Wheel and Tire Sizing

Discussion in 'Gen 1 & 2 - Suspension, Brakes & Body' started by 91 Vegas SHO, Aug 10, 2019.

  1. 91 Vegas SHO

    91 Vegas SHO New Member

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    Hoping to get some help here. Looking at wheel and tire packages for my 91 SHO and am wondering what sizes everyone is running including specifics like offset and wheel width?? Considering 17 x 7.5 with a 40mm offset and a 225/50 17 tire. I’ve read all the other threads here and have used online calculators to see where this set up will move my offset etc. and basically it’ll be very close. Anyone running something the same or larger without fitment or rubbing issues? Any pics or intel is greatly appreciated!!!
     
  2. gamefanatic

    gamefanatic SHO Member

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    I've got a Gen3 with a 17x7.5 rims, my offset was 35mm, and had to use a 3/16" spacer to get good clearance from the suspension on my 235x55 tires. Setups should match, so your 40mm offset would match my end result, and should fit perfectly with room.
     
  3. luigisho

    luigisho SHO Member

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    You want to test fit. The rear wheel clearance on GenI sho's is tighter than the other model years.

    Hit up forum member Irish Pride. He has/had a bunch of GenI (89-91) SHOs and different wheels
     
  4. NoSlo

    NoSlo GoldMember

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    Also, the problems with additional tread width or diameter may vary from brand-to-brand of tire, and may manifest only when you are going over speed bumps or taking hard corners, or when you go to put on snow chains, so anecdote of an oversize working for someone doesn't mean you'll be without problems. Different alignment or good springs means you can hit the fender earlier, and the strut's spring cup and anti-sway link bolt also vary.

    There's a tire size calculator on miata.net, where you can see how much faster (diameter) a rim upsize or width x sidewall % will be.
     
  5. blk\blk90

    blk\blk90 SHO Member Supporting Member

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    I have 18x7.5 with 225/40 on there and it clears fine. Speedometer is off, but I plan on getting 235/45 to get closer to factory size.
     
  6. PaulTAutoX

    PaulTAutoX Member

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    A 225/50-17 is a pretty tall tire. For instance, using Tire Rack's info, with a Michelin Pilot Sport A/S3+, it's 25.9 inches tall. The OEM SHO tire, I think was a 215/60-16, someone may correct me here, was about 0.2" taller. So you won't have a problem fitting /under/ the springs, it will just be width. On a 7.5" wheel, the rim space is just a hair wider than the tire, so the tire will be a little wider at maximum section than if you had it on a 7" and the tire will be closer to the outer fender lip, and your offset moves it in that direction too.
    FWIW: I have a Gen II. Have only had interference issues in the rear. When I put on one set of new brake rotors years back, I think they were aftermarket, I started getting rear fender interference on my 225/55-16's on 7.5" wheels that had a wider offset. The hats on the rotors were thicker, it was like adding a spacer. It only showed up with a rear seat/trunk load.
    Later I trimmed those fender lips back to the middle of the spot welds and later rolled those lips up a little with the baseball bat/Mag light method.
    Anyway now I have 17x8" / 38mm offset, have used 235/40-17 and 235/45-17's with no interference, but do have lowering springs, and coil-overs/camber plates in the front. I was autocrossing the car and wanted the shorter tires for better gearing/cornering response anyway.

    Have been told that 245/45-17 will fit the car on these wheels, it will depend on what is available in which tire when I next get new ones. So my guess is that your 225/50-17 on 40mm will fit OK. (It's really annoying that the top tires are often only available now in 17, or even only 18+, but that is another rant.)

    Yeah, even at a given size the tread width and tire section width can vary between tire models and brands up to several millimeters. Even before you take wider wheels into account. I have paid re-stocking fees for wheels/tires that did not fit (on a different car); the general view is once they are mounted they are yours and you'll have to reimburse them % for wheels that have been mounted, even if the only difference is that the paint at the bottom of the hole is scratched from bolting it up. If you have a relationship with an installer, you could order 1 of tire+wheel and put it on the corners (usually rears in my experience) while on the lift and then put it on the ground, inspecting the clearances, and then buy the rest if you are satisfied. Less re-stocking fee that way.
     
    DJSHO91 likes this.

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