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Old 08-31-2017, 04:28 PM   #1 (permalink)
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Question Calculated MPG significantly (consistently up to 15mpg) lower than FCD MPG

Second try, this time in the correct subforum ...

I'm seeing a difference of between 10 and 20 mpg between what the FCD shows and what the pumps show. To head off some of the common gotchas: I know that the FCD doesn't always report the same in different Insights, and also that gas pumps are inconsistent and that you have to try and remove that variable as much as possible. I log my MPGs fairly religiously (offline) and always ensure I'm using the Trip buttons correctly and not accidentally using the Lifetime mpg, etc etc.

Having said all that, I still think I have some sort of problem due to the large difference between my reported and calculated MPGs. I've read some other threads regarding differences between FCD and manually calculated MPG on this site, and in those threads, other users report maybe a 5-10% difference. But what I'm seeing is FCD calculations on the order of, say, 60-63mpg with manual calculations approx. 45-50mpg - a much larger difference on the order 20-30%.

I'm concerned that I might have a fuel leak somewhere between the tank and the injectors, but there are no obvious signs such as a gasoline smell etc. I'm going to get the car up on jacks and take a look underneath/under the hood the first chance I get, but until then... anyone else had a similar experience? Thanks!

EDIT: Car is a 2000 manual, RE92s on stock wheels inflated to 45psi. No MILs. I run a Calpod and typically have the IMA battery (functional but ageing, approx. 9 yrs old) disabled when cruising at constant speed and enabled when accelerating hard or when ascending hills.

Last edited by Carbonator; 08-31-2017 at 08:46 PM. Reason: Wrong thread
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Old 08-31-2017, 04:39 PM   #2 (permalink)
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You don't accidentally have a UK model with Imperial mpgs ?

All I can say is keep a fuel log like on Fuelly and note your indicated mpg and compare to the mpg calculated from the odometer and pump volume.
You can only tell for sure if you have the data to prove it.

And, are you sure the volume as indicated by the gas pump is accurate?
It should be, it would be illegal to overindicate the volume provided. And very lucrative for the pump owner.

I actually encountered the opposite case. One pump on a gas station seriously underreported the volume. After the second fillup (confirming to me that the pump had that 'feature') I informed the employees of the station about it, but they just shrugged. I was a loyal customer until it finally got fixed months later
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Old 08-31-2017, 04:43 PM   #3 (permalink)
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I don't use Fuelly but I do keep my own logs of fuel economy - this is a fairly consistent behavior irrespective of fuel station or pump. What I'm hoping folks can suggest is any problems that might cause this behavior if it doesn't turn out to be just inaccuracies in the volumes provided by the gas pump meters, the FCD or both. The discrepancy in my case seems to be somewhat larger than the discrepancy Insight owners typically report from those causes, which is what's causing me concern.
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Old 08-31-2017, 05:05 PM   #4 (permalink)
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Your location ('United States') gives no clues about the weather. Anchorage or Guam makes a difference

When it is really hot the gas evaporation from the tank may be a major factor. It should be captured in the overflow canister, from which it gets sucked into the air intake.
This will enrich the air with fuel, helping combustion, while the ECU does only register fuel consumption by measuring what went through the injector pump.
So the ECU sees better than real fuel consumption. After all, not all gas you tanked went through the injectors.

This is the case with all cars using injector quantity so all of them will be optimistic on FE. A 5% positive economy reading is my average here in cool-mild Dutch weather; but during a heat spell 8 to 10% is not uncommon.

Methinks 60 mpg is exceptionally good for a 2nd gen Insight. I am an avid hypermiler, but everything needs to go my way to get a per tank economy that high - and only in summer, trailing the semis at 50 mph.
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Old 08-31-2017, 05:11 PM   #5 (permalink)
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Quote:
Methinks 60 mpg is exceptionally good for a 2nd gen Insight.
Yes, me too. Which is what made me realize I posted this in the 2nd Gen MPG forum by accident Thanks for your help anyway! Now time to go put this where it should have been... apparently my diligence in economy logging doesn't extend to checking the generation of Insight subforum I'm posting in
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Old 08-31-2017, 06:43 PM   #6 (permalink)
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It's all a bunch of percentages adding up, some possibly by design*.

One thing I noted via GPS reading was a consistent ~3% discrepancy between satellite reading of speed vs. dash indicated speed in the car [using stock size re92]
*Possibly a design feature 'within tolerances' to soften corporate gas figures?
fwiw: my '00 as acquired initially ran on 175/65r13 tires until the re92's were acquired. GPS and dash display agreed. Now, with stock 165/65, indicated speed is ~3% under gps velocity.
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Old 08-31-2017, 06:50 PM   #7 (permalink)
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My discrepancy has been FCD mpg only +1-2% over fill-up mpg calculations, for last 5 years or so. So OP's discrepancy is huge. Not sure what would cause that, though...
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Old 08-31-2017, 08:39 PM   #8 (permalink)
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Quote:
Originally Posted by eq1 View Post
My discrepancy has been FCD mpg only +1-2% over fill-up mpg calculations, for last 5 years or so. So OP's discrepancy is huge. Not sure what would cause that, though...
Since there is no info about the car, tires, inflation psi, transmission, any MILs, maybe he's got a load of moonshine in the back it is just about impossible to help.

My speedometer with RE-92s was exactly the same as my GPS reading last time I checked it. My mpg when calculated by hand vs the car display is slightly different but I just don't worry about it since it's so high either way.
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Old 08-31-2017, 08:44 PM   #9 (permalink)
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Well, olrowdy01, I don't wish to render diagnosis impossible, so: It's a 2000 MT. RE92s at 45psi. No MILs. The IMA battery is old (9 yrs) but functional, albeit with a robust regen schedule. I typically run it disabled via a Calpod for cruising and leave it enabled for accelerating from a stop or when ascending hills.

Last edited by Carbonator; 08-31-2017 at 08:51 PM.
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Old 08-31-2017, 09:27 PM   #10 (permalink)
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Quote:
Originally Posted by Carbonator View Post
Well, olrowdy01, I don't wish to render diagnosis impossible, so: It's a 2000 MT. RE92s at 45psi. No MILs. The IMA battery is old (9 yrs) but functional, albeit with a robust regen schedule. I typically run it disabled via a Calpod for cruising and leave it enabled for accelerating from a stop or when ascending hills.
Have you checked the speedometer vs a GPS?

Do you try to put as much gas as you can get in the tank at fill ups?
By chance do you fill up way before the tank is near empty?
It's known that the Insight sometimes shows inflated (like 150+ mpg) right after a fill up for a short time.

A common fuel line rust spot on salted road Insights is under the hood where the fuel line is attached to the alum bar from one shock mount to the other.
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