I went to discount tires and got Conti's DWS 255/55/18s which fit perfectly (I had stock Michelin's 235/55/18) - The tires are great, no problem with fitment, I can feel the car handling way better (no more swings cornering) - I think I lost a few MPGs but too early to tell … I can feel the engine 'grunt' a bit accelerating from start (the tire weighs more ?) … The car looks more like a truck now, more fuller around wheels, I'll post pics (sorry my rear side marker's broken & bumper has scratches)

Any suggestions how to calibrate my speedometer ? at 80 MPH I'm actually doing 82.44

Also, Discount Tires gave me a great deal, $221 per tire - I also bought their lifetime replacement plan for $30 per tire

-Adam

 

your gas mileage could be due to weight and the fact your speedometer is reading slower due to the larger overall size tire...

calibration, definitely dealership material.. or maybe go with the factory tire size?

 

The 255 width won't affect the speedometer, at 55 the stock diameter stays the same.

 

I don't think so.
55% of 255 ain't the same as 55% of 235 (255 and 235 being millimeters, for the records).
Net result is about 3 inches more of circumference with the 255.
You can try this website to check the math:
http://www.csgnetwork.com/tiresizescalc.html

 

as spacecraft mentioned above....

if you want 255 tires.. you want 50 sidewall... 255/50/18 would work..

 

Oh, so very, very wrong. Sidewall width is measured in terms of an "aspect ratio" for the tire as spacecraft has pointed out. It is a percentage value of the tire width. Change the width and you change the height of the sidewall.

Being there are so very few tires, perhaps one or two and very much at a price premium, at 255/50-18 you have no real choice but to go with a 255/55-18. However, this tire size is will definitely throw your odometer and speedometer off by significant values.

 

Here is a link that I've used. It allows you to see differences between stock tire size and what you are proposing plus a crude visual to go with the numbers.
http://www.miata.net/garage/tirecalc.html

 

Actually it should be opposite, 80 on speedometer but 77-78 actual speed. I had same on my 08 rx350 with 235/55/18 till yesterday, I put 235/60/18 and now 80 is 80. 07,08,09 RXs has two sizes 225/65/17 and 235/55/18 so, I assume 55 on 18 giving slower reading but 60, which is not recommended by dealers, I don't know why, giving actual speed (according to my gps)

 

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/60-18 5.6in 14.6in 29.1in 91.4in 693 0.0%
255/55-18 5.5in 14.5in 29.0in 91.2in 694 -0.2%

 

I wouldn't worry 'bout it, just figure on going 2-3 mph slower if your worried ;bout the speed limit.