Im getting 18x7.5 rims in the mail soon and i havent bought tires. Now what size should i get.I currently have 17s with 215/255 tires. I had a friend get bigger rims on his maxima but the tires were smaller then his stock rims and tires so it made his speedometer read wrong because the tires were spinning faster then they would with the bigger tires. so basically i dont want this to happen to me, but i dont want them to be really big so you cant even notice a difference in wheel size.
Im planning to drop the car 1 inch all around with z springs .

so basically whats a good size tire that wont make the rims look small in the wheel well and not make my speedometer read wrong? (if thats even possible in the g).

 

huh?

 

Im not sure if this is right but i know it happend in my friends car. he went from 15" rims (not sure on tire size) to 17 but the whole tire it self was smaller then the stock wheels it came with so his speedometer read wrong by a couple mph. is this even possible to happen to me?

 

Im no engineer, but I don't see how something like that would even be possible just by changing your tire size. I laughed when i first read that just cause it sounded soo silly to have someone believe that...but i dont know too much about cars sooo who knows

 

I believe the stock tire is 215/55-17" which is a diameter of 26.3"

225/45-18 diameter is 25.9"
235/45-18 diameter is 26.3"
245/45-18 diameter is 26.7"

For 7.5" rims I would not go higher than 245 wide, but be sure to check the tire manufacturer's recommended rim size range for each tire you are considering.

Note that although the 235 width is the ideal, they are also hard to find as only one or two manufacturers make this size.

 

The speedo looks at tire rotation and calculates distance travelled based on circumference (diameter) of tire.

In simple terms, if the circumference changes so does the speedo reading as it actually measures the number of tire revolutions/hr.

Also note that the allowable speed error for the ECU is in the order of 3%.

 

Sounds like a bunch of mularky to me

 

im confused now, isnt say... 235= width and 45= the height of the tire from the edge of the rim?

so why would increasing the width of the tire increase the height which is still staying 45?

 

The number 45 is % of width = Aspect Ratio
So for a 225 (mm) tire with a ratio of 45, the tire wall height would be .45 (225mm) = 101.25 mm or approx 4".
So simple math says that overall diameter is 18" + 2(4") = 26".

Sorry to have lost the people who struggle with these complex engineering calculations.

 

Tire sizes are very confusing. The aspect ratio is dependent on the tire width. so a 245/45/18 is taller than a 225/45/18. Also realize that tire width and heights of a 245/45/18 tire will vary between manufacturers.

 

Sorry shodog... I have to disagree...The aspect ratio is NOT dependent on the tire width -- if it were then all 225 tires would be the same ratio. The side-wall height is dependent on the aspect ratio and the tire width.

The height (or tallness or diameter) of the tire will vary between manufacturers but you would only expect to see 0.2 - 0.6" difference between brands.

Once you've decided on a size then you need to consult the manufacturer's specs to determine which tire will give you the handling you desire. Even tires with the same sizing specs can perform differently depending on compounds and sidewall/tread design used in tire manufacture.

There is an excellent link to a site run by tire experts who don't work for a particular tire manufacturer --- I'll see if I can find it back and post later.

You are correct, shodog, in saying that is can be confusing -- at best it is a "guideline"

 

mmmmkk, i kind of get the idea. still a little confusing but im sure the tire guys can help me out with real life examples. so out of those three tire sizes what would u guys recommend?

 

If you are running 215/55R17 tires then the tire that will match in diameter the closest is 235/45R18. Your speedo should read the same at highway speeds.
That is the tire size that comes standard with the 05 sedan 18" sport wheels.

 

as long as you get a tire with the same "rolling circumference", you should be fine. You may see a 1 to 2 mph difference, but not much.

KayGee56, yes it will change your speedo! It's the law of physics, smaller tire turns faster (more turns per mile) for a given road speed (speedo reads fast), larger tire turns slower (fewer turns per mile) for the same road speed (speedo reads slow).

 

I am not sure what your getting at. The Aspect Ratio is An expression representing the height of the sidewall of a tire in terms of a percentage of tire's width.