GPS vs. Speedo?

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Brant 1

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Evening all,



I just got a new GPS navigation system for christmas and I have a question. I recently swaped over to my winter tires and used my SCT tuner to reflash the tires to the proper revs per mile (according to the internet). Now when I am driving the GPS says that speedo is off, for example when I am going 100km/h on the GPS the speedo reads 111km/h. My question is, can the GPS be trusted more than the apparently correct revs per mile number? If so, any thoughts on how to come up with the proper number to flash the truck to without doing trial and error?



Many thanks



Brant
 
I wouldnt trust the GPS more. The GPS gets the speed pretty close but not exactly. My Garmin is + or - 1 mph with accuracy. So it is decent, i dont know about your case and if that GPS is reliable or not



I dont know if this helps you or not. Just wait, more members will help you, that know about this more than me
 
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Your GPS should be dead on. Especially over a pretty long highway run. A GPS updates every few seconds so a nice long maybe 5 mile run on the highway at a constant speed and your GPS should be accurate to the 1/10. It would be less accurate for block by block driving in the city. Sorry I don't know how to adjust for your tires.
 
The one I borrowed was dead on going to louisville..... I have not checked it since but I was happy with the results..



Todd Z
 
This is so simple. Divide the actual MPH from the indicated MPH.



100/111=.9009



Take the rev's per mile (i.e. 350) and multiply it by the factor above.



350*.9009=315



Go from there. This is just simple math here.





Tom
 
Thanks all, I will mess with it tonight. :D



Caymen, I guess hindsite is 20/20, that was pretty simple. I guess all the turkey and christmas goodies fried my brain ;)



Brant:banana:
 
Welcome back Tom !!!!



Hope the trip and Vacation was a great time !!



Todd Z
 
GSP is pretty accurate. My TomTom GSP and speed has 1 mph difference of all my vehicles.
 
As stated above the GPS will be very accurate, especially on the highway where you can drive for a distance at a consitent speed. After you do the speed conversion calculation, send your number to the tuner and they should be able to update your tune to be more accurate.
 
My speedo isn't very accurate at all.



It keeps riding up in the back where it's not supposed to go, and/or leaving a berry hanging out in front.



But for some reason, it does help me get the entire hotel pool to myself pretty often.



Is this "GPS" a new competing brand, like "BVD"? I may have to check them out.



:lol::lol::lol::lol: :banana:
 
My speedo reads 2mph more than my scangauge. I tend to think the scangauge is right. When I go thru those portable radars they read the same as my scangauge.
 
The typical GPS system is more accuarate than the Speedometer for several reasons.



Most newer GPS navagation systems are accurate to withing 15-20 feet depending upon how many satelites they are able to lock in on. Also, most GPS systems will refresh your location every few seconds or less. The faster your system refreshes you new location, the more accurate it should be. Some GPS units even give MPH readings down to a tenth of a MPH...I think they do that just to show off ! :D



Automotive speedometers are notoriously inaccurate since the Federal government only requires they be accurate to within 3% of the speedometer readings. Most speedometer adjustment mechanisms are only accurate to 5%, so it can always compensate for a +/- 3% over or under reading.



Something many people do not know about GPS systems is that the software does require updating because the satelites do have a slight drift and are not in exactly a fixed or parked orbit.the programs have algorythms to adjust for the drifting for some predetermined time. It's important to keep the date and time fairly accurate in your GPS unit for the algorythms to work accurately.



I had my Garmin go out and thought it had died. I called Garmin tecjh support and they told me that my algorythms were our of date and I needed to downloand the latest software/firmware updates (not the same as map updates). I did that and it worked perfectly. He is the one that told me that the periodically the algorythms to locate the satellites must be updated because they do move/drift slightly as well as new satellites are launched and old satellites are taken out of service.



...Rich
 
Thanks for the tips Richard. I never thought about them having to be updated, but now I see why.
 
Automotive speedometers generally show 1 or 2 miles per hour faster than your actual speed. If it were the opposite, the car companies would get sued by every lawsuit-happy idiot who gets a speeding ticket. Tire wear has an impact on speedometer readings as well. As the tires wear, the diameter of the tire decreases, resulting in higher RPMs for a given speed and a higher speedometer reading. At the risk of opening a can of ugliness and discontent, my work vehicle is equipped with a traffic RADAR unit that shows my actual speed. Every vehicle I have ever driven at work has a fast speedometer reading compared to the RADAR speed.
 
Fast Eddie, if using the ScanGauge to check speed, the manufacture of the gauge has a procedure in the operating manual which would indicate that they think GPS is more accurate. I followed the tweaking procedure and made a slight adjustment on the Scanguage to agree with the GPS



One accurate way to set the speed is to use a handheld GPS. With one

person driving, the other person observes the speed on the GPS and

adjusts the % until the lower right value agrees with the GPS speed

indication.
Click to expand...
 

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