speedo calibration

They made 379's for a long time, a year would narrow it down a little. VSS driven speedo or ECM?

 

To check your speedometer run 60mph and check your watch. If it takes you 60 seconds to run a mile it's right but if you run a mile in 59 seconds you are running 61 or if it takes you 61 seconds to run a mile you are running 59. This test run is between mile markers. At 60 it takes you 60 seconds to run 5,280 feet and that is one mile. Another way is with a GPS, most tell you your MPH and that is about as close as you can get. If your speedometer is off then a gear change that goes in your transmit ion for your speedometer cable will correct your problem.

 

I don't think they used gear speedometer for quite a long time now. I have a 99' pete and it uses the dip switches and the magnetic pickup at the back of the transmission.
I've been told, you just loosen the locknut on the magnetic pickup and turn it in or out a turn to adjust.
I beleive with the dip switches, it is not ecm controlled.
I do believe a gps is the most accrurate way to check but you have to check it several times to see how far off it is. As most gps is on accruate over a distance like 10-30 miles not accurate in a mile.

 

I am a pilot and I like the GPS approach, When weather is so bad you can't see the airport many Pilots trust their GPS to keep them on the right path and speed, yes we have speed limits in controlled air space. it's called terminal velocity and we get this reading from several instruments one is our trusted GPS. ATC is watching us on radar and that also gives a read out as to our speed. Airlines us GPS speed read outs to comply with terminal velocity, you can't have a airplane on final running one speed and another behind him running faster, he simply over takes the first aircraft and that's not good. The other instrument is DME and I don't use it in class A,B or C airspace, the other is ISA or indicated air speed but a tail wind will give you a false reading as apposed to GPS ground speed. I have checked my GPS with mile markers and have found no problems. Now as to how they gear speedometers I'm not sure but I do know about the gear driven systems and we change gears to adjust speeds. If you check mile markers and your GPS using the system I gave you will find they are right on. Some NC state troopers will give you a radar report but most don't like to. You can simply say you've made changes to your gearing and want to know how far off it is. Garmin GPS have a problem as well as most GPS systems and it's the mapping language they use, But there has never been a problem following a target and it's ground speed. Finding airports have never been a problem or they would not certify them for IFR or instrument flight rules and trust me each aircraft is checked out on a regular bases. I have several GPS systems, one show truck routes and to keep me away from low underpasses and to keep me on the truck route and the other is a E Tech that shows tracking and miles driven. The other is a G1000 and is hooked to my autopilot and I've never had a problem with any of mine. Every day airlines trust GPS for true ground speed. You just can't beat mile markers and watch or your GPS, Keep your GPS updated as they make changes almost weekly.

 

I forgot one thing, you don't need to run a mile to check it our as it give you a instant report as you drive be it a mile or less. It's updated each second as per Garmin.

 

I am also a certified pilot and I know the GPS for planes are not the same as one would buy for a truck or car.
Now sure where you training at, but far as my ratings as a pilot, I can tell you, I've never seen a Plane or car gps certified. As a professional pilot, we use a gps as a conveince and a backup. It is not certified and is not supposed to relied on for take offs or landing.
As a pilot you, you are not suppose to by using a gps on approach. You should be using VOSA or instrument-IFR or VFR.
Not saying, I don't use my GPS while flying. I do use it and find it to be most accurate and most conveint. But a gps for the plane (not certified) is 1200-$6000. The GPS in our DA-40 is instrument rated and picks up the weather but FAA does not certifiy them.
I am not wanting an argument. I just hope being a pilot, you don't mislead in comparing a plane gps and a car gps. Also in a plane a garmin is the most common desired because of the live weather maps.

 

Speed Calibration

 

Is the speed not sent from the engine ECM to the dash even for a 2003? All the speed calculations should be done in the ECM. You will have to change the pulses per mile calculation in the ECM. I believe Cat has a formula for this. Cat engine in your truck I guess?