MPG reading not matching up... anyone else??

[93civiccpe]

Hello Everyone,
First of all, I've read a bunch on here but this is my first real post. I have switched jobs so now I'm commuting around 80 miles each way to work every day, and my average speed is probably around 65mph as it is all highway. (I cannot drive slower and remain sane.. the commute to and from work is long enough). Anyways, I picked up a 2000 MT Insight a few months ago specifically for this commute. When I got it the whole hybrid battery system was shot and Honda replaced it under warranty. When I bought the car, it had right around 5,000 original miles. (It had sat in storage for many years). I'm now up to 15,000 miles and there's something I've noticed which seems to be a problem. First off, the car has non-oem tires which I know robs me mileage, but I'm fixing that issue. Also, I suspect the EGR is getting clogged so I have a brand new one and I'm going to clean the EGR plate as well.

That being said, I've been averaging in the mid-to-low 60's as far as MPG is concerned. (Or at least that's what the FCD is telling me). Anyways, I always keep track of how much gas I have to put in the tank to get it completely full. I top it off the exact same way at the same station, and for the point of argument I actually started using the exact same pump every time. What I've been doing is looking at how many miles I've actually driven, and then how much fuel I used to go that distance, and so I calculate a real-life MPG as far as what I'm actually getting. Here's my problem... the actual value is ALWAYS 8-10MPG LOWER than what the car is telling me I'm averaging. So I'm actually only getting around the mid-50's MPG. That's not the worse thing, and it's much better than my Civic was getting, but it's upsetting and deceiving at the same time if it's telling you that you are getting a certain MPG which turns out to be completely false. It also is frustrating that I can barely get 500-520 miles on a tank before the engine warning light is on, when I was told that 700+ miles per tank was possible. I haven't read about this issue yet, so I'm hoping it's just something I need to calibrate on my end and I'm hoping someone can please point me in the correct direction.
Thanks!!
-Sean

[Insightful Trekker]

Non stock tires must be compensated for the difference in revolutions per mile. Maybe only a small percentage + additional friction losses, maybe larger. Look up the specs on tire rack or the manufacturers website.

Other factors have recently been discussed here:

Yet another FCD discrepancy post

HTH!

[joecvt]

Originally Posted by 93civiccpe

Anyways, I picked up a 2000 MT Insight a few months ago specifically for this commute. When I got it the whole hybrid battery system was shot and Honda replaced it under warranty. When I bought the car, it had right around 5,000 original miles. (It had sat in storage for many years). I'm now up to 15,000 miles and there's something I've noticed which seems to be a problem. First off, the car has non-oem tires which I know robs me mileage, but I'm fixing that issue. -Sean

The original owner must have hated the OEM tires if they were replaced before 5,000 miles on the odometer. How much larger in diameter is the tire you have on the car now compared to the OEM tire? You have to add those extra miles the car actually traveled into your MPG calculation. The display guage and the ECM does not "know" that you have larger tires installed (assuming they are larger).

Also.....Try slowing down just a bit and increase the air pressure in the tires to what the max is listed on the sidewall of the tire.

See: Tire size calculator

JoeCVT = Just your average CVT owner

[cainsight]

I also keep track of my mileage with an excel spreadsheet. It varies, but at most 1-2 MPG per tank.

[RobertSmalls]

The FCD doesn't always count fuel burned while idling before your trip begins. Chaning a thermostat or charging the IMA battery in the driveway can use a good amount of gasoline, but it won't always be recorded.

Sometimes, when I cycle the ignition at speed, the car comes back on pegged at 150mpg / zero fuel consumption, which is pretty disruptive. Cycling the ignition again fixes it every time.

[Insightful Trekker]

Originally Posted by RobertSmalls

The FCD doesn't always count fuel burned while idling before your trip begins. Chaning a thermostat or charging the IMA battery in the driveway can use a good amount of gasoline, but it won't always be recorded.

Huh? I've always seen a 0 (zero) recording. Sharply dragging down the recorded MPG average since the last reset.

Originally Posted by RobertSmalls

Sometimes, when I cycle the ignition at speed, the car comes back on pegged at 150mpg / zero fuel consumption, which is pretty disruptive. Cycling the ignition again fixes it every time.

Yup! Its a known FCD glitch. Happens even without cycling the key as a cause. As a "fix" the ignition key cycle is the cure (until next time).

HTH!

[uhtrinity]

From my observations.

When my car was still stock the FCD would show numbers up to 5% high, that was when I was averaging ~60 - 65 mpg per tank. I would also see numbers up to 5% low if I did a lot of mountain driving with extremely long down hills.
Now that I have mima and additional battery capacity the FCD always reads low.

For example, when I went to Oregon last summer, the stretch from here to Portland showed 72 mpg, but the calculated value was over 76mpg. The next stretch showed as 68mpg on the FCD, calculated came in around 72 mpg and so on.

My thinking is the sensors have issues with ultra high mpg numbers (80mpg +) but are consistently low at numbers in the normal range (40's to 60's). As long as I have owned the car I have also used OEM tires.

OP, you sound like you are driving your car 'normal', and your non OEM tires may be also adding to that discrepancy.

[93civiccpe]

I figured I would update this. I put new tires on this, and used the OEM Potenza RE92's. I inflated them to 44psi, and also cleaned my egr plate (which was clogged) and put a new EGR valve in. I put new spark plugs (using the OEM plugs with the correct letters), and a new air filter. I am now getting much better MPG's, and have learned how to drive the car to get more efficiency, but on my car, I'm still seeing my FCD show an average that is between 6-10MPG higher than the calculated value. I may go check the speedometer to see if maybe my speedometer is off, but at least now I'm getting 60+ MPG every tank so I'm not that concerned that the average is off. (And I do not let my car warm up in the driveway.. I just jump in and go so no fuel lost there.)

[02insight]

It is known the speedo is off on these cars, that is why Honda changed the battery warranty from 150,000 to 157,500, because of the ~5% error.

Although in my car, I have noticed only a ~3% error, verified by GPS and calculating MPG at the pump. Slightly taller tires would correct this error and give a more true MPG reading on the dash even if its slightly lower than with the stock RE92's

[jonnyvtec]

If you are using the odometer miles for your calculated MPG then the tyre size wont be skewing the FCD to real as the miles driven for both calculations comes from the same source.