How accurate is the MPG detector computer thing?
Platinum Member
Joined: 12-06-2009
Posts: 11,721
From: Alabama
Jay. Do you reset it after you fill up? If not, then it will average all previous miles back to the last reset.
Senior Member
Joined: 09-15-2011
Posts: 90
From: Florida
The trip computer readout for inst MPG is a good gauge for monitoring driving style. My baseline is 40 at 40. Sometimes my in town (simi-rural) average MPG is as high as 27 per tank which is about the same as on the freeway at 70, about 29. I have taken trips on two lane roads averaging around 55-65 and clocked mid 30s. At 80 I only get about 22mpg, worse than in town. Speed and driving style can make a huge difference at the pump. Here locally 93 octane is around $4.50 so my foot is a little lighter on the throttle.
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Joined: 08-06-2008
Posts: 1,761
From: Grand Haven, MI
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Joined: 01-23-2009
Posts: 26,533
From: Fredericksburg,VA
Somewhere back there somebody said it uses airflow, injector cycles, and wheel revs. I think. If you have different size tires than the computer thinks you have MPG is wrong. Tire size can be set with a TechII scanner.
Senior Member
Joined: 04-07-2008
Posts: 1,051
From: Wisconsin
The MPG calculated by the computer cannot be wrong, if the diameter of the tires is the same as came from the factory, and the fuel injectors have not been changed.
Each pulse of a fuel injector injects a precise amount of fuel into the combustion chamber. Each rotation of a wheel advances the vehicle a specific distance.
To determine how much fuel is consumed, the computer counts the number of times the injectors are pulsed and multiplies that figure by the injector's delivery volume per pulse. (there may be pulse duration factors in there somewhere)
Simultaneously, the odometer is counting the revolutions of the wheels, and determining how far the vehicle has traveled. Computers cannot be partially correct. They can only be correct, or non-functional.
The trouble is that engines spend a lot of time consuming fuel while the vehicle is at a standstill. However, since standing still is also a significant part of vehicle operation, you are getting the REAL MPG from the computer.
Each pulse of a fuel injector injects a precise amount of fuel into the combustion chamber. Each rotation of a wheel advances the vehicle a specific distance.
To determine how much fuel is consumed, the computer counts the number of times the injectors are pulsed and multiplies that figure by the injector's delivery volume per pulse. (there may be pulse duration factors in there somewhere)
Simultaneously, the odometer is counting the revolutions of the wheels, and determining how far the vehicle has traveled. Computers cannot be partially correct. They can only be correct, or non-functional.
The trouble is that engines spend a lot of time consuming fuel while the vehicle is at a standstill. However, since standing still is also a significant part of vehicle operation, you are getting the REAL MPG from the computer.
Here's the thing. It seems to me that people make far too much of trying to find exact mileage on a daily or weekly basis. There are more important things in life to lose sleep over. Take readings both from the DIC and by calculating at each fill up. Average the results over a period of time. That's going to provide the most accurate picture of how your vehicle is performing because conditions change every time you drive.
Even then, take that number with a grain of salt. Don't get too worked up because someone on this forum or another reports that they average 38 mpg and you only average 31 (never forgetting that not only are someone else's habits are different, but not everyone on the Internet is being honest. Shocking thought, isn't it?). What's important is that you be consistent in your driving habits, and that when the mileage over time drops by more than a few percent without going back up, you'll know its an indication that something isn't right.
Senior Member
Joined: 02-22-2012
Posts: 376
From: Baton Rouge
True only if all conditions are met perfectly. The reality is that those calculations are based on controlled lab conditions, which rarely, if ever, are going to be met in the "real world". So yes, computers can be wrong, and no, you are rarely seeing perfectly accurate "real world mileage" on the DIC. You might see it close, but its unlikely that it will it be exact. The computer can only calculate what's fed into it. GIGO, garbage in, garbage out. Too many variables for the programmers to take it all into account (things like temperature and humidity have an effect not only the air/fuel mixture, but on tires as well, just as an example). All one needs to do is the old fashioned method of calculating mileage at each fill up and you'll find that over a long period of time the DIC will NOT match those calculations. The real mileage is going to be somewhere in between the "hand" calculations and the DIC.
Here's the thing. It seems to me that people make far too much of trying to find exact mileage on a daily or weekly basis. There are more important things in life to lose sleep over. Take readings both from the DIC and by calculating at each fill up. Average the results over a period of time. That's going to provide the most accurate picture of how your vehicle is performing because conditions change every time you drive.
Even then, take that number with a grain of salt. Don't get too worked up because someone on this forum or another reports that they average 38 mpg and you only average 31 (never forgetting that not only are someone else's habits are different, but not everyone on the Internet is being honest. Shocking thought, isn't it?). What's important is that you be consistent in your driving habits, and that when the mileage over time drops by more than a few percent without going back up, you'll know its an indication that something isn't right.
Here's the thing. It seems to me that people make far too much of trying to find exact mileage on a daily or weekly basis. There are more important things in life to lose sleep over. Take readings both from the DIC and by calculating at each fill up. Average the results over a period of time. That's going to provide the most accurate picture of how your vehicle is performing because conditions change every time you drive.
Even then, take that number with a grain of salt. Don't get too worked up because someone on this forum or another reports that they average 38 mpg and you only average 31 (never forgetting that not only are someone else's habits are different, but not everyone on the Internet is being honest. Shocking thought, isn't it?). What's important is that you be consistent in your driving habits, and that when the mileage over time drops by more than a few percent without going back up, you'll know its an indication that something isn't right.
