Alrite, im sure im gunna get flamed for this becuase theres probably about 100 different posts with this topic with my luck. but any way here goes.

I have always put in 89 octane into my car, from mobil/exxon or citgo religiously. Once i put in half a tank of 93 just for the hell of it, no problems didnt really notice a difference, that was back when the car had like 2500 miles on it, around. now my cars got close to 4700 miles and i have been getting about 26 mpg, i live out in florida and the majority of my drive is a 45mph road where theres almost no traffic lights and i ususaly cruise from 45 to 55 (seems to be pretty nice, 5th gear with revs around or below 2k). So, ive been 26 mpg? wtf... and the past couple fillups (probably in the last month) i get this (i fill up ususaly when it gets near half tank).

So now this time i fill up sometime around sunday n the needle has gone a bit past half but not quite at 1/4 tank left, so i decide to put in some 93 octane again (from mobile the same station i always fill at). And now the tank has been running a bit lower than usual. Im curious to see what my mpg will be but i have a feeling its lower since im past a 3/4 tank with about 80 miles on the od.

now i ask you, is it the gas? does changing octanes make the computer rework the maps? does it really use that much gas in doing so (everythings stock with regards to the engine). could it be the ambient temperature, it has been in the 70s but also back up to mid 80s? what about the engine wearing in? (this should be done tho right?) one last thing the oil change hopefully will be done this week maybe this weekend. (i dont think the two are related). but if any ones got ideas it would be helpful....hmmm if i forgot nething lemme know

 

Here's a readup on Octane, I've only put 93 (can't find 91 in the midwest). And yes mixing 89 and 93 will produce 91 octane.

howstuffworks.com
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The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

 

interesting..

 

very interesting.

I dont know if that answers your question though. It seems as though you would get better gas mileage with the higher octane verses the lower. To truly test it out, you should put in a full tank off 93 instead of mixing the two. I know my car performs better (smoother acceleration, more butt HP) with the 93, so it would make sense that that would equal better gas mileage. Plus the mpg rating for the TSX are with premium gas, so you should expect to see those numbers. I dunno..

 

yeah, i plan to put in 93 next fillup (i really dont like letting it get low dunno why tho). n we shall see what happens. i realized that the 5 or 10 cents more is like a dollar more a fillup. and if i get better gasmillege it should balance out, or even a few cents more a fillup.

 

I beg you. Never put anything less than 91 in your car ever again. You'll kick yourself for trying to save a few cents when your check engine light comes on because you fouled your plugs and clogged your injectors. Not to mention a knock is also bad your your engine. Put some V Power in there. You'll thank me later. I can't tell you how many people came in after having cheap gas in their cars. Not only are you looking at hours of labor to clean out your lines, but also about $300 in parts to get it back to normal.

 

putting lower grade fuel into your car has more effect than just how it feels or your MPG. Its very short-sighted to saving $2 each fillup for the long-term health of your car

 

The difference in the energy per gallon of fuel should be negligible between 89 and 91 octane. However, the manner in which the engine handles it might vary a bit, particularly at higher power settings.

Keep in mind that low-octane kerosene contains a lot more energy per gallon than high-octane alcohol. Not that I recommend you using either in a TSX, but octane number and energy in the fuel are not particularly related.

 

true, well i hear where yall are comming from, but now i really wanna know why that half a tank of 93 is now a little above half....and i only did about 100 miles. thats about 15mpg or something disgusting like that. argh! and ive only brought the car about 3000 rpm twice since fillup. like i said i ususaly cruise around 2500 and accelereate under 3... argmmm any ideas?

 

why is there like 100 diferent threads on gas. lol

 

well this really isnt about gas, this is more or less why my car is sucking down fuel like a V8. and if its worth the extra few bucks for the high octane. well yall convinced me on that, no more cheap assin it on gas but why is my miledge crap? should i just let this one go to hell n start fresh?

 

Because "search" is not their friend.

 

"search" hates me. lol im just foolin, i dunno i had more of a specific thing, just so happened this week gas was bad

 

Probably because the fuel gauge isn't all that accurate at any point other than "full" or "empty".

Run the tank to the bottom, and see how it works out.

 

Probably because this is a subject of great personal interest to me (more for it's impact on airplane engines than car engines) so I've been adding fuel to the fire in all these threads.

Haha, "adding fuel to the fire."

 

the knocking in the engine will occur most when you are going up hill because you put stress on the engin and it needs more fuel and can not combust in time...I would really stick to 93 or higher...its up to you if its worth it messing up ur engine for a .20 diffences... luxury cars needs luxury gassss

 

Good to see another Floridian in the forum. Doesn't seem to be very many. Stick with the 93 octane..........I've always used it and I've had no problems. Hope your MPG goes up!!

 

Before choosing octane grade, you have to understand the meaning of RON and MON hehehe

MON and RON are determined by standardized ASTM laboratory tests. The details of the tests are not as important as what they mean in terms of performance. Low to medium-speed knock characteristics are determined by the Research (RON) method, while high-speed and partial throttle heavy load knock characteristics are determined by the Motor (MON) method. MON testing is conducted under more stringent conditions with the timing on the test engine advanced and run with a higher inlet air temperature, so the MON number tends to be lower but also more valid for high-performance applications. There are a number of more valid tests that have been developed to determine the anti-knock characteristics of fuels used in high performance engines, but the aren't in general use at this point so we are stuck with the old reliable pump octane number.



http://www.eric-gorr.com/techarticles/Fuel_Basics.htm

 

and i think most station use PON.... which is the average of RON and MON. so if you hear we got 98+ octane here in europe... well big woop. that translates to 93 in the US.

 

Exactly my point hehehe.