Sponsored

Confused on new 35s and speed difference

jlgaustin

Member
Joined
May 28, 2019
Threads
6
Messages
12
Reaction score
4
Location
Austin, Texas
Vehicle(s)
2019 Jeep Rubicon
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?
Sponsored

 

nerp

Well-Known Member
First Name
Terry
Joined
Jan 10, 2018
Threads
16
Messages
121
Reaction score
97
Location
Denver, Colorado
Website
interface.ninja
Vehicle(s)
2018 Jeep Wrangler JL
Occupation
Software Architect / Security Engineer
Vehicle Showcase
1
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

Well-Known Member
First Name
Max
Joined
Mar 14, 2019
Threads
5
Messages
87
Reaction score
107
Location
New Hampshire
Vehicle(s)
2019 JLUR (SOLD), RAM 1500 LARAMIE
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

Well-Known Member
First Name
ryan
Joined
Apr 2, 2019
Threads
0
Messages
45
Reaction score
173
Location
seattle
Vehicle(s)
2019 rubicon, others
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.
Sponsored

 
 



Top