Speedo vs GPS
01-20-2008, 04:06 PM
#1
1st Gear Member
Thread Starter
Join Date: Jul 2006
Location:
Posts: 50
Speedo vs GPS
just finished a 1000 mile trip, I noticed that my speedodmeter and my navigation system (garmin 650 ) speeds dont match but are of 2 to 3 mph off, speedo reads 70, gps reads 68...speedo reads 85, gps is 82, speedo 120, gps 117, not much difference but I wonder which one is wrong.Anyone have a gps and do you see a difference¿
01-20-2008, 08:44 PM
#5
5th Gear Member
Join Date: Mar 2006
Location: ATL
Posts: 2,955
RE: Speedo vs GPS
My speedo and GPS were dead on at 75mph when I checked it on my way to work one morning. My GPS is not a navigational one, just a simple Etrex. https://buy.garmin.com/shop/shop.do?pID=6403
I bought it for tracking my RC car's top speed.
I've also used the same GPS in a Hyndai rental car. Car was maxed at 117mph and the GPS said the exact same.
I bought it for tracking my RC car's top speed.
I've also used the same GPS in a Hyndai rental car. Car was maxed at 117mph and the GPS said the exact same.
01-20-2008, 10:24 PM
#8
Join Date: Mar 2005
Location:
Posts: 11
RE: Speedo vs GPS
The GPS figures out speed by using a rather complex mathematical formula based onposition received from communication with at least fourof the31 GPS satellites in orbit. The speedo's in these cars calculate speed based on wheel revolutions. Since I suspect there are more variables affecting the cars motion across the ground I'd think that the Speedo has a better chance of being off than the GPS. On the other hand I'm not a mathematician so my opinion is likely more an uneducated guess. [sm=gears.gif]
01-20-2008, 10:28 PM
#9
5th Gear Member
Join Date: Apr 2007
Location: The High Country
Posts: 3,547
RE: Speedo vs GPS
ORIGINAL: martimus
The GPS figures out speed by using a rather complex mathematical formula based onposition received from communication with at least fourof the31 GPS satellites in orbit. The speedo's in these cars calculate speed based on wheel revolutions. Since I suspect there are more variables affecting the cars motion across the ground I'd think that the Speedo has a better chance of being off than the GPS. On the other hand I'm not a mathematician so my opinion is likely more an uneducated guess. [sm=gears.gif]
The GPS figures out speed by using a rather complex mathematical formula based onposition received from communication with at least fourof the31 GPS satellites in orbit. The speedo's in these cars calculate speed based on wheel revolutions. Since I suspect there are more variables affecting the cars motion across the ground I'd think that the Speedo has a better chance of being off than the GPS. On the other hand I'm not a mathematician so my opinion is likely more an uneducated guess. [sm=gears.gif]
True, but consider the terrain. Driving in hilly or mountains roads will throw off the GPS. It calculates as the crow flys, not as the donkey runs.
