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Before posting this I waited to see if the dealer could correct...they can't

*My SKY's Speedometer needle shows I'm going SLOWER than I actually am. Oooppps... that will really increase my chances of a ticket.

Testing multiple times, using 2 different GPS on flat interstate:
Cruise control set so the Speedometer Needle is set EXACTLY on 70MPH, the GPS shows I'm actually going 72.7mph. That's almost 3 mph faster than it shows. At lower speeds the difference is not as drastic. Only about 1.5 mph over at 35mph.

Neat test: Set your cruise on an exact speed on a flat road, then RESET your Average Trip Speed. The mph it shows is the speed the car thinks it's going. With my speedometer at 70, my Average Speed shows 71.

Just passing it on for those pushing the limits...you may be further over the limit then you think.
 

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Sunny Ragtop said:
Before posting this I waited to see if the dealer could correct...they can't

*My SKY's Speedometer needle shows I'm going SLOWER than I actually am. Oooppps... that will really increase my chances of a ticket.

Testing multiple times, using 2 different GPS on flat interstate:
Cruise control set so the Speedometer Needle is set EXACTLY on 70MPH, the GPS shows I'm actually going 72.7mph. That's almost 3 mph faster than it shows. At lower speeds the difference is not as drastic. Only about 1.5 mph over at 35mph.

Neat test: Set your cruise on an exact speed on a flat road, then RESET your Average Trip Speed. The mph it shows is the speed the car thinks it's going. With my speedometer at 70, my Average Speed shows 71.

Just passing it on for those pushing the limits...you may be further over the limit then you think.
In the long run you will actually have fewer miles on the odometer!
 

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Sunny Ragtop said:
.....*My SKY's Speedometer needle shows I'm going SLOWER than I actually am. Oooppps... that will really increase my chances of a ticket.....Cruise control set so the Speedometer Needle is set EXACTLY on 70MPH, the GPS shows I'm actually going 72.7mph.......
Sunny,

Run down the profile of your rear wheels. With every percent of diameter you loose on the wheels, the speedo will show one percent faster :thumbs: and you'll a lot of fun :D
 

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Think of it this way...you have a defense to a speeding ticket.....sort of.....:D
 

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have the dealership hook up the scan tool so they can compaire the cluster with what the spped sensor is actually reading. That way they know if everything is ok. How accurate are those GPS systems by the way?
 

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Just for everyone's edification, your speedometer should be more accurate than GPS at measuring your speed. The location error for commercial GPS is about 30m or 98ft. Even if you got the error down to 10m, thats 32.8 ft. The difference in location over one minute between 70 mph and 72.7 mph is only 4.3 ft. That is much smaller that the margin of error expected with the GPS system.

I also bet it would be difficult to tell if you set your cruise at 70 or 70.5 mph. 70.5 mph would most likely be rounded up to 71 by the comp.

I'd say its a wash.
 

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This is getting technical..... :D
 

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SKYtime said:
Think of it this way...you have a defense to a speeding ticket.....sort of.....:D
No, there is some truth to that statement. Many courts will accept proof that your speedometer was miscalibrated (can be gotten from some repair shops and oil change locations) and if its upwards of 3 or 5 miles you have a pretty decent defense even if you were going 15 or so over.
 

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AUsomTiger said:
Just for everyone's edification, your speedometer should be more accurate than GPS at measuring your speed.
I wonder if it would be more accurate to get on an Interstate and note the mile markers. If you use a separate stopwatch to start when you cross a mile marker and stop when you cross the next mile marker, you can calculate your speed. Then, you can compare it to what you think you've set for the cruise control.

(Doing this was my dad's favorite road game on family vacations when I was growing up!)

- Jeff
 

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SKYtime said:
This is getting technical..... :D
'Scuse me if I doze off here :lol:
 

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SkyTech said:
have the dealership hook up the scan tool so they can compaire the cluster with what the spped sensor is actually reading. That way they know if everything is ok. How accurate are those GPS systems by the way?
GPS systems are not that accurate for speed calculations as they are triangulating against satelite signals and thay come down at different speeds. I have found the the GPS in the various cars we have had has been off by up to 9km/hr. Depends on teh quality of the GPS and how many satelites you are connected to also.
I wouldn't take the GPS speed calc as gospel.
Cheers
OAKVILLESKY
 

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Try Mile Markers

When I used to travel a lot, I would go to an interstate and start out at mile marker crusing at 60 MPH. At the first mile marker, check your watch second hand and the cruise set as acurately as possible on 60 miles per hour, it should take exactly 5 minutes to the second to reach the 5th mile marker. Repeat several times. If you reached the 5 mile marker sooner than 5 minutes your speedometer was reading is too slow, whereas if you reached it later, just the reverse. If you are good at math, you can figure precisely, or do what I did. Keep adjusting the speed until the five miles marker and the speedometer were in agreement.

I do some geocashing and I agree that GPS's are prone to up to 60 feet difference in the aggregate.

I think I got that right....I'm getting too damn old!
 

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I just went thought a ' Flashing speed sign ' at 35 mph , and the sign read 35.
not very scientific, trying to stay at 35 while looking at the road, but I think at 35 its pretty close to being accurate.
 

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AUsomTiger said:
I also bet it would be difficult to tell if you set your cruise at 70 or 70.5 mph. 70.5 mph would most likely be rounded up to 71 by the comp.
Set your cruise control then switch your DIC over to AVG SPEED. Reset it. you'll get your current MPH pretty dang close so long as you are on a perfectly flat stretch of road. If it is hilly there will be slight variances as the CC compensates for the elevation changes to maintain that speed.
 

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skyfan said:
When I used to travel a lot, I would go to an interstate and start out at mile marker crusing at 60 MPH. At the first mile marker, check your watch second hand and the cruise set as acurately as possible on 60 miles per hour, it should take exactly 5 minutes to the second to reach the 5th mile marker. Repeat several times. If you reached the 5 mile marker sooner than 5 minutes your speedometer was reading is too slow, whereas if you reached it later, just the reverse. If you are good at math, you can figure precisely, or do what I did. Keep adjusting the speed until the five miles marker and the speedometer were in agreement.

I do some geocashing and I agree that GPS's are prone to up to 60 feet difference in the aggregate.

I think I got that right....I'm getting too damn old!
Unfortunately, the mile markers may not be exact. :D

I used to work for the highway department here in Texas and would replace signs when they were knocked down by drivers (and other things) and we would find the softest ground to put the signs back up...sometimes several yards away from the previous site. So...add up several changes of several signs over a period of time and you won't get exactly one mile between signs. :eek: But I'm sure it's still pretty close.
 

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% error people, % error. Not every device is going to be dead one, they are designed to meet specifications within a % error margin. Some cars will report higher some will report lower.

Also did you ensure your car had 29 PSI cold pressure prior to doing your testing, gotta eliminate any external sources of error first?

I'm shocked no one has complained about the speedo being dampened yet...
 
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