Flashpaq Tire Default Wrong!

[Csteve300]

The Superchips flash paq defaults to a 27" tire diameter, however the stock 22560R18 tires are NOT 27" in diameter! The calculation is as follows: 225x.6=135x2(two sidewalls)= 270 mm plus 18x25.4(number of mm's in an inch)=457.2 457.2+270=727.2/25.4=28.619921 inches! I reset my tire size ti 28.5" the closest I could get without going over! If you doubt the formula check the Tire Rack's website! Sorry if this has already been posted!

[MrHappy]

So are we going faster or slower than our speedo's would indicate? (It's Friday and I don't want to think)

[artichoke]

The speedo will understate your true speed. You are going faster than the speedo indicates, because each revolution of the tire actually carries you farther than the software thinks.

[JimTurner]

Originally Posted by MrHappy

So are we going faster or slower than our speedo's would indicate? (It's Friday and I don't want to think)

The speedo takes the diameter and applies that to revolutions of the tires, etc. For a given tire size you have so many revs/mile.

To make the math simple, say your revs per mile are 500 for tire diameter X.

If you set the speedo to X, the puter will calc based on 500.

If you're tire diameter is actually SMALLER, circumference is smaller too. When the tire turns 500 revs, you will actually cover LESS physical ground. So the distance is shorter, and the speed will be slower. The Speedo will say 65 but the actual may be 63. Your Odometer will behave the same way. If you travel 5000 feet, the Odometer may record that as one mile.

Actual Diameter < Set Diameter = Speedo reporting too high and Odometer reporting mileage too fast.

The inverse is also true (obviously!).
"Honest Officer! I was going 55!"

jimt

[kevenj]

I tuned my 'C' with the flashpaq and I have stock wheels and tires; so I just skipped the tire size part, assuming the default value of 27" was correct.

Last Sunday I had the pleasure of conversing with a nice Idaho State Policeman on the shoulder of U.S. 95. In the course of this conversation he happened to mention to me that his radar showed my speed was 70mph, which for some reason concerned him since the posted speed limit is 60 mph

I actually did have the cruise control set at 70, so the speedo is pretty darn close. I suppose I could have been going a bit faster because as soon as I saw him I hit the brakes and disengaged the cruise so I'm not sure exactly when his radar locked in - - but the speedo isn't far off.

(He ran my plates and upon finding that I was not a fugitive from justice let me go with a warning - - Good thing he hadn't seen me 30 minutes earlier when I was doing 105.......)

[kenm]

Over on the LX Forum SuperChips has recommended not to use formulas for tire measurement. They said to take a board and put it across the highest point of the tire from the ground and measure it. Tire wear and manufacturers don't always make the tires to formulas.

[CRUZR]

Originally Posted by kevenj

I tuned my 'C' with the flashpaq and I have stock wheels and tires; so I just skipped the tire size part, assuming the default value of 27" was correct.

Last Sunday I had the pleasure of conversing with a nice Idaho State Policeman on the shoulder of U.S. 95. In the course of this conversation he happened to mention to me that his radar showed my speed was 70mph, which for some reason concerned him since the posted speed limit is 60 mph

I actually did have the cruise control set at 70, so the speedo is pretty darn close. I suppose I could have been going a bit faster because as soon as I saw him I hit the brakes and disengaged the cruise so I'm not sure exactly when his radar locked in - - but the speedo isn't far off.

(He ran my plates and upon finding that I was not a fugitive from justice let me go with a warning - - Good thing he hadn't seen me 30 minutes earlier when I was doing 105.......)

The default of 27" is not very close. Here is the stock and the optional tire size (per the Tire Rack), for the 300 C, and the closest to 27" size tire. Just for a comparison to show how far off the 27" is.

Stock tires size of 225/60 18:


Sidewall, 5.3”
Radius, 14.3”
Diameter, 28.6”
Revolutions, 704/mile

Stock optional tires size of 225/65 18:


Sidewall, 5.8”
Radius, 14.3”
Diameter, 29.5”
Revolutions, 683/mile

Tires size of 225/55 18:

Sidewall, 4.4”
Radius, 13.4”
Diameter, 26.9”
Revolutions, 751/mile

Hope that helps,
Gary

[Csteve300]

Out of curosity I took a tape measure to a mounted tire and realized that the diameter was in excess of 27", so despite that they say I think the default is wrong for the stock tire and rim. Since I have 10,000 miles on the tires and the are mostly shot(easy driver!) they are less but still in excess of 27". I'm not rreally upset because I love the preformance afforded with the Flashpaq and no problems or error codes!!!

[artichoke]

I wonder what our stock software thinks the diameter is.

[JimTurner]

Originally Posted by artichoke

I wonder what our stock software thinks the diameter is.

I wondered EXACTLY the same thing. You just posted it first!

Anybody?

jimt