How fast should you be able to go?

Hi, I just looked up this thread because I was wondering what the prior owner could have done to my daughter's jeep. So there is supposed to be a speed limiter? In the cold desert, as I was driving it back to Pennsylvania, it easily goes 100. My daughter admitted running up to 113 MPH (GPS reading) on level ground. It has JL wheels and 31s, but that's about all I can see other than a very expensive AEM intake that everyone says does nothing. 1997 4.0 Sport with all new OEM suspension.
 
TominAZ said:
Hi, I just looked up this thread because I was wondering what the prior owner could have done to my daughter's jeep. So there is supposed to be a speed limiter? In the cold desert, as I was driving it back to Pennsylvania, it easily goes 100. My daughter admitted running up to 113 MPH (GPS reading) on level ground. It has JL wheels and 31s, but that's about all I can see other than a very expensive AEM intake that everyone says does nothing. 1997 4.0 Sport with all new OEM suspension.
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What did the speedometer on the gauge cluster say?

The rpm limiter is at 5200rpm. The speed limiter is said to be 93mph as read by the computer.

Tire size, axle gearing, and speedo calibration are the reasons why the speedometer might not match GPS.
 
I had a shop change the little plastic gear in the transmission so the speedo would match the GPS. It reads accurately to a phone GPS, but I know I've never taken the RPMs over 5000. It was always reading low before.
 
Well, I didn't know you had to do that on a Galaxy S10e. Perhaps the reading needs to be verified. I was thinking one of those engine tunes might have been used to remove the limiter. I have a banks automind that works good on my Silverado but not on a jeep. I would put the limiter back if that was the case.
 
TominAZ said:
Well, I didn't know you had to do that on a Galaxy S10e. Perhaps the reading needs to be verified. I was thinking one of those engine tunes might have been used to remove the limiter. I have a banks automind that works good on my Silverado but not on a jeep. I would put the limiter back if that was the case.
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Did the speedometer match GPS at 100mph?
 
Yes, it did as far as I could tell. I stopped looking at 95. With that tight steering box, one twitch and its over. I don't drive that fast.
 
I tested mine up a bit over 75 today after my skid install. It’s is one hand on the wheel, but I would not want to be part of any evasive maneuvers.
 
GPS speeds are almost exact, the error is in fractions of a mile per hour. You can't calculate position unless you know the speed to many decimal points. GPS receivers output speed long before calculating position, you can always assume if you have any sort of position lock that the GPS speed is accurate. Sometimes handheld devices jump around a bit because well, your hand jumps around and they don't really average the readings in software. You don't need to calibrate GPS speed.
 
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pagrey said:
GPS speeds are almost exact, the error is in fractions of a mile per hour. You can't calculate position unless you know the speed to many decimal points. GPS receivers output speed long before calculating position, you can always assume if you have any sort of position lock that the GPS speed is accurate. Sometimes handheld devices jump around a bit because well, your hand jumps around and they don't really average the readings in software. You don't need to calibrate GPS speed.
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In the last 20 years that I have had a telephone or a navigation unit with GPS, I can't remember a time where it wasn't closely matched with the speedometer. Nor have I ever had to calibrate one.

The only times I have seen small discrepancies are on tight twisty roads.
 
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jjvw said:
In the last 20 years that I have had a telephone or a navigation unit with GPS, I can't remember a time where it wasn't closely matched with the speedometer. Nor have I ever had to calibrate one.

The only times I have seen small discrepancies are on tight twisty roads.
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I calibrate my GPS with those highway construction radar signs. You know the ones: "Speed Limit 25mph, Your Speed 65".
Yup, that's what the GPS says, I'm good.
 
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pagrey said:
GPS speeds are almost exact, the error is in fractions of a mile per hour. You can't calculate position unless you know the speed to many decimal points. GPS receivers output speed long before calculating position, you can always assume if you have any sort of position lock that the GPS speed is accurate. Sometimes handheld devices jump around a bit because well, your hand jumps around and they don't really average the readings in software. You don't need to calibrate GPS speed.
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This is correct. I'm amazed at the number of speedometers that read slow - based on my limited, yet still fairly substantial sample size; I'm of the belief that most speedos are off. The Mercedes was the only one I'd ever seen that agreed with the GPS - until I got the Jeep. Much to my amazement, its dead nuts on as well. Every other car I've verified the speedo against a GPS has read slow on the speedo.
 
Zorba said:
This is correct. I'm amazed at the number of speedometers that read slow - based on my limited, yet still fairly substantial sample size; I'm of the belief that most speedos are off. The Mercedes was the only one I'd ever seen that agreed with the GPS - until I got the Jeep. Much to my amazement, its dead nuts on as well. Every other car I've verified the speedo against a GPS has read slow on the speedo.
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The speed shown on the needle vs the speed read by OBDII are different numbers.
 
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jjvw said:
The speed shown on the needle vs the speed read by OBDII are different numbers.
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Not applicable to the MBZ - but I didn't know that. That's pretty stupid, I'd expect the needle to read correctly on any/all vehicle(s). Its not rocket science - but I've known some electronic dashes to not even be able to keep the correct time! *shrug*
 
You can do a cluster test and it'll show how far off the needle on the cluster is. I prefer digital displays for speed. You don't need to see it move like engine speed. I've owned a few with digital readouts, if the vehicle has a computer with the information already then I'd rather it just show me the number.
 
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