Check Tyre Size setting in COMAND and in Dash Cluster on SDS Coding, I think it might be something like do a "first time start on the COMAND" Unit :wink

Speed Limiter is in Developer Mode, all in German, "Implizite Kodierung" look for "V-MAX" or something similair, this is all from a very tired and fried ageing brain atm :wink

HTH,

Cheers Dave
 
#25 ·
OK, here's the comparo using Dad's car.

The car, with relevant info:
--------------------------


  • 2000 Mercedes-Benz S430
  • Tires: Vogue Custom Built Radial Wide-Trac II Whitewalls, 225/60-R16 (H-rated) now with about 2,000 miles on them
  • Overhauled front suspension, with original M-B AIRmatic springs (they're still good)
  • Meyle 236.10 fluid, all 8 or so litres
  • New trans filter and transmission electrical connector, all M-B brand
  • Mobil-1 engine oil, 0W-40 European formula, with about 2,000 miles on it
  • Costco 93-octane gasoline
  • Ambient temperatures: highs of 30 deg. Celsius (86 Fahrenheit), lows of 16C (61F).

--------------------------

Here's today's fill-up.

Miles: 393.1
Gallons (US): 22.095
Calculated efficiency: 17.79 mpg
Computer said: 16.9 mpg

This was combined city/highway driving, I'd say about 70% city. This particular car has always been pretty fuel-efficient, especially given its nature of a full-sized, 4,000+ lb sedan with a potent V8 under the hood. It's got to be the tires.

Note that the car's computer is slightly pessimistic, by just short of one mpg. I seem to recall this car doing better real-world vs. the computer's calculation. This is about what the silver '03 S430 used to do, on average, before the new conductor plate. I'm getting my STAR setup back from my buddy tomorrow so I can do adjustments on the silver car.
 
#26 ·
Does anyone remember where the computer correction for fuel mileage lives in SDS? I'm having trouble finding it for some reason. I've checked out ME-SFI, the CGW, the Instrument Cluster, and I'm not seeing it.
 
#27 ·
Hi Terrell, @cowboyt ,

IIRC Tyre Sizes are in ME, ABS / ESP and possibly Cluster, maybe also Transmission, I can't think of anything else that would need to know that info ................................

The other thing that may affect it is there is a setting in the Cluster for "Ski Bag" Fuel Tank, or "Regular" Fuel Tank, which could be wrong, there is a 10 litre difference between the 2 tanks :wink

It's been a long while, (touches wood), my W220 has not needed to go on SDS for a while now, so I haven't been poking around :devil

HTH,
 
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