SRT-8 Speedo 2% Slow?

[tbyrne]

I guess this is the only thing about the car that is slow! I had my Garmin portable GPS with me tonight so I figured I'd see how accurate the stock speedo is. I found that mine reads about 2% slow. An indicated 100MPH on the speedo showed 102.3 on the GPS. I later went for a top speed run but I came up on a couple of cars so I shut down at 156. People that have posted going 168 might have actually broken the 170 barrier.

Happy driving!

[330toSRT8]

Originally Posted by tbyrne

I guess this is the only thing about the car that is slow! I had my Garmin portable GPS with me tonight so I figured I'd see how accurate the stock speedo is. I found that mine reads about 2% slow. An indicated 100MPH on the speedo showed 102.3 on the GPS. I later went for a top speed run but I came up on a couple of cars so I shut down at 156. People that have posted going 168 might have actually broken the 170 barrier.

Happy driving!

How can you be sure your portable GPS is giving better accuracy? Most consumer GPS devices are accurate to about 40-50 feet, but I'm not sure what that would mean for the MPH reading. It probably is though.

[GoofyTimL]

I had a similar indication in my 300C with my Magellan Roadmate 700... essentially a constant 2 mph faster than speedo was showing. Didn't know whether to believe it or not. Next road trip I'm going to time between highway markers as a manual check of both speedo and GPS.

[Farns514]

Yes i dont believe the GPS systems are accurate. I was in a mustang using the TomTom GPS and we had that screen you are talking about on. Everytime it did not record the accurate speed. It was very laggy. I was typically 3-8Mph off.

[frank29]

Originally Posted by 330toSRT8

How can you be sure your portable GPS is giving better accuracy?......

I have an observation unrelated (?) to the topic of this thread: I have the REC indash nav unit. Recently I had the misfortune of having to sit in a parking lot for 20 minutes that turned into 90 minutes, ARRRGHHHHH!! Anyway, as I was sitting there, I accessed the screen that shows the number of satelites and the elevation. Over about an hour, the elevation changed over 90'! It wasn't constant in one direction, it went higher and lower at various times.

[JMatt]

The computer has a setting for the 245 tires. It does not have a setting for the optional 255 tires. The actual speed, when using the 255's will be about 1%-2% higher than the speed reported by your speedo. If you set cruise control on a level road over a period of a mile or so, you can verify this.

GPS altitude is not very accurate. GPS speed - at slow speeds and over short distances isn't as precise either. However, at higher speeds and greater distances, the GPS will be incredibly accurate.

(pleae don't make me type another mathematical dissertation to prove all this.)

[330toSRT8]

Originally Posted by JMatt

The computer has a setting for the 245 tires. It does not have a setting for the optional 255 tires. The actual speed, when using the 255's will be about 1%-2% higher than the speed reported by your speedo. If you set cruise control on a level road over a period of a mile or so, you can verify this.

GPS altitude is not very accurate. GPS speed - at slow speeds and over short distances isn't as precise either. However, at higher speeds and greater distances, the GPS will be incredibly accurate.

(pleae don't make me type another mathematical dissertation to prove all this.)

I agree with you, but I would be really impressed if you could write a dissertation factoring in all the variables such as the number of satellites, nadir angle, rotation of the satellites and the earth, etc. just off the top of your head.

[JMatt]

Originally Posted by 330toSRT8

I agree with you, but I would be really impressed if you could write a dissertation factoring in all the variables such as the number of satellites, nadir angle, rotation of the satellites and the earth, etc. just off the top of your head.

I knew someone would ask.....

Imagine a satellite right on the horizon exactly west of your location. Now imagine another directly opposite the first, directly east of your location. You are now in a straight line between these two. The difference in the amount of time it takes for the signal to reach your GPS will tell you exactly how far east/west you are. They can't tell you anything about your altitude, because changes in altitude would affect both satellite signals the same. Further, they can't tell you anything about your latitude (north/south) because any changes would again affect both satellite signals the same.
These two satellites are in the best possible configuration to tell your east/west position. Any thing less than a 180 degree line, and the accuracy drops. (it gets made up somewhat by using multple satellites - more on that in a moment)

Now - do the same drill with a satellite on the horizon directly north and another directly south. This gives the best possible latitude, but can't tell you anything about longitude or altitude. (same reasons as above)

Imagine 10 satellites, all directly on the horizon. No matter what your elevation, the distance to all of the satelites will be constant, thus no data about elevation is possible. You do have VERY accurate readings for lat/lon though.

To get the best possible altitude, one needs a satellite directly overhead, and one directly below - through the earth. Obviously not possible. By the time we can "see" a satellite on the horizon, it has moved from 180 degrees opposed to our perfectly overhead satellite, all the way the horizon - a mere 90 degrees from the overhead satellite. Much of the potential accuracy is already gone.

Just like a bunch of satellites on the horizon, if we had 10 satellites, all 30 degrees up in the sky, we still wouldn't have any altitude data. We have to derive altitude from satellites at diferent degrees above the horizon. Because the earth is in the way, our best possible is 90 degrees, but typically only 60-70 degrees. This compares with lat/lon, where we have a maximum of 180 degrees (perfect) and can usually get a difference of 150 degrees or more.

Phew - that's the simplest version I can draft. Beyond that I have to draw charts. And I ain't gonna!!

[Rambit]

Originally Posted by JMatt

I knew someone would ask.....

Imagine a satelite right on the horizon exactly west of your location. Now imagine another directly opposite the first, directly east of your location. You are now in a straight line between these two. The difference in the amount of time it takes for the signal to reach your GPS will tell you exactly how far east/west you are. They can't tell you anything about your altitude, because changes in altitude would affect both satellite signals the same. Further, they can't tell you anything about your latitude (north/south) because any changes would again affect both satelite signals the same.
These two satelites are in the best possible configuration to tell your east/west position. Any thing less than a 180 degree line, and the accuracy drops. (it gets made up somewhat by using multple satelites - more on that in a moment)

Now - do the same drill with a satelite on the horizon directly north and another directly south. This gives the best possible latitude, but can't tell you anything about longitude or altitude. (same reasons as above)

Imagine 10 satelites, all directly on the horizon. No matter what your elevation, the distance to all of the satelites will be constant, thus no data about elevation is possible. You do have VERY accurate readings for lat/lon though.

To get the best possible altitude, one needs a satelite directly overhead, and one directly below - through the earth. Obviously not possible. By the time we can "see" a satelite on the horizon, it has moved from 180 degrees opposed to our perfectly overhead satelite, all the way the horizon - a mere 90 degrees from the overhead satellite. Much of the potential accuracy is already gone.

Just like a bunch of satelites on the horizon, if we had 10 satelites, all 30 degrees up in the sky, we still wouldn't have any altitude data. We have to derive altitude from satelites at diferent degrees above the horizon. Because the earth is in the way, our best possible is 90 degrees, but typically only 60-70 degrees. This compares with lat/lon, where we have a maximum of 180 degrees (perfect) and can usually get a difference of 150 degrees or more.

Phew - that's the simplest version I can draft. Beyond that I have to draw charts. And I ain't gonna!!

Can you give us a more detailed version? This was way too simple for me! Anyone who didn't understand that is just dumb, like NR for instance. What the hell are you talking about anyway? And here I thought satelites were just for watching TV. Way over my head...hehe.

Rambit

[marnepup]

I had a handheld GPS in my car a few years ago...it was off from my speedo by a good 2-3 MPH...at 60-80mph...that's like 5% difference...I'd have to suspect that the GPS was wrong, not the speedo.