Dodge Charger Forum banner

1 - 19 of 19 Posts

·
Registered
Joined
·
174 Posts
Discussion Starter #1
So I'm wanting to go from a factory 275/40zr20 to an 285/35zr20
If I do that will I have to have my speedo recalibrated?
 

·
Registered
Joined
·
175 Posts
Yeah I think with a smaller sidewall the total circumference changes and can throw off the speedometer. I don’t know by how much and if it would be worth it. Smarter folks than me will surely chime in.

If you could find the tires with a width you like with the same sidewall you wouldn’t have to worry at all.
 

·
Registered
Joined
·
2,327 Posts
will I have to have my speedo recalibrated?
The difference on the speedometer between these two tires sizes is 2.8%
You can recalibrate the speedo if you want to eliminate the 2.8% error. At slower speeds (e.g. 35 MPH) the speedo difference is 1 MPH but at higher speeds (e.g. 105 MPH) the speedo difference is 3 MPH.

Here's a online calculator for estimating speedometer readings based on differences in tire sizes.

https://www.blocklayer.com/tire-size-calculator.aspx
.
 

·
Registered
Joined
·
175 Posts
I'd like to find someone that could tell the difference in wear on 2.8%.
c'mon, really?
2.8% is the difference on the speedometer. I was just saying if it turns over more times per mile it’ll wear out faster, I can’t imagine actually noticing the difference.

Edit: Maybe Rain Man driving slow in the driveway could tell. And you’re the one that posted the chart! Did you even look at it first!?
 

·
Registered
Joined
·
2,327 Posts
How would I go about recalibrating the speedometer?
You need to update the settings in the PCM (powertrain control module). The PCM monitors the vehicle speed sensor to determine the vehicle road speed. The PCM sends the speed information to the BCM (body control module) and the BCM sends a speed information message to the Instrument Cluster (IC). The IC then repositions the speedometer pointer accordingly.
To update the PCM you need a tuning device like the DiabloSport. The DiabloSport has a 'Modify Parameters' menu that lets you change the tire size settings in the PCM so it will make the correct calculations for the speedometer.
The PCM in the 2015+ models is encrypted (software locked). A 2019 model will need a unlocked PCM, plus the tuner, in order to reflash it or change any of it's parameters/settings. The unlocked PCM plus the tuning device make it an expensive proposition for only recalibrating the speedometer. You can contact the dealership to see what they charge to recalibrate a speedometer and compare this with the cost of doing it yourself. If you're going to change other parameters in the PCM (e.g. disable MDS, install a custom tune, etc...) then the cost of the PCM+tuner may be worth it ... not so much if you're only going to recalibrate the speedometer.

https://www.diablosport.com/
.
 

·
Registered
Joined
·
77 Posts
312/35/20 is the best way to size up from 275/40/20.

Tire Size 275/40/R20 315/35/R20 Difference
Diameter 28.7″ 28.7″ 0
Width 10.8″ 12.4″ +14.8%
Sidewall 4.3″ 4.3″ 0
Circum 90″ 90.1″ +0.1%
Revs/Mile 703.7 703.2 -0.1%
 

·
Registered
Joined
·
13 Posts
312/35/20 is the best way to size up from 275/40/20.

Tire Size 275/40/R20 315/35/R20 Difference
Diameter 28.7″ 28.7″ 0
Width 10.8″ 12.4″ +14.8%
Sidewall 4.3″ 4.3″ 0
Circum 90″ 90.1″ +0.1%
Revs/Mile 703.7 703.2 -0.1%
Here's the visual:
upload_2019-10-5_10-53-31.png

upload_2019-10-5_10-54-0.png
 

·
Registered
Joined
·
77 Posts
up until today I ran Nitto 555's. Just had Atturo AZ850 installed 315/35/20 for the rear and 275/40/20 for the front. 10.5" rear 9" front width.
 
1 - 19 of 19 Posts
Top