245/45r vs 245/40 on a 17x9
 

Are there any advantages to running a 245/40 over a 245/45?

Im specifically looking at Continental extreme contact DW

This would be for my DD and occasional spirited drive

One advantage is that 245/40s are significantly closer to the OEM tire/wheel overall diameter than the 245/45s.

If your speedo says 75 and you're on 245/45/17s, you're going to actually be going 78 mph but with 245/40/17s, you'll actually be going 75 if the speedo says 75.

Huh, yeah youre right....

I thought that since our stock tire has a sidewall of 45 that getting a 245/45 would the same.. weird. why would increased width affect diameter??

245 is the width of the tread in millimeters

45 means the sidewall height is 45% of the tread width.
Stock tire: 215 * 0.45 = 97MM
Tire 1: 245 * 0.45 = 110MM
Tire 2: 245 * 0.40 = 98MM <-- Winner! Winner!:happyanim:

So the 245/40 is closer to the oem sidewall tire.

^^^^

AHH i see, i always thought the sidewall number was a fixed measurement like mm or something like that.

Thanks for clearing that up

I was thinking about this in the shower just now too..

Generally speaking, the lower profile the tire (shorter the sidewall), the stiffer it will be. It translates into better handling/acceleration/braking usually - so aside from the speedometer accuracy, the 245/40/17s should have a performance benefit over the 245/45/17s as well. The downside is of course decreased ride-quality going over pot holes/bumps due to the stiffer ride.

What are people's thoughts on 17/40/255's?
Compared to 245's?

I'm thinking about going 255's and a little worried about clearance issues... It'll be dropped fairly low on coilovers :)

40's will have less compliance than 45's.
40's (of the same tyre) will have less ride comfort.
40's will have less grip over typical roads.
40's may provide better suspension/arch clearance.

So the bottom line; if the 45 clears the suspension/arches, then it's the better tyre.

Wheel specs? 255 on a 9" is not a prob, just offset matters

I'm running 18x9 wheels with 255/35 ZIIs ... no issues with rubbing. I'm running RCE Tarmac 2 coilovers as well with a 1.5" drop ...

I would think a 17x9 +40 would be fine if you dropped an inch ...

technically anything lower than a 235 you would want a 45 or more sidewall. Anything more than a 245 tire and you would want a 40 or less sidewall.

RPF1's 17x9 +35
17/40/255's all round.
Lowered on coilovers so there is no wheel gap (hopefully), maybe a little tuck if possible