It sounds like a lot of people don't have a clue how to measure their gas mileage accurately. Let's start by measuring gas mileage correctly, then let's discuss what mileage we are getting and under what conditions in what year and model with what mods. That may be a more useful conversation.

You do not look at your cars measurement of your mileage. Your cars estimate of your avg mpg is horrible.
You do not need to look at when your light comes on. The estimated capacity of your gas tank is a horrible way to measure mpg.
Here is how you accurately measure gas mileage:
1. Reset trip odometer when you fill up at the gas station
2. At the following refill at the gas station, look at the trip odometer and the gallons pumped into the tank
3. Divide the miles on the trip odometer by the gallons pumped
4. Reset your trip odometer again before leaving the station
5. Repeat and average the results

I had some tuning and transmission issues for a while and I averaged 7 MPG with mixed highway, city and track driving! All on 91, C16, or Q16. It was lots of fun! I couldn't even make it 150 miles on a full tank. It was literally impossible to do any long distance driving, because stations are too far spaced on most interstates.

Now that the issues are resolved, I'm able to get about 12 mpg with mixed highway and city (no track) driving. This is on low boost, very light driving. 2006 Enthusiast Roadster, Built Longblock with low compression, Twin Turbo, built 5AT.

 

~23ish MPG with mixed city/hwy miles. has been as low as 20 and as high as 27 (all highway). 06 manual

 

I calculated mine manually for a while, but just kept finding that my car's readings were dead on target. I'd guess that the MPG reading is calculated based on how much fuel the car knows that it has sent through the injectors compared to how many miles it knows you have driven, which is going to be a pretty accurate calculation.

However, if you're running non-stock tire sizes, your speedometer (and thus your odometer) readings are going to be off, and this is going to affect your calculations whether you are doing them manually or trusting the car to estimate your MPG. The only way that you could do it accurately with non-standard tires would be to GPS log and verify your driving distance between fill ups.

Also, if you really want to be picky, are you confident that the gas pump's margin of error is accurately measuring the amount of gas to a level that wouldn't affect your calculations if you're using the gas pump reading for your manual calculations?

Because all of the systems introduce their own margins for error, it's probably best to use both the car's calculation and the manual calculation.

 

The built-in MPG measurement was never accurate in my 2005 Base Coupe or my 2006 Enthusiast Roadster. I understand why it couldn't possibly be accurate in my roadster (nothing is stock), but many people have found it to be horribly inaccurate. Look back at this thread and look at how many people went along these lines...."well my light turns on after XXX miles, and I have about XX miles after the light turns on, then I pump about XX gallons in, so I get about XX miles per gallon." That's such a horribly ineffective method, that it can't even be called measurement.

If your car's calculation of MPG has been consistent, that's cool. Many cars will not. The majority of us can't just look at the car's reading and go by that. It doesn't take any significant effort to calculate your mileage. I did it yesterday and it literally took 3 seconds. I don't normally measure it and therefore didn't do anything to prepare for it, other than the habit of resetting my trip odometer at refill.

 

Measurement of fuel left in your tank before DTE says you're at zero or your fuel light comes on is not the same as measuring fuel that has passed through the injectors.

I'm no expert by any means, but this is how I'm looking at it -

Fuel tank level is a float meter. It can vary due to vehicle position and is not extremely accurate.

Fuel through an injector is a factor of duty cycle and time and can be measured in very small increments of time & accuracy. I am hoping that the MPG calculations are based off of the fuel that is being sent through the injectors and not a rough guess at how much fuel is left in the tank based on a float.

Also, the amount of miles left after DTE is intentional so that people have a cushion. It isn't meant to be accurate to the point that if it says you have 3 miles DTE that you drive 3 miles and your car shuts off.

I also did not advocate trusting the computer reading alone, my point was that looking at both provides you the most info - quoted "it's probably best to use both the car's calculation and the manual calculation."

 

OK. So I'm not sure why you have a problem with what I said. I said that a lot of people in this thread aren't measuring their mileage properly. They were either just relying on their computer (without verifying it and possibly with non-stock tire sizes and modification to their vehicle), or just mentioning DTE. Both of these you agree are not suitably accurate especially for comparing to other drivers. Also, no mention was made of which year, model, and modifications were made to the vehicle and under what driving conditions they see these results. Props to you for not being one of the many I was referencing. I see no reason you need to continue to defend your use of your cars calculations, as you have verified its accuracy. That doesn't, however, make someone else's horribly misguided guesses any more accurate.

 

No problem with what you said, I'm just advocating people use both and compare your results, you're just advocating people only use the manual calculations.

Yes, I agree that some people:

A) don't understand how to properly calculate their own MPG
B) aren't going to be disciplined enough to remember to reset their trip odometer each time
C) just want to "guess" at it instead of doing the necessary work
D) will record the mileage, fill the tank all of the way up to the top each time (just stopping the first time the pump kicks off isn't an accurate way of determining that you're filling to the same level each time), will use math to calculate the results, and will then adjust those results by the percentage difference that their tires' overall diameter is different from stock overall diameter

I would guess that the same people that ask a question like this are in category C and not in one of the other categories. It's not that I disagree in your reasoning on how to calculate mileage properly. We probably disagree in our understanding of why people aren't calculating their own mileage properly, because it sounds like you feel that the problem is that the majority fit into category A above.

 

Well, I have been trying to see just how far my 03 enthusiast will go till about dry, and I'm at 490 miles, gas light has been on for about 80! Last time I filled up i averaged 34.9 MPG (however, I'm Canadian so I use our 4.54 litre gallon). 29 mpg on a 3.78 litre American gallon.

 

Not really here to b!tch about the gas mileage, but I recently read a news article about Hyundai over-inflating their MPG estimates. http://www.wdef.com/business/story/EPA-finds-Hyundai-Kia-overstated-gas-mileage/NhOVLjsWpk249nf3F6D_8g.cspx

So it got me thinking, is our 350z's inflated too?

Granted I drive mostly highway to-from work each day, w/ slight stop n go traffic. Still I'd say I'm doing about 60-70% Highway, 30-40% Local. I've only managed to avg around 20mpg. I understand that with some performance mods + tune, the car would use more, but even before modding, I've still avged only about 20mpg moderate driving (not flooring it everywhere).

So I've read a few threads and I've seen people avging like 25+mpg with mixed driving. And some even managed 30+ full highway. When I took a roadtrip and done completely highway, I was struggling to keep above 24-25mpg goin @ 60-70mph goin slightly downhill. What's goin on?

 

You took a road trip that was all slightly downhill? Where was that, from Mt Everest to the valley floor, lol? Regardless, no two people are ever going to get the exact same mileage on the exact same trip - way too many variables ranging from the car to the driver.

 

It was generally at a decline where I didn't have to give it a lot of gas to maintain speed. Regardless, there are people on this forum that stated they get near 30mpg highway driving. Just curious what they are doing differently

 

I love these threads! It always makes me feel better about my Z. I average around 30-33 on the hwy and around 26 in the city . I have a full exhuast, intake, a plenium spacer, and I'm sure my 18lb LMGT4's don't hurt either.

edit: hwy I drive typically drive around 70mph

 

I averaged 28 from Ohio to Carrollton for the ZNats and we were busting it pretty good for some short stretches, averaging over 70 for the entire trip. Including a 20 minute period spent idling while conversing with one of Kentucky's finest, lol. Can't remember whether I shut the car down for that or not.........
'03 with 126,000 miles, usual bolt on mods.

 

wut lol how you guy's 03s get so much better? Perhaps the wheels?

 

Most of us have aftermarket rims so i doubt it is that, lol. The '03 also had only 287 HP at the crank so that might have something to do with it. Some mods work better than others - there are mods that actually cost you HP and so maybe MPG too? Don't really know.....

 

the ppl getting 30 mpg readings are those using the OEM meter and are not resetting it everytime they reset the trip meter..
Its an average.. so if you never reset it, its an average of 10 years of driving.. if you reset it at Cabazon and then drive the fwy back to Phoenix, you will get 29-30mpg reading... try resetting it in town adn see what happens.

THAT is why its not accurate. the meter itself is usually pretty good aside from variable slike wheels etc.. but its the user error of not resetting it that gets the crazy data floating around the internet. And it is again a variable of how you drive and other things.

 

I get 190 Miles to my 04 tank. All ive got going on are a pop charger, plenum spacer and test pipes. With a bit of suspension work.


Welcome to the club.

 

BOOM! You'r wrong, because I reset it every time I get on the hwy . Also if I am going to go from one side of town to the other I reset it if I am feeling curious.

 

Keep in mind folks. When your MPG reads ---- you still have 5 gallons of fuel left. I never go buy these stupid gauges because they are terrible. I go by mileage. For example: I can drive about 500 miles on a tank before I run out. (trust me on this one, I tested that theory)

If you are only gettinh 260 miles on a 20 gallon tank, there is something seriously wrong.

 

I average 17MPG around town/some HWY. 25MPG HWY ONLY. I typically put in 13-14 gal on a fill up from empty, although it could take another 3 gal or so if I really ran it down.That would give me 300 MI TOPS on a full tank. That's typical COMBINED driving. I would be lucky to get 400 miles on a full tank HWY ONLY.

SO......YOU'RE GETTTING 25MPG? AND...can put in a full 20 gallons??

My wifes 2012 Hyundai Sonata averages 24 MPG..and you're Z *ain't no Sonata*