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I know that there has been plenty of discussion on here about the DIC being higher than manual calculations on the average mpg and I think I found something interesting that could explain another reason why the reading is a bit high.

I was traveling back home from work the other day and I was messing with my gps when I noticed that the mph reading on the GPS was 1 mph lower than what I had my cruise control set at. Normally when I am driving I leave the DIC on the average mpg page so I switched over to the speed page and it said I was going the speed I had my cruise control set at. I happen to have a radar detector that has a gps chip in it so I switched the display to show my speed and it also showed me going 1 mph slower than my car speedometer. Finding this interesting I decided to download a speedometer app on my iPhone and it also showed me going 1 mph slower than my car speedometer.

This 1 mph difference was rather consistent the whole time including while driving in town to my house. I was only monitoring the difference when on level ground. The speed difference was more consistent at the higher speeds on the highway but still fairly consistent on the slower city roads. When the speed did change on one the three devices the other two would change to the same reading within a second or two.

If the car's speedometer is reading any percentage higher than reality then that means your trip meter is also reading high. Ironically though my fuel used reading has been dead on for my last 3 tanks.

With it being Christmas weekend I am thinking there are a few people on here that can test this out on their travels and see if this is just my car.
 

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GM is known for "adding 2 mph" electronically to the speedometer display (but, not to the odometer), to compensate for changes in tire diameters.

People with ScanGauge II's or Ultra-Gauge's have also noticed this difference, ie: speedometer showing ~2 mph faster than SGII or UG readings (which are read directly from ECM & wheel sensor).
 

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Interesting! next time I get on an open stretch of highway, I will measure vs the mile markers(time and distance) and see if I can get my wife's GPS back from my brother in law(I don't need it and she only drives local but he uses it for driving a school bus). Since I have a LS, I never really worried to much about getting 40+ anywhere as long as it seemed reasonable. But minor discrepancies drive me nuts (I am a "techy" in IT).
 

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...all you need are:

1) a nearly flat highway where you can drive 60 mph with cruise control "on" (for constant speed).
2) a watch with a second hand, or stop watch.
3) ability to divide XX-seconds into 3600, ie: MPH = 3600 / XX-seconds(per 1 mile).

...if it takes you 61 seconds between the highway 'mile-posts,' then your speed was actually 59 mph, or one-mph slow.

...conversely, if it took you 58 seconds between the 'mile-posts,' then your speed was actually 62 mph, or two-mph fast.

...for better "accuracy" measure between 2 mile-posts, and simply double 3600 to 7200, ie: MPH = 7200 / XX-seconds(per 2 mile).
 

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I was driving on I-75 they have the your speed is signs. From what I saw, my LS with 16 in wheels and tires, the speedo was right on or both are wrong. I saw a post a short time ago about wheel sizes and speedo discrepancies, maybe all cars are calibrated for the 16 in wheels and the 17s + 18s are as close as they could come to the calibration without being "noticibly off"? I have heard tire dealers say when trying to sell sizes other than stock: "Whats a mile or 2 off"! Just an idea!



Interesting! next time I get on an open stretch of highway, I will measure vs the mile markers(time and distance) and see if I can get my wife's GPS back from my brother in law(I don't need it and she only drives local but he uses it for driving a school bus). Since I have a LS, I never really worried to much about getting 40+ anywhere as long as it seemed reasonable. But minor discrepancies drive me nuts (I am a "techy" in IT).
 

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1 mile per hour at highway speeds is hardly anything, GM considers anything under 4% different once to be acceptable. Tire pressures, tire manufacturing tolerances, things can easily add up to 1mph at speed, and that's less than %2 error. ****, undulations in the road over long distances can attribute to mileage markers being off.....:lol:: it
 
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My 2011 6spd ECO is off 1 mph against my Garmin GPS...sometimes :)

:eek:fftopic:On the good side, the MPG is going up a little. New it was 33.2 now it's 33.6 (for my driving) :eusa_clap:
 

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@Patman,
I asked the Chevy Parts dude about changing wheel size (from 18" eco) to a smaller steel wheel for snow tires. He said there are 3 computers for the 3 rim sizes. They aren't swapable starting with the 2011 models also not having the PSI wheel sensors would set off the check engine light. That's all I know and forgot that idea.
 

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...there are NOT three different computers -- just one -- but, there are three different firmware/software program loads, one for each different tire/wheel combination.
 

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Interesting, I don't think I've ever had a car with a speedometer that was within 1% of actual. I guess with all of the electronics available, a new car should be dead on, but I just don't see a huge need. Even if the speedo is right on from the factory, the first set of replacement tires will likely throw it off.
 
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