I want to know if I can safely go from a 215/45 to a 245/40, retaining the stock 17X7 wheels. Does anyone know if you can increase your contact patch by 30mm without adversely effecting something else?

 

Can't be done. Meaning that the overall diameter of the tire is going to be a lot bigger than what you had before.

Maybe 225/40/17, but I'm not sure if they make that size.

 

I'm not an expert on tires but why so little sidewall? I think the 17s might look a little funky with such a low profile.

 

no you cant.
Thats to wide of a tire for that rim.

You can go 225/40 maybe even 235/35 but no way 245/40

Just remember if you go one size wider you gotta go one size down on the profile.
I think the max is +2 .

my stock accord rims are 195/65/15....I put 205/60/15 falken tires.

 

ummm....not exact. the overall diameter will be less.
Since he is going wider the profile needs to compensate for the extra width which is why you go one size lower.
If you retained the same profile with wider tire the diameter will still be the same.

225 - width 40- profile 17 - size

 

So you're saying that the overall diameter of a 245/40/17 is going to be smaller than a 215/45/17 tire?

 

40 profile = o
45 profile = O

 

allright people, I guess I misunderstood how tires are measured. I ASSUMED that the profile (second number) was X% of the width (first number). therefore, 215/45, the stock tire, would have a width of 215mm and a height (from the rim) of 96.75mm (45 percent of 215). I haven't actally measured my tire- maybe I should have. UNDER the following assumption, I found a tire with what I thought was approximately the same profile- 245mm width and 98mm height (40 percent of 245).

I don't know much about tires. one site that I read on the internet said that this is more or less how you read the numbers, and a friend told me that in this case, the tire would simply have less of a curve between the contact patch and the sidewall.

I checked and found a wiki article here that explains it the way I thought it was. what am I missing?

 

That is true if the first number (the section width of the tire) is the same.

But it isn't. He wants to increase the width by 30mm and only reduce the aspect ratio by only 5.

 

look, as far as I can tell the actual diameter of the tire should be within 1.25mm of being the same. you wouldn't be able to see a physical difference between the two tires if what I'm reading is correct. The only thing I might be worried about is that tiny difference in tire effecting my odometer over a long period of time, as I doubt it would effect my speedometer significantly.