ROTARYROCKET7

Well i filled up my tank with 87 octane gas , and been driving around. Driving hard and normal, and i got about 18o miles per tank which equates to 10.84 miles per gallon (MPG) which is ridiculosy low.

So if the tank is 16.6 and theres 4 notches that means that each space between notches holds 4.15 gallons.

Well my gas was low so i stopped by the gas station. I was on the first notch before the light comes on. I filled it up with 89 this time, and put 6 gallons. The gas tank went up past half (lets say 9 gallons per say) but i only put 6 gallons. Is the reserve before empty 3 gallons or am i just messing this thing up.

Cuase 10.85mpg is terrible if each space between notches is 4.15 gallons.

Jason NYC

sar

iTrader: (1)

The reserve space is about 3 gallons.

The way that you are supposed to calculate mileage is by filling it up, then at your next fillup see how many gallons you used and how many miles you went. So if I go 300 miles on a tank, and fill up wih 16 gallons, I've then gotten my 18.75miles per gallon.

Keep in mind that the gas tank sender is just a float on a lever and can easily be bent over the last 20 years.... that and the gauge isn't necessarily weighed equally between each quarter (aka the top runs quicker than the bottom).

ROTARYROCKET7

Hmm true it can be many things, i just want to see how much MPG im getting.

I did fill it up, maybe not to 16 exact but damnnear close. THen i ran it to the notch right before the gas light comes on. So 13 gallons for 180 miles you would say

Jason NYC

sar

iTrader: (1)

No, not at all... to break it down step wise
1. Fill tank up all the way
2. use as much gas as you wish
3. fill tank up (see how many gallons it takes to get full again)
4. based on gallon used to fill and miles on before fill you can calculate your mpg.

It's all based on the volume of your gas tank and not your meter that way. Calculating mpg accurately has nothing to do with what your gauge says.

again, say i fill up my tank on tuesday and reset my odometer, on friday I need more gas. Say I put in 5.5 gallons before the pump clicks. I then take the reading on my odometer, divide it by 5.5 and calculate my mpg. Then I reset my odometer and recalculate next time based on how many more gallons I have to add for it to click off.

NZConvertible

The level shown on the gauge doesn't accurately represent the quantity of fuel in the tank. It's a general indication only and shouldn't be used in any calculation or or even estimate of fuel consumption.

SirCygnus

iTrader: (1)

yeah. like nz said. if i punch the **** outta it, it will go hella low. then if i start driving normally, it will get low. its weird. cus i know that the fuel slushes around, but it will stick and not do anything. the moment i downshift and hit high rpms just cruizing, the gauge plummits.

jgathome

Iv been meaning to calculate mine for some time now
always forget about it when I fill up (tends to be when Iv been getting out of the night shift and Im just to tired to consider thinking about anything)

Jigsaw

If I'm lucky I get 150-170 miles from a full tank, why the hell is it taking my gas so fast?

DeclareYrWar

poor gas mileage from an rx7, no way! lol just messin guys. we all know our cars are anything but economical. i tend to get like 14 mpg citty and like 17 or so highway. its not good and it doesnt help that i refuse to run anything but premium but i can live with it. i have noticed tho that the first half of my tank seems to go by faster but its whatever. i usually run my tank down to a quarter then fill back up

ROTARYROCKET7

See i dont have $$$ to fill it up to full, so 80% of the time its just filling it up to halfway. I filled it up at work, with some shitty gas they had here and i filled it up till it clicked..Then i reset the odo and been driving it since, hard and easy depending on where i was. The gage hit the lower mark and i put 6 gallons yesterday, and got it up to a lil more than half a tank.

Well ima do what you said. Ima fill her up tonight and drive around for a bit under normal conditions for about 30 minutes and then hit up the gas station again and fill her up and see how many miles i put on her and how many gallons it takes to full it up again and i will calculate my mpg.

Thanks

Jason NYC

mort2002

iTrader: (1)

i have an 89 TII and i drove from yakima wash. to seattle, i only raced 1 guy in a subaru wrx for like 45 miles on and off, i got about 18 MPG, in the city its dramaticly worse like 10 to 12.

emac

i get about 20 mpg. it's not that bad. My gauge is way off though. When it says a little under half, i need 12 gal to fill up then it sits at over the full line for 100 miles then plummets.

gxl90rx7

iTrader: (1)

your n/a will run better on 87 and probably get better mpg too

ROTARYROCKET7

87 i thought the better the octane the better the gas....I usuallyput 89 but gas prices have gotten me to use 87

NYC

TitosToy

my 88 n/a, from full to the fuel light turns on completely i get about 235 miles with 0% correction to my safc. when i fill it up normally (from F to almost E according to the gauge) i get about 12.7 gallons and it lasts about 220mi. it's not so bad considering i do no highway driving.

gxl90rx7

iTrader: (1)

the higher the octane, the harder it is to ignite, which is good for high compression/forced induction engines, b/c it prevents pre-detonation. for regular engines it just means less power and less gas mileage

ROTARYROCKET7

**** man rotarys are totally different huh on all basis. SO the shittier the gas the better. I put shitty gas my car was iffy, put 89 she felt better

I dunno , il go back to 87 cuase of gas prices

Jason NYC

emac

any engine will run better on the lowest octane that it can handle without knocking, not only rotarys. lower octane fuel also has more energy per volume than high octane fuel.

Icemark

That is not correct.

The BTUs are the same regardless of Octane.

ONCE AGAIN The Octane number is the referance to how much the gas can compress without exploding. Higher the number, higher the compression can be before exploding prematurely. So simply: the higher the octane, the more resistant to burning until ignited.

See you want the gas to explode/burn when the spark ignites it. If the gas burns before the spark, you have detonation or pre-ignition (often called ping or knock).

Higher compression or forced induction motors use higher octane to simply prevent that from happening. They are not getting any more BTU per liter from the gas.

So... all that given, on a stock or near stock motor you want to use the lowest octane you can get. You want that gas to burn as much as possible as soon as it is ignited. Using too high of an octane may result in not all the gas being burned in the combustion and deposits from that not being burned.

and ROTARYROCKET7 back to your original question.

Seldom does anyone use the full amount of gas in the tank. Even if you wait until the light comes on, there is generally 2.5 to 3 gallons still in the tank. When figureing gas mileage, You see how many miles you drove on the last tank full, then you divide that by the amount of gas it takes to fill the tank back up.

For example if you drove 225 miles until the light came on, and then fill your tank and put in 13.5 gallons you would be getting approx 16.67 miles to the gallon (perfectly normal). Now even that method has some flaws because of minor filling differences, but that should get you a ball park figure. It does not make any sense in just driving for an hour, you won;t get a full driving cycle.

Do about 3 or 4 tank fulls, and then average out the numbers... that will give you a pretty close ball park number of what your mileage really is, despite bad gas or differences in fill or driving styles (highway vs city etc).

emac

"It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane."

http://en.wikipedia.org/wiki/Octane_rating

i wrote it backwards, but what you're saying is not correct based on the way gasoline is blended.

also, using a higher octane gas in a car that cannot preperly burn it will result in a smaller amount of energy released per volume injected, as unburnt fuel is being expelled in the exhaust.

Icemark

Doubtful but okay. I'll have to do some further reseach on that to confirm it or not.

yes, and no... again the BTUs (actual possible power of the fuel) stays the same regardless... its the engine utilizing all the possible BTUs or not that creates more power or lack of power. If the engine is designed to run on 86 octane, but you put 91 in, it is possible that the fuel does not get fully ignited if the compression chamber allows for mutiple burns (as our rotary engines do as well as the CVCC style Honda engines). Lack of full burn would indeed result in a lower power output of the engine, but not of the gasoline itself. Just trying to make that clear.

For example if you put a gallon of premium in a milk bottle in a middle of a sandy feild, and lit it on fire, it would burn the same exact height and way as a gallon of 86 octane in a milk bottle at the other side of the feild. Power of the fuel is the same.

emac

Exactly, so for his car (s4 n/a) it's likely that unburnt fuel would be expelled out the exhaust, thus providing less output. So with higher octane fuel less energy is released.

In terms of Btu/gal of fuel, the difference between octane is tiny, maybe 3%, which is essentially saying the output is the same. Output of gasoline mixed with 10% ethanol (or 15% MBTE) is 111,400 BTU/gal.

I don't think a 3% change would mean that much difference in a rotary engine operating in the 20-22% efficiency level (Bosch Automotive Handbook, 4th Ed.).

jkimbro

iTrader: (3)

Another heated gas conversation.

spot_skater

Wack city. All highway driving I've managed to pull 330 miles to a tank of gas. Without the light coming on. Every other day doing a 3rd and 4th gear red line. Well...4th gear to like 65-6800.



Sorry you're mileage sucks dick Jason. Try calculating it correctly, you might be a little happier.

James

13b4me

Isn't that normal?