MPG Observation

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Peter V

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This weekend, I went camping with a few friends and noticed a very strange phenomenon. Hopefully someone can shed some light on the situation.



Scenario #1:



- my daily work commute is roughly 30+ miles each way, relatively flat highways. I am NOT a lead foot, do not haul anything in the truck. Average HWY MPG, 18MPGs at BEST.



Scenario #2



- took the truck on a 140mile trek, relatively flat highways, same driving habits, same gas type, this time, truck loaded with AT LEAST 400lbs of camping gear and 2 additional passengers. Average HWY MPG, almost 20! TWENTY!!!



Can someone please explain to me why my MPGs seemed to improve by almost 10% and I had close to 700 additional lbs of weight?
 
How did you calculate MPG?
 
Without any additional detail, I would imagine the reason is that on the trip you had long stretches at a constant (and efficient) highway speed, while the majority of your 'commute' involved many instances of acceleration (inefficient) from stop and go traffic.

 
I agree with Uncl Bob,

Driving at a constant speed is what really gives you the best mileage. On your communte to work, you probably have more traffic, more stop lights, etc and that constant slow down, then accelerate is what eats your gas mileage.



 
To add to this I believe the more the torque converter stays locked ( driving at a constant speed ) the better your fuel economy will be. So if I am on the highway and I take my foot off the gas pedal to coast, then press it again, the converter will come unlocked and take a few seconds to engage again. You will see it happening on your tachometer. The RPMs will rise slightly and as the torque converter locks up they will settle
 
Flat run yesterday in Alberta (flat country) in the 09 Adrenalin for 350 km at a rock steady 110 km/h and the mileage was 10.6 l/100 km or 23 MPG, around town I average 13.6 l/100 km or 17.6 MPG.



The 2001 gets worse mileage than the 2009 V8.
 
Is that 140 mile trek one way or total (not specified)?



my daily work commute is roughly 30+ miles each way, relatively flat highways
Click to expand...



This is why he is asking - similar circumstances, but with more weight on the trip.



This noted, both suggested explanations make sense ...



Think of that 140 mile trek as four trips to work (or eight if one way) without stopping and without all the city street driving done to get from the highway to work.
 
Thanks for all the feedback guys....



Here's more info of my scenarios:



MPGs calculated using the computer's system. (I know it could be off +/- MPGs)



Scenario #1: my 30ish miles of commute (90% fwy, 10% city), I am not stuck in traffic as I drive against the flow. I generally keep the truck between 60-65mph.



Scenario #2: during this weekend's drive, I took virtually the EXACT same route I would take to work (roughly 20-30miles of the 140 miles one way to campsite). I observed in that 20-30miles stretch, the MPGs differences. It was substantial enough for me to make a comment to my buddy and his wife who were carpooling with me). Same driving habits, same speed, etc. From the time I filled up and drove past work, I easily averaged 19-20mpgs.



Since returning and unloading my ST to conditions PRIOR to the camping trip, I'm once again back to roughly 17 mpgs.
 
30 mile commute still equals starting and stopping, cold engine starts every day, etc. Starting and stopping takes a lot of power. Just try pushing something heavy around. I can barely get my trailer going if I need to push it around the driveway, but once it's going, I can push it with one hand fairly easily. It wouldn't take a lot more effort than normal walking if I had to push it across a large parking lot as long as it was flat.
 
The most accurate way to check milage is to do the math yourself. The cars computer can only estimate consumption based on what the tank reads. I once found a Volvo that was displaying 3 - 4 more MPG than it actually got.
 
Along the same lines a Phill R's comments.



The fuel gauges on most cars a very inaccurate and most computers will use the signal from the fuel level sending unit to calculate MPG and Miles till Empty.



Couple that with the usual speedometer/Odometer error (showing faster speed and higher mileage than the vehicle is traveling makes for very inaccurate calculations.



I have an UltraGauge which plugs into the OBD port under the dash. What makes it better than the typical onboard comuter is that speed, mileage, and fuel level can all be adjusted to compensate for speedometer, odometer, and fuel level errors.



It's also harder to accurately calculate mileage on a partial tank. Even with a full tank you really don't know exactly how much gas is actually in the tank. I always do mileage tests after I fill the tank to the brim....When I see the gas at the top of the filler neck restrictor (yes, that is called "Over Filling" and I don't recommend doing that every time you fill up). I then run my mileage test and go back to the same pump and refill the tank to the same level of gas that I started with...and use the exact pump reading (to the tenth, or hundredth of a gallon) to tell me how much gas I used during my mileage test. I also don't round the odometer mileage up or down...I use the exact mileage down to the tenth of a mile.



The more miles you drive on your test, the more accurate your results will be.



...Rich
 
I also get around 17 mpg driving "normally" (that's when I try to drive conservatively), and get over 20 mpg on most long road trips. Usually, a long road trip includes extra people, luggage, cargo, or even a trailered motorcycle. I once got such a ridiculously high average on my trip computer with the cruise control set on 55 mph, that I didn't brag about it on here for fear of reprisal for my gaff from the counter-anti-error zealots (which I am a member of).



So, with all that's been said on this forum about mpg calculation, estimation, or speculation; zcspec's numbers look about right to me.
 
The original post was about the difference he noticed in mileage, not about getting a specific mpg. As long as he's consistent in how he fills up, and how he's doing the math, the mpg he's coming up with doesn't matter much in this case. If he was claiming he got X mpg, then the method he used to calculate is more crucial. He just got noticibly better mpg on the longer trip and is wondering why.
 
It seems to me that the computer takes the most recent AVG MPG and multiplies it by the level of fuel in the tank for the Total Miles Remaining number.

The reason that I feel that this is the way you are seeing it is that on a recent trip I reset the "A" Miles Driven before going on a long freeway trip. Prior to that I was always seeing about 375 Total Miles Remaining on the full tank. Average MPG was usually 17 or so.

When I stopped for gas I was seeing Average MPG at around 20. And Total Miles Remaining 435 on the full tank.



But I do agree that freeway miles versus city miles are different due to the type of driving. The added weight once the truck is up to speed would be minimal.

Just doing my part to add to the confusion... :cheeky:

Hey I'm waiting for the paint to dry in my living room and have nothing else to do.

Lou...
 
Duane Foster,

My comment was to tell zcspec that traffic varies and that probably had the biggest impact on his MPG between his daily commute vs his weekend trip. Many people do not realize that the timing of traffic lights is often different between weekends and week day...even if you are driving against rush hour traffic. Simply catching a few more traffic lights, or if he had to wait at a red light for 30 seconds longer can effect gas mileage more than people realize. It is those slow dow and speed up cycles that eat up your gas mileage. If you use something like a ScanGauge or UltraGauge, you can see that accelerating from a stop light, even slowly will show a drop in gas mileage until you get up to cruising speed....then you come up to another red light and that repeats the slow down speed up cycle.



I also wanted to point out that there are errors in the fuel level, and odometer readings. Finally, if one takes a lot of liberty and estimates or rounds off the pump readings, or odometer reading, you can get very inaccurate calculations.



...Rich



 
My display average MPG currently says 13.1 MPG. I have never reset it since I purchased it nearly 4 years ago. I have 39,000 miles on my trac right now.
 
I am going to try another experiment today. This afternoon, I will be taking a 280+ miles road trip to pick up a set of rims/tires for my ST.



The way there should be 140 miles. I will run into some traffic over a 5-10 miles stretch on the way there. On the way back, it should be a non-issue. Already knowing that I'll be in traffic, I plan to "RESET" my MPG calculator once I've passed traffic and will see how my MPGs look for the trek down. Keeping all things as close to constant as possible (IE, speed) on my way up and loaded with 4 rims/tires (I would guess about 300+lbs) I will once again, "RESET" my MPG calculator to see what MPGs I'll average.



Given my recent experience, I am guessing that the drive back home will net better MPGs due to the extra weight.
 
Given my recent experience, I am guessing that the drive back home will net better MPGs due to the extra weight.
Click to expand...



This is flawed logic. The higher mileage is not because of the increased weight; it is because of the driving characteristics (city vs highway). The same trip could be made with no weight difference and the result would be similar.



Here's an experiment that may shed light on the fundamental principle:



Walk one block and then sprint the same block in 10 yard intervals. How's your mileage?
 

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