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Discussion Starter #1 (Edited)
Hi Everyone!
Got a shadow black 2016 Focus S (5spd mtx) that I've been toying with. I've done just about all the "upgrades" I plan on doing to it. RS airbox w/ AEM filter; Mustang TB; MST intake hose; Steeda RMM; Steeda short shift bracket; Cobb shifter cable bushings; Steeda jacking rails; MagnaFlow Cat-Back exhaust.

Last thing on my list was to replace the 15" steelies with something better looking. Original plan was for Konig Hypergrams, but a change in jobs and cut in salary put those on long-term hold. Instead I landed on a set of craigslist 17x7" OEM Focus 5 spoke black/machined alloy rims. They look pretty much brand new and I got em for a song so I'm happy.

I've been toying with the "tire size calculators" online to double check what 17" tire size would be the closest match to the current 15" tires. So as not to throw the speedometer off.
The OEM 15" tire size is 195/65 with an outside diameter of 25.0".
I've found that for 17", the closest tire size is 205/50 with an outside diameter of 25.1".

That's cool. Nearly identical!
However, everything that I've found online says that the OEM tire size for OEM 17" rims is 215/50. Outisde diameter is 25.5". In theory, at 70 mph that'll cause a difference of something like 3mph. So, in theory, I'm better off with the 205/50. I'm just curious as to why, if anyone knows, Ford would upsize their OEM tire size to something that isn't the closest match to other OEM wheel/tire combos. I presume their speedo settings are the same across all Focus models. Am I wrong to presume that ya think? Or are there other forces at play here like sidewall bulge that may play a factor in "reducing" a tire's real-world diameter?
 

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You can go into the BCM using Focccus or Forscan and change the wheel size within the programming. You can select from the available OEM sizes and this will look after the rotational difference in size and speedometer calibrations. I'm not sure why they selected that particular size. Without doing anything it's generally acceptable to go 3% either way.
 

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You are fine either way, get whatever size has the tire you like between those. Its less than ~2% difference, you will not notice it. If you want more grip, the 215 is going to be better choice and is the tire size Ford chose for its 17's. I would go with that and not be concerned at all. When changing sizes, most suggest less than 5% difference and you should be fine.

Most cars arent accurate with their speedo anyways. Get a gps app that shows your speed. Find a straight/flat stretch of road and set your cruise, see what the gps says.
 

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I also have an updated rear motor mount(the ERMM), the snorkel delete, the hi flow air filter, and the 67mm throttle body so our mods are similar.

You seem to want a level of performance for your mods.To follow in that vein, regarding the tires you get for the 17x7 rim, my recommendation is to get a W rated tire. A V rated tire will not have the feedback/grip that the W will have.

And I'd stay with the factory recommendation of 215/50R17. No need to stay perfect on the OD of the tire. The extra grip will be worth it.

When I had my 13 SE and put new tires on it I made the mistake of putting a V rated tire on it. Granted, it was only a mistake to me as I wanted more road feedback. Luckily it was a Continental tire and I figured this out right away and took advantage of their satisfaction guarantee and swapped them for a set of DWS06 in W speed rating which gave the best performance. My son ended up with that car and replaced the DWS06 with another set of the same when the first set wore out. I'm on my second set of DWS06 on my 15Ti for the 18" wheels that came on my car and am just as satisfied with them.
 

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Discussion Starter #5
Hey thanks for your replies guys. Ya I think I was overthinking things a bit when I originally posted. One can lose their mind in tire size deep dives.
I went ahead and bought a set of 205/50 17, Continental Pro Contact Touring today. Largely talked into that size by a friend who obsesses about his own rim/tire combos and "stance" and "flush fitment". Though he drives a Hellcat so his budget is way different. Anyway, they get installed tomorrow. You may have a point about the V vs W rated tire, Mike; but I'm hoping the ones I got are good "all around" tires... and the fact that I'll be running with much less sidewall now will, I imagine, greatly increase roadfeel as it is. I will keep that Continental Tire Guarantee in mind though if I'm not liking the feel of these.

I'll post a pic after they're installed.(y)
 
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