What are the implications of changing tire sizes???

[clintj]

Edit: Can a mod please move this to the wheel and tire section please?

So I am checking out some used rims with tires for sale in my area and had a few questions. I have found a set of 18"s with 235/40R18 which will give a diameter of 25.40 inches and a set of 17s with 205/40R17 which will give a diameter of 23.45 inches all according to tire rack's website.

Our Corolla S factory tires are 205/55 R16 which gives us this information according to tire rack:
Section Width: 8.07 in 205 mm
Rim Diameter: 16 in 406.4 mm
Rim Width Range: 5.5 - 7.5 in
Overall Diameter: 24.87 in 631.69 mm
Sidewall Height: 4.43 in 112.52 mm
Radius: 12.43 in 315.72 mm
Circumference: 78.13 in 1984.5 mm
Revs per Mile: 836.3

The main thing I am looking at is overall diameter. Which our factory is 24.87".
Here are my questions:
What are the implications of going with a tire size difference of about an inch up or down (gas mileage, etc)?
Can my speedometer be reset for different tire sizes?
If not can I figure the % difference in my speed from the revolutions per mile given by tire rack?

[wylee]

A smaller diameter tire will give you a little shorter final drive ratio putting a little more torque to the ground resulting in quicker acceleration but also a higher engine rpm on the highway...probably reducing gas mileage.

A larger diameter would give you less torque to the ground but lower your engine rpm on the highway...in theory in might increase your gas mileage because the engine is turning fewer revolutions per mile. In practice, the engine is turning fewer revs per mile, but it is also working harder which will likely more than use up the theoretical gains of the taller gearing.

1 inch either way isn't going to be a huge difference in gearing. You can calculate the % of error by using the tire revs per mile but I'd recommend using a GPS nav unit to get the most accurate reading.

The combined weight of the stock wheel/tire compared to the replacement will also be a small factor in acceleration and gas mileage.

[REN69]

http://www.miata.net/garage/tirecalc.html

usually the higher the overall height of the wheel, the more you throw your speedo off.

[RandySolara]

Dont listen to shop, those are headache info and useless, listen to the car n consumer test, they said rim size 17 is for the best speed, its faster and any size of rims.

Dont use 235/40/18 u'll get rub issue, your original factory size is 205/55/16 so i recommend for 225/40/17 if you want ur wheel to look cool n nice, use 225/35/18.

As i said though, rim 17 is the best speed, its up to you bro, Different Tire/Rim shop give you different info and suggestions, they will just say yes it will fit, but when u run over a pothot, you will say F word. I like to answer simple and fast, too much info would make u blind !

Are you using ur STOCK suspension or aftermarket ?