speedo vs gps
Race Director
Joined: Jul 2007
Posts: 18,681
Likes: 47
From: Reno is so close to Hell you can see Sparks , State Of Confusion
St. Jude Donor '12-'13-'14
Mine both read the same, But I agree with the above, Too many variables in the GPS.
Burning Brakes
Joined: Jan 2003
Posts: 984
Likes: 0
From: New Smyrna Beach Fl
One more thing to consider here......GPS uses a satalite signal to calculate your speed from point "A" to point "B". It also calcultes speed from "A" to a nonspecified point. BUT it is all calculated "As the crow flies" A to B may be 10 miles apart as the crow flies (straight line) but may be 15 miles apart by the exixting road. The speedo reads how fast your covering the asphalt, the GPS reads how fast you are getting closer to "B", leaving "A" etc.
Thread Starter
Drifting
Joined: Mar 2007
Posts: 1,589
Likes: 1
From: Leelanau County
just a couple of points.
i have compared gps to the suburban-dead on
my v-max is off.
when in the vette all day i have compared them numerous times throughout the trip. always the same discrepency(sp?)
also, are mile markers always exactly placed?
any road commission employees here?
thanks for all the input
i have compared gps to the suburban-dead on
my v-max is off.
when in the vette all day i have compared them numerous times throughout the trip. always the same discrepency(sp?)
also, are mile markers always exactly placed?
any road commission employees here?
thanks for all the input
Race Director
Joined: Jul 2007
Posts: 18,681
Likes: 47
From: Reno is so close to Hell you can see Sparks , State Of Confusion
St. Jude Donor '12-'13-'14
just a couple of points.
i have compared gps to the suburban-dead on
my v-max is off.
when in the vette all day i have compared them numerous times throughout the trip. always the same discrepency(sp?)
also, are mile markers always exactly placed?
any road commission employees here?
thanks for all the input
i have compared gps to the suburban-dead on
my v-max is off.
when in the vette all day i have compared them numerous times throughout the trip. always the same discrepency(sp?)
also, are mile markers always exactly placed?
any road commission employees here?
thanks for all the input
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
if two trains... leaving in opposite directions...
http://forums.corvetteforum.com/show....php?t=2166177
http://forums.corvetteforum.com/show....php?t=2166177
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Racer
Joined: Jun 2005
Posts: 481
Likes: 0
From: Lake Forest CA
One more thing to consider here......GPS uses a satalite signal to calculate your speed from point "A" to point "B". It also calcultes speed from "A" to a nonspecified point. BUT it is all calculated "As the crow flies" A to B may be 10 miles apart as the crow flies (straight line) but may be 15 miles apart by the exixting road. The speedo reads how fast your covering the asphalt, the GPS reads how fast you are getting closer to "B", leaving "A" etc.
But I got my degree in backyard physics at the College of Small Minds.
Melting Slicks
Joined: Oct 2006
Posts: 3,447
Likes: 6
From: Los Angeles
One more thing to consider here......GPS uses a satalite signal to calculate your speed from point "A" to point "B". It also calcultes speed from "A" to a nonspecified point. BUT it is all calculated "As the crow flies" A to B may be 10 miles apart as the crow flies (straight line) but may be 15 miles apart by the exixting road. The speedo reads how fast your covering the asphalt, the GPS reads how fast you are getting closer to "B", leaving "A" etc.
Speedos are generally accurate, but can vary from vehicle to vehicle due to mechanical difference... motorcycles are certainly a good example of this. My bike reads 167 mph when the GPS only reads 150 mph. Given I still had a couple hundred RPM to go in 6th gear and it is mechanically limited to 155 mph, it's only logical the GPS was correct. Granted, the error increases the faster it goes.
Now if your GPS receiver is only tracking 3 satellites at a diminished signal (weather, mountains, tall buildings etc), yes your accuracy will decrease.
As far as the OP goes, 10% is quite a bit... I'd get the speedo/settings checked and trust what the GPS is displaying.
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
this is true; and any GPS made in the past 5 years or so has a fast aquisition repeat rate.
But I can't help to further remind people that a GPS measuring speed ...is based on a calculation; and nothing beats actual on-the-ground direct measurement.
Granted, the two may be so close to each other that it's not worth haggling over... but the GPS "guesses" and can't be used as a standard or end-all measurement device.
But I can't help to further remind people that a GPS measuring speed ...is based on a calculation; and nothing beats actual on-the-ground direct measurement.
Granted, the two may be so close to each other that it's not worth haggling over... but the GPS "guesses" and can't be used as a standard or end-all measurement device.
Safety Car
Joined: Jul 2001
Posts: 4,661
Likes: 67
From: Ocean Springs MS
this is true; and any GPS made in the past 5 years or so has a fast aquisition repeat rate.
But I can't help to further remind people that a GPS measuring speed ...is based on a calculation; and nothing beats actual on-the-ground direct measurement.
Granted, the two may be so close to each other that it's not worth haggling over... but the GPS "guesses" and can't be used as a standard or end-all measurement device.
But I can't help to further remind people that a GPS measuring speed ...is based on a calculation; and nothing beats actual on-the-ground direct measurement.
Granted, the two may be so close to each other that it's not worth haggling over... but the GPS "guesses" and can't be used as a standard or end-all measurement device.
Team Owner
Joined: Jul 2006
Posts: 22,182
Likes: 4,178
From: Kannapolis NC
The GPS is not an approximation, it is a measured distance change in a designated amount of time, converted to mph. It is not subject to the variables of tire size, tire wear, etc. I'll go with the GPS every time. By the way, mine matches all three cars within 1 mph. If something is off 10%, it's not the GPS.
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
My speedo measure fast compared to the GPS. But, my DIC cruise setting measures right on. If I set it to 60mph or 70mph or 80mph the GPS says that is the speed within about 1/2 mph. But, my speedo itself is still reading higher a little bit.
Really? And the C5 doesn't do a calculation based on the number of times the differential rotates per unit of time??? How is that calculation any better. Besides, the GPS units sold today will have about 12 or 13 years newer computing technology compared to the C5.
Peter
But I can't help to further remind people that a GPS measuring speed ...is based on a calculation; and nothing beats actual on-the-ground direct measurement.
Granted, the two may be so close to each other that it's not worth haggling over... but the GPS "guesses" and can't be used as a standard or end-all measurement device.
Granted, the two may be so close to each other that it's not worth haggling over... but the GPS "guesses" and can't be used as a standard or end-all measurement device.
Peter
Vette Dan
Joined: Sep 2004
Posts: 1,043
Likes: 1
From: OKC Oklahoma
That's not really how it works, it updates very quickly. GPS is very accurate, most civilian receivers can track more satellites (making it more accurate) than most of the military equipment I have used. Even our stuff was good to less than 1 meter when tracking the max amount of sats, the garmin type stuff is currently no different (if not better)... it is the same signal regardless of the receiver you use.
Speedos are generally accurate, but can vary from vehicle to vehicle due to mechanical difference... motorcycles are certainly a good example of this. My bike reads 167 mph when the GPS only reads 150 mph. Given I still had a couple hundred RPM to go in 6th gear and it is mechanically limited to 155 mph, it's only logical the GPS was correct. Granted, the error increases the faster it goes.
Now if your GPS receiver is only tracking 3 satellites at a diminished signal (weather, mountains, tall buildings etc), yes your accuracy will decrease.
As far as the OP goes, 10% is quite a bit... I'd get the speedo/settings checked and trust what the GPS is displaying.
Speedos are generally accurate, but can vary from vehicle to vehicle due to mechanical difference... motorcycles are certainly a good example of this. My bike reads 167 mph when the GPS only reads 150 mph. Given I still had a couple hundred RPM to go in 6th gear and it is mechanically limited to 155 mph, it's only logical the GPS was correct. Granted, the error increases the faster it goes.
Now if your GPS receiver is only tracking 3 satellites at a diminished signal (weather, mountains, tall buildings etc), yes your accuracy will decrease.
As far as the OP goes, 10% is quite a bit... I'd get the speedo/settings checked and trust what the GPS is displaying.
And I hate to disagree with Mike but a GPS is very accurate. We use them to fly bombs in windows.
Last edited by vette dan; Oct 26, 2008 at 05:30 PM.
