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Discussion Starter · #1 ·
GPS is always 2mph higher no matter what the speed.
 

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Hi vidmo. That is pretty normal. The speedometers in modern vehicles are not precision instruments, so the GPS is most likely more accurate.

There is virtually always a margin of error in speedometers, due to manufacturing tolerances in the tires, gearing, etc. Then the constantly changing differences in overall wheel/tire diameters between new tires and older/worn tires, etc. etc. This means the speedometer will almost always be off some small percentage, like yours.

Good luck.
 

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Discussion Starter · #3 ·
I'm wondering if my MKZ cluster is contributing to this. Maybe the individual who programmed it did not put in the correct tire size.
 

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Discussion Starter · #4 ·
I'm wondering if my MKZ cluster is contributing to this. Maybe the individual who programmed it did not put in the correct tire size.
Update: This is not contributing. "there’s nothing in the IPC that designates wheel or tire size, that’s all derived from the BCM" .
 

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Update: This is not contributing. "there’s nothing in the IPC that designates wheel or tire size, that’s all derived from the BCM" .
Hi @Vidmo ,

It might still be though. You might try doing a BCM Learn in the diagnostics menu against your IPC module. I have heard until you perform that the modules will tend to stay with their current values.
 

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What radar detector are you using? The Uniden DFR9/R3 all seem to be 1-2 mph faster than actual. My R7 is closer but not accurate.

Best way to check speed is use a GPS speedo app and set your cruise on a flat road. The smartphone GPS is 1 Hz which isn't ideal for aviation or track use, but it is good enough if the road is flat. I've used a 10 Hz aviation grade GPS and the results were the same as the 1 Hz GPS.
 

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Discussion Starter · #7 ·
Uniden R3, so maybe that's the reason.
 

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Discussion Starter · #8 ·
I appreciate all of the replies and ideas. I've conducted two more tests to confirm what's going on. I used a GPS based phone app along with a couple (yes two, our city likes these things) radar based speed signs. The speedo, GPS app and both of the radar based speed signs all showed the same values. The problem is the Uniden R3 is fast by 2mph. So there is the answer should anyone need this in the future.
 
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I mentioned this on the Uniden forums in one of the radar detector groups and everyone was on my case asking why a radar detector GPS needs to be accurate, or how many angels can you fit on the tip of a needle, etc... Bottom line is that I found the DFR9, R3, and R7 GPS to be woefully off from other GPS's that run on 1 Hz or 10 Hz refresh rate. So the issue is with Uniden and not the other devices. The latest R7 update seemed to make it closer. I've found it to be about 1 mph faster instead of 2 mph faster.
 

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Discussion Starter · #10 ·
To me this is a representation of the quality of the product. If they can't even get the GPS based speed correct, what else are they getting wrong.
 
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Well actually (well achktually LOL) the Uniden R3 and R7 are one of the best detectors on the market right now. The GPS speed might be off, but GPS is otherwise really only used for remembering lock out locations or for automatic lockouts on the R7. The Ka and K range are very good. I can pick up legit Ka band signals up to 2 miles away. K-falses are a problem with any detector though, namely due to the widespread use of blind spot monitoring systems in the K-band spectrum. Stellantis/Jeep is notorious for this thanks to its 24.158-24.160 GHz frequency. My R7 has pretty much silenced the Japanese and Cadillac BSM's but the Jeep's are the worst for triggering 24.158-24.160 especially when they are right next to me for a long drive.

The R3 and R7 have comparable Ka range, but I love the arrows on my R7 with v1.37 firmware (it reacts very quickly now) and the GPS auto lockout works very nicely for the K-band falses from stores/etc...

I ran the DFR9 for awhile and gave it to a friend. I bought a R3 and compared it to the DFR9, and the R3 offers maybe 2-3 seconds faster reaction to Ka band. The R7 is about the same as the R3, but with arrows and a rear antenna horn, and v1.37 has the auto lockouts.
 

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Discussion Starter · #12 ·
I tested the R3 last week on I-80 in Wyoming and the number of false positives (with laser of all things) was a little disconcerting. A lot of K band false positives too, probably due to the same things you mentioned. It did pick up on the one and only alert on Ka when a sheriff nailed a truck that flew past me a few mins earlier. In the end its not worth it to me, there are just not that many wide open roads anymore to even worry about a detector.

NOTE: Yes I used the recommend settings from Vortexradar.
 

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I’ve picked up enough legit K and Ka that makes it worthwhile. We did over 20k miles of cross country driving and it wasn’t that useful but for daily driving in the suburban area they come in handy.

There's a strategy to using a good radar detector. If you're by yourself out in the open, the detector isn't going to help unless the operator keeps the radar blasting. If you're in traffic, bouncing radar signals is what your detector will pick up and can greatly assist. My R3 and R7 will pick up Ka up to 2 miles away in traffic around slight curves, etc... My R3 has falsed on laser a few times and the general consensus is the laser detection is worthless on any detector. The V1 Gen 2 might be OK but laser jammers are pretty much the only useful defense against laser.
 

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Update: This is not contributing. "there’s nothing in the IPC that designates wheel or tire size, that’s all derived from the BCM" .
There is a setting in ForScan "easy mode" for the IPC that affects speedo calibration. Speedometer "Calibration". There is a EU and US setting. For me, EU will always read about 5km/h higher than the GPS speed. US will be within 1 km/h of the GPS speed (I think supposedly to be the accurate reading).
 

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Another thing that indicates a discrepancy - i use cruise control, and the arrow is usually a tiny bit lower on the speedometer. So If i set cruise control to 80km/h, my speedometer shows about 78/79. I am fully stock.
 

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Hi @Vidmo ,

It might still be though. You might try doing a BCM Learn in the diagnostics menu against your IPC module. I have heard until you perform that the modules will tend to stay with their current values.
The tire size is stored in the BCM (Body Control Module). It is measured in millimeters. If you look through all the IPC settings in easy mode or any spreadsheets, you will see there’s no tire size in there. However, if you look at the BCM, regardless of the vehicle, you’ll see settings for the tire size in mm. For example my car is set to 2084mm. Speedometer matches what my phone as well as GPS shows.

Should someone ever change the tire size in the BCM, you will have a DTC in the PCM because you need to do a certain procedure to relearn. Until you do that procedure that DTC will show. It is also important to know that there are small variances in tire size by different manufacturers. There’s a margin of what’s “in spec” and deemed acceptable +/- x % to still fall in that range. Two tires of the same make and model could be slightly different in size.
 
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