Speedometer Calibration Question

Down the Tubes · Jan 26, 2021

Hey Everyone!
I replaced my lift and tires and need to calibrate my speedometer.

I got the truck with the old lift and tires installed.
BDS 4" Lift
37x12.5x20 Nitto Trail Grapplers

It now has
Carli Back Country kit
35x12.5x17 Nitto Ridge Grapplers

The place that installed the lift set the tire size at 34.5".
My speedometer says that I am doing about 2 miles more per hour than the radar station at the nearby school.
I am guessing that I need to set the tire size to 34.75 or 35.
I just wanted to make sure that I'm going in the right direction.

I'll post pics in a different thread when I have time, working 7x12's right now and don't have time to think.
It's great for the wallet!

Grand Mesa · Jan 26, 2021

I'm questioning the accuracy of the school radar station. Check it against GPS.

G. Mcpherson · Jan 26, 2021

Put a Level on top of your tire and measure ground up to bottom of level.

muddy12 · Jan 26, 2021

If the speedometer reads faster than actual speed, set calibration to a smaller tire size.
If the speedometer reads slower than actual speed, set calibration to a larger tire size.

Sent from my iPhone using Tapatalk

retired · Jan 26, 2021

set the tire size at 33.75"

UTAHPWRWGN · Jan 26, 2021

Speed limits are only suggestions. Just ignore them.

olyelr · Jan 26, 2021

muddy12 said:
If the speedometer reads faster than actual speed, set calibration to a smaller tire size.
If the speedometer reads slower than actual speed, set calibration to a larger tire size.

Sent from my iPhone using Tapatalk

This^^^^

Sounds like you need to lower the tire size. And also, check it with another gos for good measure, not just the school one. You can download an app for free.

LeesEvoX · Jan 26, 2021

So what is a good app to use for GPS speed?

I ask because i downloaded one from the app store, but checked it against the GPS speed on google maps and it was off by 1-2 mph.

I want to say the google maps GPS would be more accurate, but who knows lol.

Overlander · Jan 26, 2021

I know that Waze is off when connected to CarPlay.

retired · Jan 26, 2021

I have a Garmin gps and also use Ulysse speedometer app and it is spot on. I know from experience with a nitto ridge grappler in 35x12.5-17 that 33.7 will be spot on

Mace Peveler · Jan 26, 2021

I have a free app called "Speedometer". Always seems to be accurate.

Down the Tubes · Jan 27, 2021

Thanks everyone!
I should have thought about the level on the tire.......

I'll finally have a day off this weekend, so I can set the calibration to 33.7.

CheechDogg.0n37s · Jan 27, 2021

I use Ulysse Speedometer and it is dead on ... I drove side by side with my wife and her speedometer and mine were only off by 1 mpg compared to the app and my speedometer ...

You are not going to get your speedometer with the precise speed ever .. you will either be off 1+ or 1- ..hell even 2 mpg is good lol ...

62Blazer · Jan 28, 2021

Two things:
- As mentioned above I would not 100% rely on the traffic sign for setting my speed.
- For the tire height measuring from the ground to the top of the tire will not be accurate in regards to speed readings. You need to measure from the ground to the center of the wheel and then times it by 2. Because of the normal tire "squish" under weight the center of the tire to the ground will always be less than the measurement from the center to the top, and the center to the bottom measurement is what affects the speedo (and torque multiplications/gearing/lever arm or whatever you want to call it).

boblonben · Feb 1, 2021

UTAHPWRWGN said:
Speed limits are only suggestions. Just ignore them.

NOPE, people like you are the problem. Follow the limits set and we will all be safer.

Breaker Alex · Feb 1, 2021

CAG CheechDogg said:
I use Ulysse Speedometer and it is dead on ... I drove side by side with my wife and her speedometer and mine were only off by 1 mpg compared to the app and my speedometer ...

You are not going to get your speedometer with the precise speed ever .. you will either be off 1+ or 1- ..hell even 2 mpg is good lol ...

You know you're talking Hemi's when speed is calculated in mpg :anitoof:

boblonben said:
NOPE, people like you are the problem. Follow the limits set and we will all be safer.

You know that was a joke right?
:chillpill:

retired · Feb 1, 2021

boblonben said:
NOPE, people like you are the problem. Follow the limits set and we will all be safer.

but I don't want to drive 85mph.

UTAHPWRWGN · Feb 1, 2021

boblonben said:
NOPE, people like you are the problem. Follow the limits set and we will all be safer.

Typical 5.7 owner

Chap · Mar 17, 2021

What are you guys using to calibrate your speedo? Tuner? I never bought one for my Power Wagon. My speedometer is 3mph over actual. I had so many issues with the Diablo I had for my 14 Ram 1500 5.7, I didn't wanna deal with that crap again. Had to send my pcm in 3 times because the tuner messed it up.

GTyankee · Mar 17, 2021

When i went to Discount Tire to get a new set of Michelin tires
The manager told me that the new tires would be 3 % larger than the Original tires.
The factory original tires only had about 700 miles on them

There places near me, where someone put signs along the road
The signs are to check your odometer against a 3 mile section of road

Speedometer Calibration Question

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