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Discussion Starter · #1 ·
I bought the Latitude on 11/29 and did a 500 mile round trip this week. I noticed that when I would set the cruise for 74 mph I was actually doing 72 mph as indicated by the GPS. I've used this GPS with three other vehicles and it has always been within +/- 1 mph with them so I think the GPS is probably accurate. Has anyone else experienced this?

I also thought it strange that on the TO part of the trip I was averaging just over 29 mpg but on the FROM part of the trip I was only getting 27.2 mpg. This was traveling close to the Gulf Coast and therefore relatively flat in both directions.
 

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Discussion Starter · #5 ·
I'll need to look. I didn't have the digital speedometer displayed. I was trying to monitor the mpg carefully to get some idea what I could expect
 

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In the US, speedometers may not show more than 5% deviation from true speed. At 74 mph, the allowed range would be from 72.15 to 75.85 mph. So you are for all practical purposes within the legal bounds. I presume, but do not know for sure if this rule applies to new tires or to the tires regardless of wear. As tires wear, and as their circumference decreases, the error shows the vehicle increasingly going at speeds above true speed.

In Germany, the land of strict rules, a speedometer may deviate up to 10% of displayed speed + 4km/h. However, the speedometer is not allowed to show the vehicle to be going slower than it actually is. Otherwise drivers would have a convenient excuse for going too fast. ;)
 

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The cruise control knows jack about what speed your gps thinks things are going.

It should track to the speedo +\-2mph depending on hills. I had pretty early revision tcu firmware, and it did not do that. It was always under the set speed. Got the tcu and other stuff flashed and now it works like cc has on every other car I have driven.

As for speedo accuracy vs gps, I have a trailhawk, and it is pretty close to spot on. There is some discrepancy, but it is under 1mph at 60mph.
 

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With WAAS enabled, and intentional degradation of public GPS signals disabled, and good reception, GPS should be accurate to around 3 meters on the horizontal plane. Presuming a dithered reading under constant speed, GPS should be considerably more accurate than a car's speedometer. The lower the speed, the less accurate the displayed speed from GPS. Any GPS user has probably noted that while standing still, the GPS does sometimes display a speed due to readings that are off.
 

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I own a Dodge Sprinter van (Mercedes Benz) and my wife used to have a Smart car and both had dial type speedometers that were off more than Lamont says is allowed in the US. In the case of the Sprinter the GPS and the output displayed on the scangage attached to the on board diagnostics port are in agreement whereas the dial is "optimistic." When the dial reads 60 the van is going 55......

I would trust GPS to display accurate speed under most circumstances. Part of the differences referred to by runnergeek may have something to do with refresh rate or??? A vehicle with the cruise enabled tends to hold speed pretty well, at least on the flat stretches so I would hope GPS and the speedo would agree. I don't know if Jeep makes any adjustment to the speedometers for the different sizes of tires installed by the factory but of course, that could make a difference as well.
 

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I own a Dodge Sprinter van (Mercedes Benz) and my wife used to have a Smart car and both had dial type speedometers that were off more than Lamont says is allowed in the US. In the case of the Sprinter the GPS and the output displayed on the scangage attached to the on board diagnostics port are in agreement whereas the dial is "optimistic." When the dial reads 60 the van is going 55.....
It sounds like the speedometer in your Sprinter needs to be adjusted so that what it displays more closely matches the sensor readout data. That can be done with the proper equipment and knowledge.
 

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Between the GPS and scangage I have two speed references so I don't worry about the analog speedo. I just set the cruise and forget it :) If I were to follow the analog at least I'd never get a speeding ticket....
 

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Between the GPS and scangage I have two speed references so I don't worry about the analog speedo. I just set the cruise and forget it :) If I were to follow the analog at least I'd never get a speeding ticket....
Now don't you wish you also had MetFan's clock that's running fast? Then you'd never be late even though your are going slower than you think! ;)
 

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I would recommend taking a test drive with someone from your dealership, kaba. You can point out the difference to them and have them diagnose it from there to ensure your speedometer is accurate.
 
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