In my 2009 BMW 128i chronicle I'm pondering increasing the tire width from "stock" 205/50R17 on 7" factory wheels to 225/45R17. Some reading of multiple tire specs at TireRack and reviewing the calculator at https://tiresize.com/comparison both seem to support this (easily).
But... I am also wondering if 235/45R17 would work. They seem to just barely exceed the limit of a 7" wheel (based upon reading the specs of several different summer and all season high performance tires). I swear I have seen tires with a slightly different profile that make wider tire on a narrow rim still feasible.
This is admittedly a bit of analysis paralysis since 10mm more tread on a DD could be a waste of time. So please allow me to waste your time, too and discuss.
Will it fit and not debead? Most likely.
Why though?
For best performance, a 205 goes on an 8 inch wheel. (As found through testing by GRM as well as many others).
Pinched tires look very dated to me (70s or 80s).
I'd either get wider wheels or stick with 205.
In reply to wvumtnbkr :
Interesting. TR specs say 7" wheel is fine for 225 width. I will have to search for this GRM "ideal width" testing.
I like a little bulge for a DD to protect my wheels.
We used to race with a 225 on a 7" wheel and it was fine
OHSCrifle said:
In reply to wvumtnbkr :
Interesting. TR specs say 7" wheel is fine for 225 width. I will have to search for this GRM "ideal with" testing.
I'm sure it's "fine". It will just be a bit squishy and indirect versus a tire that is square or slightly stretched. Probably not as important for a daily driver.
The rule of thumb is 205 on an 8. 225 on a 9. 245 or 255 on a 10.
Lemme see if I can find the article.
https://grassrootsmotorsports.com/articles/wheel-width-is-wider-always-better/
235/60, on a seven inch wheel is no problem. 235/40, not so much.
Here's how 225/50R17 on a 17x7 looks, a slightly squeezed fit. Fingers intentionally in frame to block out sun glare 
In reply to wvumtnbkr :
Thanks for chasing down and posting the link!
wvumtnbkr said:
OHSCrifle said:
In reply to wvumtnbkr :
Interesting. TR specs say 7" wheel is fine for 225 width. I will have to search for this GRM "ideal with" testing.
I'm sure it's "fine". It will just be a bit squishy and indirect versus a tire that is square or slightly stretched. Probably not as important for a daily driver.
The rule of thumb is 205 on an 8. 225 on a 9. 245 or 255 on a 10.
Lemme see if I can find the article.
https://grassrootsmotorsports.com/articles/wheel-width-is-wider-always-better/
Those are the competition setups. For street use, I go an inch narrower on the wheel.
I autocross my civic on 225/50-16 Re71rs tires on 16x7 Enkie rpf1 wheels.
I bought a set of 17x7 rpf1 wheels to improve my tire choices, then ran them last week at the annual GRM Challenge track day with a set of lightly used 225/45-17 Falken RT660 tires (old version, not the plus).
The car has been great at both autocross and at the track day. I see no reason to go down to a 205 for my car.
NickD
MegaDork
4/7/25 11:09 a.m.
For SCCA E/Street, guys cram 225/45R15s on 6"-wide wheels. My daily driver, from the factory, comes with 225/55R17s on 17x6.5s from the factory. I wouldn't sweat it.
Driven5
PowerDork
4/7/25 11:57 a.m.
For a 128i on 17x7's, I see zero reason to go any bigger than the 225/45. The 235's are entirely pointless IMO. They're more expensive, have fewer options, and offer no dynamic advantage on the street.
Adding tire width without increasing the wheel width means that the actual tread width change is less than the section width change, because you're bending the tire edges around (pulling away from the ground) that much more too. That's why they specify a 'measured rim' width with the published dimensions.
