Car's computer agree with fuelly? - Fuelly Forums

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Old 03-26-2016, 04:47 AM   #1
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Car's computer agree with fuelly?

I've noticed my car's computer is giving me mpg higher than I get with fuelly.
I'm confident fuelly is calculating right. It's not rocket science. Maybe I should start double checking the calculation?
I could understand the computer being off. I also think calculating a tank like this on fuelly is dead correct...can't be off.

EDIT: Just an example....most recent...
Computer said I got 34.8 mpg. Fuelly says 33.4 mpg.
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Old 03-26-2016, 08:11 AM   #2
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There is a big topic I started about this here:
http://www.fuelly.com/forums/f8/dash...ual-17936.html

A cars computer will never be 100% accurate, they are nearly always a few percent optimistic. There's no need to check your calculations, fuelly is the best method of gathering data.
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Old 03-26-2016, 01:27 PM   #3
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Thanks....That was an interesting informative read.
I do have to admit. As long as the computer is consistent. It is doing a bunch to help me with mileage. I'm very thankful for it.
If I just goof off driving. I've gotten only 30.6mpg. I'm confident I'll get ~34 this next fill up.
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Old 03-26-2016, 03:22 PM   #4
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Yes regardless of the accuracy, it's still a good tool to use whilst driving as a reminder to save fuel. It becomes a game after a while, you'll start chasing numbers if they're there right in front of your nose!
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Old 03-26-2016, 04:39 PM   #5
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Hehe.... chasing numbers.. I can be an obsessive cuss...Doh!
I've already decided on Pirelli P7 tires this year. They should gain me some mileage. .... hehe
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Old 03-26-2016, 05:10 PM   #6
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There are plenty of tyre topics here do, always pays to do a bit of homework, I've learnt alot since being on this forum. Here, our tyres are rated A to G for economy, so it makes choosing easier.
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Old 03-27-2016, 02:45 AM   #7
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Originally Posted by 14Corolla View Post
Hehe.... chasing numbers.. I can be an obsessive cuss...Doh!
I've already decided on Pirelli P7 tires this year. They should gain me some mileage. .... hehe
If you're really in to numbers and mpg, consider this: One thing to consider after your new tires are mounted is how the true diameter vary from tire to tire and how this variance affects the accuracy of your odometer (trip meter), and therefore, sometimes, the difference in mpg is much less or more than what it seems.

If you care and if you can, you can check your worn, existing tires mounted on your vehicle against a GPS a few times, and then do this again once the new tires are mounted. From my experience, there will most always be at least some error and some differences from tires to tires, and also from my experience, the odometer/trip meter check versus a GPS is usually a very consistent result.

Currently, I'm commuting in an F150 with the all new 2.7 Ecoboost engine. I have the original factory tires Michelin LTX 245/70R-17s. Every single time I've checked the trip meter against the GPS, the result has been 1 mile vehicle-recorded loss for every 54 miles, or roughly 1.8%. I've checked about three times before my 1st rotation and once after. All four have given the same variance. In my case, if the GPS is the more accurate of the two, then I can multiply my fuel-tank miles by 1.018 before dividing by the pumped volume to refill and my actual mpg is overly pessimistic w/o considering this common error.

What I do is add 1.5% so that I have a conservative record, however, I am not very physics or tech. minded, so what I can't tell you is how accurate GPSs are with respect to tracking miles, but I can say that mine is very consistent as long as it does not lose satellite reception.

I realize that with regards to my current vehicle, there is not that much error caused by tire diameter throwing off the accuracy of the trip meter. However, in my last vehicle, I once mounted new tires, and the error went from 8% to 1% as to what I had to subtract from the refill miles. W/O accounting for this error, I would have thought that the new tires caused a 10% mpg loss, but the actual difference was far, far less.

One last thing to consider... it is my experience that new tires will usually hurt mpg even beyond the extra diameter that new tread might add versus bald tires, because all that tread and newness will give lots of grip, and also, in the 1st thousand miles or so, there will be a break in period, so if you're unhappy with the first few recorded tanks, don't get too discouraged, as it may not be long term.
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Old 03-27-2016, 04:58 AM   #8
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Interesting.... I do have more than one GPS. Might do some obsessing on that. hehe
My choice of tires I got mostly from Consumer Reports. I don't want to get into dedicated summer and winter tires. My car calls for high performance, all season, V rated (speed rating), tires. I want to get the V's to match what Toyota designed the suspension for. The Pirelli's have a lower rolling resistance than the others listed. I would love to get the Michelin's they listed. They do steal a little mileage though. They track on grooves also. The Pirelli's don't do that.
I really don't think I want to get into searching for the best mileage tire. Sacrifice performance. Especially in winter.
Want to do well in mileage. But not overdo it.
I am considering getting the anti sway bars from Toyota. That's to try to up my speed on cornering. While not driving above mellow. Up the speed for better mileage. Don't have to accelerate from lower speed. Besides, I live in the Hills. Typical hilly, mountain, type roads. Lots of curves. People come from all over for Rally's here. Just to drive around the Hills. Most well known is the Sturgis Bike Rally.
I have noticed here on fuelly. The 2014 Corolla's lean toward better mileage. I don't know exactly why. We just seem to do better on average than other years. I like that. Am happy I bought used from that year.
Originally, my brother had offered to grab me one of those good year end deals in the Sunday paper in So. Cal. on a Corolla. As time went on, he backed off. I ended up telling him; "I don't want a wanna be xxx's car". That offended the donkey. He bought himself a new Accord last year. I know how he drives. He can't be getting the best mileage that Accord can offer. I want to walk all over his mileage as best I can.
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Old 04-08-2016, 01:45 PM   #9
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Just for fun, before filling up the tank, I started to record mileage calculated by the car's computer. The computer automatically resets after a fill-up so this should be directly comparable to the values one can calculate manually (or via fuelly). Will try to attach a plot of the result, based on 10 fill-ups -- which indicates the car's computer is poorly calibrated.

(actual mileage in MPG) = 0.76 x (mileage reported by computer) + 6.8

So, for example, when the car's computer says 35 mpg, the actual mileage is more like 33.4 mpg.

I wonder if there is a way for owners to adjust the car's calculation?

Link to plot: https://docs.google.com/spreadsheets...4&format=image
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Old 04-08-2016, 03:15 PM   #10
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Quote:
Originally Posted by 14Corolla View Post
I've noticed my car's computer is giving me mpg higher than I get with fuelly.
I think trip computers are calibrated using 100%(ethanol-free) gasoline. Additions of ethanol(E10-10% ethanol, E85-85% ethanol) & the accuracy of the trip computer over-estimates mpg. 114 octane ethanol needs high octane, high compression ratio(16:1) ethanol engines to release its energy effectively. Used(not burned efficiently) in a low 87 octane, low compression ratio(9:1 to 11:1) gasoline engine, ethanol fails to deliver high mpg.
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