Calibrating for new rim size?

[arabbit]

Hey guys, I've got a question. Up until now, I've had the stock rims with 235/50/17 tires on it, and today, I finally had my 18" bullitts mounted with 235/45/18's on the front and 255/45/18's on the back. My question is, even though it was only an inch jump, is there something I need to change in the tune to account for the change in size? (aka to make sure my speedometer is reading the correct mph?) I have an SCT X3, is this a setting I can change?

thanks

 

[JEWC_Motorsports]

Your new tires have 746 revs/mile. Yes you can change it with your tuner. Its not the new rim size your changing, its your tire size.

 

[Swarzkopf]

Exactly. You can usually look up the revs/mile for any specific tire by visiting the tire manufacturers web site and looking up the specs for the specific model/size you're running.

 

[JeremyH]

The speedo goes off tire diameter of the rear wheels. So when change the overall height of the tire your speedo will be off. You have to calculate revolutions per mile based on the diameter of the tire and adust it acordingly with your handheld.

Note, it is possible to go to a bigger rim with a lower profile tire and still have the same overall diameter as before, in which case no adjsutments are needed.

 

[stkjock]

 

[gil_t2]

The 255/45's are only .3" shorter than stock tires. No adjustment should be needed 764 vs 772 RPMile

 

[BruceH]

There is only one reliable way to get accurate revs. Measure your tire mounted on your rim and car. Take tire height x 3.142 and divide 63360 by the result. This will give you your revs per mile. Tire manufacturers data is usually off since they aren't getting it from your car and rim combo.

 

[arabbit]

yeah i did do a search, all i got was crap about people wanting to show off their new rims. but the info in the replies was super helpful guys, thanks!

 

[b's stang]

Bringing an old thread back to life. I just put sve drifts on my car 18x9 18x10 tire size is 255/45/18 and 285/40/18. Help would be much appreciated. Thanks

Edit:
I did the formula Bruceh had put on here and I got 1,120.305537873966
Does that seem right?

 

[AnotherS197GT]

Bringing an old thread back to life. I just put sve drifts on my car 18x9 18x10 tire size is 255/45/18 and 285/40/18. Help would be much appreciated. Thanks

Edit:
I did the formula Bruceh had put on here and I got 1,120.305537873966
Does that seem right?

Measure the wheel and tire combo, not just the rim.

 

[BruceH]

Bringing an old thread back to life. I just put sve drifts on my car 18x9 18x10 tire size is 255/45/18 and 285/40/18. Help would be much appreciated. Thanks

Edit:
I did the formula Bruceh had put on here and I got 1,120.305537873966
Does that seem right?

How tall is the tire when mounted on the rim and on your car?

 

[b's stang]

And how do you do that?

 

[AnotherS197GT]

They make a strip of metal for gauging the size of something. Its usually bound up in a metal or plastic case. They sell them at Home Depot and other hardware stores. Its called a tape measure.

 

[BruceH]

And how do you do that?

Are you kidding? Put something with an edge on to on top of the tire. It needs to stick out far enough to get a measurement. A level would work. Then measure to the ground.

 

[b's stang]

They make a strip of metal for gauging the size of something. Its usually bound up in a metal or plastic case. They sell them at Home Depot and other hardware stores. Its called a tape measure.

Thanks bro your my inspiration. I didn't ask for a smart ass remark. A simple answer would be sufficient.

 

[Fox-4]

Thanks bro your my inspiration. I didn't ask for a smart ass remark. A simple answer would be sufficient.

Common sense and a bit of Intellect would have figured it out.

Anyways, you can use this Calculator as a guide too.
http://tire-size-conversion.com/tire-size-calculator/

 

[BruceH]

Thanks bro your my inspiration. I didn't ask for a smart ass remark. A simple answer would be sufficient.

Members aren't going to spoon feed you more than a few times before giving you a hard time.

Did you measure the tire height?

 

[b's stang]

In all honesty the only reason why I asked was so I could get an answer due to it already being dark where I live so I wouldn't have to mess around with anything. I thought there might be someone on here with nothing else better to do than help someone else out with a problem, but thanks for the input.

 

[b's stang]

I got 748. Does that sound about right? Please no smart remarks. I have had long day.

 

[BruceH]

I got 748. Does that sound about right? Please no smart remarks. I have had long day.

It sounds pretty close to me. It's going to vary depending on your rim size, brand of tire, inflation, weight of the car, etc. Once you change the tune check it on the interstate with the odometer and mile markers. Try for 10 miles or so.

I'm guessing they measured about 27"?