How many gallon fillup for accurate MPG readings?

[jamminjimmy]

Hello all,
How much gas per fillup will give fairly accurate MPG readings? I am starting to pop regularly into the 40+ mpg range ("I'd like to thank the academy for this wonderful trophy- and of course, ecomodder for all the learnin"....) and am getting impatient driving 400 miles for results.....
I have a '87 CRX, so no scangauge, and I'm still researching mpguino, etc options....
- Is 5 gallons enough for a rough idea of MPG?
- I'm planning on 5 gallon fills(I still have a bunch of mods I want to test out), then if something causes a mpg spike, going back and averaging larger tanks for better accuracy.

---thoughts?

[Frank Lee]

Small fills are not only inaccurate, they waste your time, your tank is open to the elements much more when the cap is off, you spill more, and you are constantly driving in/out of service stations. No good comes from constantly going to stations for small fills unless there is a cute girl working the register that you simply must see all the time.

I wouldn't advise any particular gallon amount either because of the large variation in vehicle tank sizes. IMHO about the best one can do is only go to gas up when the vehicle is at or below 1/4 tank (as close to E as you dare to get) then fill the dang thing all the way up and be done with it until the next cycle. It is helpful to try to fill at the same pump at the same station every time, and also use the same method i.e. fill on slow auto til it clicks off, wait a bit, then do two clicks... whatever floats your boat, just do it the same way every time. Might even want to do it at the same time of day for temp reasons.

I can almost guarantee you that the potential errors from calculating mpg from small fills are greater than the potential gains from our garden variety mods. Sub-9 or 10 gallon "fills" = waste of time.

Without mpg instrumentation I think the best you can do is keep a gaslog and make notes of mods/conditions for each entry. Don't get excited about any mpg spikes on one or two fills either. We have to be content with noticing trends.

Gas tank games... will they ever end?

[moorecomp]

The inaccuracy of a fill will be magnified by the smaller the fill. If you assume a full tank will be off by say .25 gallons from one fill to another, then adding only 5 gallons to reach full means a possible 5% error. By filling with 10 gallons, the potential .25 gallon error is only 2.5%. Clear as mud, right?

[Thymeclock]

Originally Posted by Frank Lee

Small fills are not only inaccurate, they waste your time, your tank is open to the elements much more when the cap is off, you spill more, and you are constantly driving in/out of service stations. No good comes from constantly going to stations for small fills unless there is a cute girl working the register that you simply must see all the time.

Gas tank games... will they ever end?

Frank is right. Those of us who are older have kept a log book and calculated MPG for decades with each tank fill.


The only justification for a partial fill-up is either the aforementioned wooing of the station attendant, or not having enough $$$ to fill the tank. And if such marginal poverty is really the case, it won't get you very far with wooing the attendant, or other prospective dates.

[doviatt]

JamminJimmy-Congrats on the 40+
Spend the money on the MPGuino and then you will always know you MPG (instant and tank, and trip numbers). Even your full tank records can be wrong or off a bit. Go with your 3 tank or 90 day average for consistency. Be patient. Remember the goal is to SAVE gas not buy it! ;-)

BTW Frank is always right!

[bestclimb]

I will echo what was said. With an older rig a vacuum guage may be the way to go for letting you know how your doing. It won't tell you what your millage is but it will let you know when you are operating your engine efficiently.

[gasstingy]

I rarely ever buy fuel until I'm down to 1/4 tank unless on a road trip, then I buy fuel when I have the need to use their rest room. I feel the need to visit the facilities more than my car needs to belly up to the pump. It wastes fuel to drive to/from a filling station. I fill the tank every time I buy fuel for the same reason, to reduce unnecessary trips to the filling station.

I buy the lowest octane gas that will run ageeably in my car. I did a completely unscientific test in my wifes 1990 Century {back when it was nearly new} and ran a tank of premium, then a tank of midgrade and finally a tank of regular, same long day on an 835 mile road trip. The results were less than 3/10 mpg from best to worst FE, and I forget which was best. At the different prices for fuel, regular won the coin toss. It ain't just the FE numbers to me, but the out of pocket expense.

[some_other_dave]

I am in the minority--I contend that small fill-ups can give you an accurate notion of overall MPG.... If you average them over enough fills. But that doesn't do you any good if you want to try out lots of mods in a short time.

If your 87 is not an Si, then I don't think the MPGuino will work. I believe that the carburetors on the 1st-gen "DX" (or standard) and HF CRXes don't provide the right electrical signals for it. The Si's MPFI (Multi-Point Fuel Injection) should, though.

The MPGuino is probably the best way to see short-term results of any mods you make. However, you should keep in mind that temps, winds, humidity, traffic, road conditions, and other things beyond your control will have effects that are larger than nearly any mod you can make. And your driving definitely can have a much larger effect on your fuel economy than any mods.

I personally think that long-term averages are the best way to gauge what you're doing and if your mods work. But the downside is that it takes for ever to test anything.

-soD

[gone-ot]

...consider the "numbers" -- the same 1% error in a 10-gallon fillup (ie: 10.1 gal. vs. 10.0 gal.) will become a 10% error in a "short" 1-gallon fillup (ie: 1.1 gal. vs. 1.0 gal.)

[jamminjimmy]

thanks for all the input on here. For the most part- I'm on a different path of thinking, but thanks for the replies....

I will "speed up" my research into the MPGuino and probably buy one.

thanks!