Remember that math class in high school you thought you'd never need, well it's back. Today we decided to see how fat a tire we could fit in the Cambridge. Now you can measure and sketch all you want but I think I have an easier way to do it. I already knew what rim size and off set I want to run based on the test fit of some rollers I had, see above. I ended up getting a single17x7.5" rim off a 08 Crown Vic, right bolt pattern, center bore and offset to fit my Cambridge and center up the rim itself in the wheel well. I did end up having to drill a hole for the aligning pin.

First thing I did was determine the height of my stock tires, staying with this height will keep your speedometer as accurate as it is before the swap. The stock sized tires here are 27.6" tall. I looked at tire offerings in the 17" category I decided on P255/50R17's, which a a bit shorter than stock but the offerings and prices were reasonable. A P245//55R17 would have been perfect height wise, but the offers are thin and expensive. I looked at wider tires than a 255, but the heights were way off and I wanted the same tire at all 4 corners. With the slightly shorter 255's my speedometer will tend to read high, which is preferable to reading low as I don't want a ticket. I went with 17's for a good handling tire, 15's are thin, very thin in the performance category these days.

Since I don't have a bunch of spare tires lying about to test fitment and I would not want to do all those tire changes if I did, I had to make a substitute. Enter the 2x6. First thing I needed to do was measure the width of my rim, outside edge to outside edge. 18.5" in this case. Tire Rack gave me the tire specifications, the height of the tire I chose is 27.1". Subtracting the two gives me 8.59", divide that by two (tire is on top and bottom of the rim so I need half the total). This gives me the height of the 2x6, 4.29"

Tire Rack also gave me the section width of the tire, in this case 10.4", rounded up to 10.5. I cut the 2x6 so that it was 10.5" wide and a hair over 4 and a quarter inches tall. I marked the center line of the 2x6 for the next step.

I used some painter's tape across the rim to mark the center line of the rim.

I aligned the center marks of the rim with the one on the 2x6.

Here you can see how we hold it all together. Once you have it racheted down a bit you can double check the alignment with a tape measure, the 2x6 should hang past the rim equally on each side. Tie up the slack so it is out of the way.

Bolt the rim to the corner you are checking and you can rotate it around looking for tight spots. Tightest spot I had on either rear corner was the front of the wheel house opening, about an inch without any mods. Note that the 2x6 has square corners, most tires do not and in the case of my closest spot the section width is wider that the tread width, combine the two and I have more than ample clearance everywhere. I could go much wider if I wanted. if you wanted to go wider you could measure and cut a new tire substitute and run a new fitment test. I bet I could go two inches wider overall with this rim and maybe three if I got another rim with more offset.

See the video link to watch the fitment test.

Yeah, what about that? Well, I didn't get far there. The 17" rim crashes into the upper control arm before I can even bolt it on. The 15" rollers I test fit on the back will bolt up to the hub fine but on a turn to lock the sidewall rubs the upper control arm outer pivot nut, just in front of the shock in this picture, rubs it real hard and in use would shred the tire real quick, probably first time you turned to lock while moving. The 15" rim is also very close to the outer tie rod, my roller rim has a slight warp and will kiss the tie rod end, if it were true it wouldn't rub. So I did a bit of calculating and I have a cheap Chineseum 1.25" spacer coming to test fit, once I have it and can check clearances I will get a good spacer. This will allow for the same rim and tire combo on all 4 corners.

My cheapo spacer showed up and I did some test fitting. The roller P255/60R15 setup will clear everywhere. Shown is the tightest spot, which is the forward end of the upper control arm upper pivot bolt with the tire turn to right lock, about 3/4" of clearance.

On the 17" rim setup, the rim will clear everything but when I put my tire emulator (sounds more professional than 2x6) on it hits the upper pivot of the spindle. I need another inch of spacer, which is going to get real close to too much on the outside of the wheel well. Oddly enough, even though both the 15 and 17 inch combos I want to use are P255's the 17" tire section width is a half inch wider than the 15" one and that half inch is making itself felt as it is a very tight in there using this wide a tire.

Shown is the stock sized tire setup on the passenger side. On the driver's side we have the P255/60R15 combo. The view from this angle will not change if I go the 17" route.