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Discussion Starter #1
Hey all.. brain dead right now.

Just trying to figure out cost of MPG increase. Not sure about everyone but where I live, the grades of gas are usually $0.10 difference. So for example right now...

87 = $2.79 ... 89 = $2.89 ... 91 = $2.99


So say I do a mod or change Octane that increases my Avg MPG by X amount (assume 2). However it would require me to run a higher Octane. Is it worth it?

Just looking for a formula. I think our tanks are like 12-13 gallons... so to fill it up with 87 would cost $36.27. Filling it up with 89 would cost $38.87. If I got 2 MPG more ... am I coming out ahead or not?
 

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Discussion Starter #2 (Edited)
Found this site http://mpghead.com that seems to do what I think I want (at least for octane vs mpg)

Playing with the numbers is basically looks like you would have to gain at least 5 MPG to make it worth it if you have to use higher octane.
 

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Done the experiment, based on cost of e10 vs 95 premium in regional Australia, slight improvement in economy could not justify the significantly added cost.
 

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Generally you have to figure the cost of the mod in that equation, for the savings, how long before you get to the cost of the mod. After that, then it becomes a savings. For only a couple Mpg, it's not worth it imo. Unless you drive like 20k+ miles per year.
 

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I mean this in the nicest way, but if filling up costing and extra $2-$3 is a concern, maybe performance mods are not for you (and I do mean that in general, not pointed at anyone specific).

And fwiw, around here the price difference is staggering. I can find regular unleaded for under $2 if I try. Even cheap premium is $2.65. And I don`t have a choice, I HAVE to run premium. Of course I`m tuned to an E85 mix, which drops my total tankful price (4 gallons of E85 per tank and I save about $.70 per gallon over the premium I fill it the rest of the way with), so it`s pretty much a wash.
 

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TBH you'd probably wont be able to accurately measure this in the real word as you'd have to drive at the same speed, same route, same stops, same starts, same acceleration, same temperature every day. MPG would fluctuate according to many factors.
 

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You can just do percentage. If each octane increase cost 5% more, then you need at least 5% increase in fuel mileage.

For instance, regular is $2.19. Plus (at Exxon) is $2.30 (5% more). (These aren't real numbers.)

Then my fuel mileage would have to increase from the present 36.8 mpg to 38.6 to come out even.

However, another impact is how "green" you want to be. We drove a VW Jetta TDI for 7 years. It got 40.2 mpg, but the diesel costs as much as $2.69 vs. the above prices. This is 2.69/2.19 = 23% more for fuel mileage that was only 40.2/36.8 = 11% more. I still liked the TDI because it was greener, e.g. required less crude oil and therefore less pollution all down the line despite the fact that VW allowed the vehicle to put out more NOx that legal.

Ralph
 

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Discussion Starter #8
You can just do percentage. If each octane increase cost 5% more, then you need at least 5% increase in fuel mileage.

For instance, regular is $2.19. Plus (at Exxon) is $2.30 (5% more). (These aren't real numbers.)

Then my fuel mileage would have to increase from the present 36.8 mpg to 38.6 to come out even.

However, another impact is how "green" you want to be. We drove a VW Jetta TDI for 7 years. It got 40.2 mpg, but the diesel costs as much as $2.69 vs. the above prices. This is 2.69/2.19 = 23% more for fuel mileage that was only 40.2/36.8 = 11% more. I still liked the TDI because it was greener, e.g. required less crude oil and therefore less pollution all down the line despite the fact that VW allowed the vehicle to put out more NOx that legal.

Ralph

Ironically I'm in the same boat. Had my TDI for 2 years and handing it back next month. The new Mz3 is the replacement.
 

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Discussion Starter #9
All,

thanks for the input. Yes I am very realistic. I'm not one of those idiots that goes out and buys a new car to get 5 more MPG ... or buys an electric thinking I'm going to break even...ever :)


Voodoo... I totally agree. This post was about general mods, but specifically me considering doing the OV tune. I would like a smoother car and better power, but being locked into a higher priced Octane "forever" is questionable.

If I could upload my spreadsheet I would, but I just created one that allows me to input in the 87,89,91 prices for up to 3 stations I go to (Costco, Safeway, Chevron). It then crunches it all up based on how much I expect to drive per year and the estimated MPG increase for any mods.

To give an example... if I did the OV tune and it net me +2 MPG (HUGE ASSumption) but required me to up my octane... it would only cost me roughly between $50-$100 more per year.

This is solely because COSTCO is so damn cheap. Their 91 Oct is lower than than most stations 87.
 

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Discussion Starter #10
Actually I now see that I can upload a spreadsheet but even though it's only 11k it says it is too large. Tried zipping it and still no go.

Not that anyone would want my spreadsheet :nerd:
 

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I doubt the extra cost would be worth the extra mileage.

This is true especially in view of the fact that the difference in fuel cost is a definite, provable fact. However, the difference in miles-per-gallon is generally a subjective thing that is not reliably proven outside of controlled tests done by objective testers.

The Mazda engine is a marvelous piece of machinery that amazingly can operate on near-diesel compression ratios with the cheapest regular fuel while delivering top fuel economy.

Serendipity from the factory.
 

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Actually I now see that I can upload a spreadsheet but even though it's only 11k it says it is too large. Tried zipping it and still no go.

Not that anyone would want my spreadsheet :nerd:
I keep little notebooks in all my vehicles (not the tractor, in a drawer). I write down every fillup and every service. Used to keep separate sections in each notebook for Gas, Oil and TBA. Now, I just put them all in a time line. These are just little bitty spiral notebooks.

I move the fillup info to my spreadsheet which has sheets for practically every car I've owned since around 1983. I make notes on the fillup values from the oil changes or other TBA I did if it might have an affect on the fuel mileage. Don't have that many sheets because we owned the one car for 25 years. It showed a big jump in mpg when I had the timing chain replaced.

Ralph
 

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Discussion Starter #13
It showed a big jump in mpg when I had the timing chain replaced.

Ralph

That is very interesting. Funny how you hear all these commercials about "Do a tune up to save on gas". So you want me to spend $150 to replace filters, spark plugs & wires, etc to save $5 in fuel :)


FYI all.. I'm not looking to spend money to get better MPG (sorry if I gave that impression). I'm thinking of spending money to get HP but I'm just making sure I evaluate the "True Cost" of a mod. Not just the initial price. I could put nitro on the engine, but the true cost is another engine when it blows!

Besides a timing chain.. the best way to increase MPG is not to drive over 55 :p
 

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Besides a timing chain.. the best way to increase MPG is not to drive over 55 :p

On my thanksgiving road trip (3K miles round trip) I got about 46mpg at 55 and 44mpg at 70. The break point seemed to be about 75, where gas mileage was noticeably worse anywhere above that, tested again on an 800 mile round trip run to Houston (same results). I`d get 36mpg or so above 75mph, but if I dropped down to 72 it went right back to getting 40+. And that was fully tuned. And remember, if you were to have a road trip coming up and were tuned, you could just as easily flash back to the stock map and run regular unleaded.
 
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