Curses on you, fuel pump! - Fuel Economy, Hypermiling, EcoModding News and Forum

Diesel_Dave · 07-28-2011, 10:39 AM

Well, I have to admit that some of you ecomodder veterans were correct. I heard about some of you talking about making sure you always use the same fuel pump every time you fill up. I thought that probably wasn't necessary, because I didn't think there could be that much difference between different pumps. After my fill up this morning, I realize that's probably not true.

Here's what happened:

My tank is nominally 34 gallons. I always top off very slowly until I can see the fuel level in the neck (to try to get consistent readings). Therefore, my typical fill-up is between 34 & 35 gallons. The most I've even put in one fill up was 34.9 gallons. Today, I used a different pump--and it read 37.5 gal! There's no way that can be right!

Last tank was a record for me (33.9 mpg on 34.8 gal), so at first I thought that maybe I didn't get the tank completely full last time. Therefore, last tank would have looked artificially good, this one artificially bad. However, I highly doubt that's the case--there's just no way I can fit 37.5 gal in my tank. Plus, my calculated mpg on this tank was 28.5. That's the same as a tank I had 3 tanks ago. However, the average of my daily log FE was 10% higher this time than it was on that tank.

I'm going to dig up my receipt from last fill up and find out what pump I used then. From now on out I'm going to try to use that pump every time.

This thing kinda pisses me off, but maybe I should look on the bright side: this was my 3rd straight kilotank and my 3-tank avg is 113% over "EPA".

slowmover · 07-28-2011, 04:13 PM

The only trend worth noting is over thousands of miles. However much fun it is in the short term on those high mpg "tanks". Which is why my single criterion for diesel is clean and fresh. And I never worry about how well-filled is the tank. It averages out.

Now, down to business. Ever start recording average mph? I've been thinking that may be the single "transfer-able" number, one truck to another despite specification, climate and terrain. Takes a fair amount of driver input away as well (if someone can meet this bar then they already have a fair handle on how to drive for economy [for their vehicle under their conditions]).

Just a cursory understanding of road type afterwards. I don't mean this to say that a guy in northern Minnesota will get the same as a guy in South Texas year-round. But if truck spec and roads are similar, much of the years climate could be weighed in the balance if truck spec is not so far apart. (Terrain would be harder, IMO).

Back to your dilemma: I recommend you do the Tank Vent Mod as outlined on the enthusiast boards. Will make your fillups fast and easy, none of that dribble, dribble crap.

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Cd · 07-28-2011, 07:43 PM

Did you have to pay for the "extra" gas ?
Are you going to report the station for overcharging ?
I have had this sort of thing happen to me in the past, except in my case, it was the opposite scenario. I filled up my tank and it said that i was getting a huge jump in mileage.
Had I not been the type to do the math on each fill-up, no one at the station would have ever known. Being the jerk that I am, I told the clerk about it and they fixed the problem...so no free gas for anybody anymore. BWAHHAHAHA
Had I just been cheap, I never would have told. ( Cheech&Chong voice "Like, hey man it messed up my 'tank' average." )

Diesel_Dave · 07-28-2011, 08:18 PM

slowmover, this tank was the first that I've kept average speed data on. It came out to an average of 29.0 mph. I agree with you that average mph will tell you a lot, however, there's still a lot it's not telling you. 1/2 20 mph, 1/2 70 mph is much, much different than all 45 mph (even though the average is the same). Also, I'll look into that tank vent mod.

Cd, yes I paid for the extra. I don't really have any concrete "proof" that the pump was off, so I probably won't report it or anything. I might mention it to the clerk, just as an FYI. I don't suspect it's anything intentional. About the only thing I'm going to do is not use that pump again.

slowmover · 07-28-2011, 08:25 PM

slowmover, this tank was the first that I've kept average speed data on. It came out to an average of 29.0 mph. I agree with you that average mph will tell you a lot, however, there's still a lot it's not telling you. 1/2 20 mph, 1/2 70 mph is much, much different than all 45 mph (even though the average is the same).

Yeah, but I can look at your truck spec, and your description of your commute (or long distance trip) and your average mph . . and I'll come away with some intuitive thoughts about my own. I already know that if I keep my mpg at or above 27-mpg around town that my mileage is excellent. Your actual mpg number (and mine) is the "skill difference".

Ready-made kit with install instruct

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Diesel_Dave · 08-03-2011, 11:00 AM

Well, as it turns out, my fuel tank may somehow hold more fuel that I previously thought possible. I just passed 300 miles on this tank...and the needle just started to move (still on the "F" mark).

I've run 150-200 miles before, but never this high. So, I don't know how, but I'm thinking that fuel pump might actually have been right after all!

slowmover · 08-03-2011, 04:51 PM

Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.

- Richard Feynman

That I are . . . .

(It's impressive that the needle doesn't move. I expect mine to uncover the FULL mark just past 175-miles.)

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Kodak · 08-03-2011, 09:19 PM

Weights and measures periodically tests pumps. I'm not sure how often. Each pump has to be within a certain fill variance (+/- X%) to be certified. You should see a sticker on the pump.

I logged one tank a while back came out to a bit over 24mpg before I was even capable of doing 23 consistently, so I suspect filling variance played a role.

Maybe one pump is a bit generous, and the other is a bit shy.

Try to fill at the same pump, but personally I wouldn't idle waiting for a specific pump unless you have a pressing reason for needing a precise number.

As an added tip, try to take note of which pumps are the most popular, and pick a lesser-used pump as your favorite (unless you suspect it pumps a bit shy of a gallon).