Bass Mech

I took my first semi long road trip in the IS yesterday. I have a 07 350
normally you would never catch me driving the speed limit and I certainly don't pay any attention to the number of gallons or the cost per gallon when I fill up, but I did some experimenting on this trip because I was not in any particular hurry and was somewhat bored.

first let me say that I live in Colorado approximately 6500 feet ASL my trip took me to Pagosa springs via the interstate and over 3 separate passes. on my way back I filled up just before I left so I could get an accurate tank average. my trip was about 250 miles in length each way and I have to say it was a lot of fun to drive the mountain roads with all the turns.

considering almost none of my trip was on any flat roads I still managed 30.6 MPG which I felt was pretty respectable considering that much of it was up and down mountain passes. what was really frustrating is that when I had wanted to coast downhill to improve my fuel economy whenever possible every time I was in a position to coast there seemed to be an oversized hauler in my way or some idiot trying to obey exactly the speed limit or in some cases 10 miles below the limit. I am sure I could have got another 2 MPG or so if I didn’t have to contend with everyone else on the road.

anyway, enough with that... I found a simple way to significantly increase the MPG and this is assuming you have a highway not clogged with other cars. I set my cruise at 75 MPH while the speed limit is the same. if you use the instant MPG bar graph as a guide you will note that the car will get approximately 30 mpg at that speed.
if your on a perfectly flat road my suggestion will be of little use to you.

but for the rest of us, if you wait for a downhill grade use the gas pedal ever so slightly to maintain a constant MPG of about 35 MPG while going down hill.

of course the car will pick up speed. I have to hold my foot against the center hump as a guide to make sure my foot doesn’t move at all once I have the correct amount of pressure on the gas pedal.
if you hold the gas in exactly that spot the car will pick up speed while going down hill and when the car begins to go on the flat or slightly up hill of course the car will slow down and the cruise will pick up to maintain 75 MPH you could also set the cruise at 70 MPH for better results. allowing the car to vary speed with a relatively constant MPG will yield a significantly better MPG overall.
I tried resetting the tank avg MPG on previous trips while doing this and have seen MPG as high as 33 MPG over roads that consist of rolling hills.

the reason behind this is that of course if you can build some momentum between hills and allow the vehicle to vary speed a bit the engine is not trying to slow the car down when it attempts to roll past the set cruise speed. the additional speed will carry it a good deal up to and over the next hill in some cases.
having the car slow itself to maintain the set speed uses additional fuel because it has to begin adding throttle at the very beginning of a hill and will attempt to maintain the set speed which uses a lot more fuel than it needs to clear the hill.
on some occasions I actually threw the transmission into neutral so that the engine could idle and the car coast and pick up speed on its own.
any car has to use a little more fuel if the engine is forced to spin at any speed higher than idle because the injectors still have to deliver fuel at that rpm even if its not needed.

anyway just thought I would share. I would be curious to know what the best MPG anyone has seen in a 350 and at what speed.. I think 35 is possible but I don’t usually drive that slow...

Zanorath

Assuming a flat road I think the best mpg you can get is around 40mph. Just as you hit 6th gear at the lowest speed. Cruising at 40 for 15 or so miles I was able to get 38 mpg. However... at the first stoplight I of course stomped on the pedal and that instantly ruined the 15 previous miles instantly dropping me to around 28mpg, lol.

arpan176

i thought 55 mph on highway was much more gas efficent then 75?

Gernby

iTrader: (1)

I did a similar test last weekend when I drove back and forth through downtown Dallas 3 times. I was able to get over 35 MPG with about 100 miles at 60 MPH (cruise control / snow mode) and about 20 miles of mixed driving on surface streets. I took pictures of the display in case nobody believed me. The average for highway only (using the average MPG), I was able to achieve over 37 MPG for about 30 miles.

mir

Try to do the same with absolutely full tank of gas and then once the experiment is done do a fillup again. Divide the miles traveled by the number of gallons you needed to refill. What is the MPG? I bet it will be 3-4 less. The dashboard meter is very unreliable. It does gives us a false sense of *achievement* though

Perfection

doing 55 will make a lot of people mad!

inxexisten

I average about 24-25 mpg doing 85

Gernby

iTrader: (1)

I did that when the car was new. Sometimes the computer was high, sometimes low. As a software developer with an engineering degree, I think the computer is much more reliable than miles driven divided by gallons paid for. You never know how accurately the pump is calibrated based on current temperature, and you never know if you're filling it up to exactly the same level in the tank since the shutoff valve's aren't always equally sensitive, and the car may not be parked on an equally level (or unlevel) surface.

sirkfc

the computer has never yielded for me a higher MPG figure than the traditional method. I'm at around 6500 miles, mostly highway.

It's a big problem if the computer is too optimistic because people will run out of gas prematurely if they cut it too close.

Bass Mech

i couldnt agree more, the only way to know for sure is to use the same pump each time. but i also think that the MPG will be a lot more accurate if your driving the same speed roughly all the time and attempting to get the same or similar fuel economey with your driving habits. i think if you constantly stab the gas pedal for breif periods of time and then drive fuel efficient your dash readings may be off a little.

also somthing i just thought about. i wonder how much of the fuel you put in the tank gets used verses evaporated. the temperature outside could have a significant effect on the actual fuel used verses lost.

Gernby

iTrader: (1)

The amount of fuel that evaporates is VERY VERY small due to the new environmental laws. Vapors aren't allowed to escape the fuel tank. If there's a leak, the ECU knows it, and gives a check engine light.

BTW, I don't believe driving style would have any impact on the accuracy of the ECU's fuel economy calculations at all. The ECU is simply dividing the amount of mileage it thinks it has traveled by the total amount of fuel it thinks it has sprayed into the cylinders (fuel injector pulse duration). The only thing that I can see screwing this up is a clogged fuel injector, but I wouldn't be surprised if it could recalibrate itself to some extent even then.

lobuxracer

iTrader: (2)

But temperature does affect fuel density, and fuel is unfortunately sold by volume and burned by weight. So it's always going to have some inconsistencies, no matter how you slice it.

Bass Mech

i know this is true and its all pretty damn amazing if you think about it, given the millions of times the injector is opened and assuming the fuel pressure is exact there are many other variables to consider just looking at the mechanical variable of the injectors themselves. for an entire tank of fuel over hundreds of miles its amazing its as accurate as it is.

noenoe_pro

how about driving in snow mode? does anyone feel any difference..

CAIFAN_64

iTrader: (1)

This is what I dont get people buy a Lexus and they worry about MPG