double check your speedo accuracy by running a level highway with mile markers at EXACTLY 60 mph and checking your watch. mile markers should be EXACTLY 60 second intervals.

correct the speedo with hypertech, superchips, or dealer visit and adjust "revolutions per mile".

 

That statement makes as much sense as a football bat. GPS was developed for the military to use as a weapons targeting system. They can park a missile +/- 3' of your truck if needed. GPS is also how the US Navy navigates its ships today.

Last summer, a friend and I followed another friend who was driving his 1969 VW. The GPS put his speed at very close to what we said he was driving.

 

At this point I'll probably just to the mile marker thing and see what's up there. The Superchips would be nice though

 

While I'm not buying the 5-15mph story, there is a difference between military grade GPS and what the civilians are allowed to use (yes, allowed). The military signals are encrypted and, as such, are not available for civilian use. There is an error designed into what is available to civilians to prevent the signals adaptation for targeting purposes. We don't want N. Korea using our satellites against us.

 

You didn't account for Google Earth Yes, military signals are encrypted but the civilian band is very accurate.

As for the OP, at 55mph (speedometer) with a 5% inaccuracy, that results in a 2.75mph difference. Again, not much to be riled up about.

 

Hey, I live in Fargo, ND too and also drive a '99 Ram 5.9 and noticed the exact same thing the other day. A few weeks ago a buddy of mine was following me down I94 (to Jamestown) and before I realized it he was gone, we got to Valley City (an hour down the road for you non-North Dakotans) and he stated that he didn't want to get a ticket so he wasn't going to follow me any longer. I had the cruise set at 78-79, and he said I was going about 85mph. I checked with the GPS on my blackberry this past weekend while driving down I94 again and sure enough, cruise set at 75mph I'm going 79-80. For the moment I too have 265s. Followed the GPS speed and sure enough, I got better fuel economy... but I'm sure the trip computer is off too due to the inacuracy.

Oh, and how do you like your exhaust set-up? Pretty loud?

 

Got this from wikipedia:

Selective availability
GPS includes a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.
SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the US military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.
Before it was turned off in 2000, typical SA errors were 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically. Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President Clinton in 2000.
In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The amount of error added was "set to zero"[54] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the US military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.[54]
Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,[55] have stated that it is not intended to be reintroduced.
On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[56] eventually making the policy permanent.[57]