Confused on new 35s and speed difference

jlgaustin

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So I just upgraded my 2019 Rubicon stock 33s and stock rims to 35x12.50 17 and a 17.9 wheel. I used a GPS app on my phone to see how far off the speed difference was. I expected to see some difference but saw nothing. Even at 75mph both my speedometer and the GPS app read identical. I tested at various speeds and always identical.. Never even 1mph difference.

The store 4wheelparts that installed my new tires said they don't have the software or capability to update for the new tire size.

So how can this be possible? Is there a gold standard GPS app on iPhone people are using to measure speed?
 

nerp

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GPS takes just about every receiver radio on your phone (except nearfield) and is quite expensive in terms of battery life. Google Maps, Waze, and Apple Maps all manage GPS polling carefully for this reason. If any of these apps were in the background during your test, the results are guaranteed to be completely unreliable because all maps applications dramatically slow down GPS polling when switched to background (like between once a minute to once every 5 minutes). In the foreground, the speed calculation might be based on a polling speed of once every 30 seconds - which would effectively limit the resolution to determine miles per hour.

You might get a more accurate measurement with using a dedicated GPS provider like Garmin. They usually poll multiple times each second and therefore, the resolution for determining mph is better.

The speed difference in your new setup vs. your old setup is +5%. That means at 100mph, you are actually traveling at 105 mph. 60 is around 63 mph. Your phone may be trying to be smart and rounding down.

Just a thought.
 

flyingmonkey

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I just changed out to 35s from 33 stock on my Rubi and the Waze app was identical to the speedo. I thought I didn’t need to adjust anything. I then downloaded an app called Speedometer and it showed a discrepancy as mentioned above. 55 speedo showed 59 on app. 60 showed 65, etc.

Just got my AEV ProCal Snap in the Mail as I write this. We’ll see how it goes...
 

rutiger

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Using waze on my iPhone I saw the ~5 mph difference when I upgraded to 35’s. Adjusted with tazer and retested with waze and it was pretty much dead on.
 
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