Tire size question

Hi folks, My 86 boxie needs new shoes. The car is bone stock and want to basically keep it that way. I would like to put wider tires on it, but want to keep the stock rims. What is the widest I can go with out swapping rims, or modifying anything. Thanks for the help.

well that depends on if you want to have 4 identical tires or separate front and rear tires.

if you want all 4 identical tires, then you can go as big as 235. maybe 245, but that would be pushing it i think.
if you want separate front and rears, you can put 275+ out back and stick with 235 up front.

the outer diameter needs to stay in the 27.5-28 inch range if you dont want to calibrate your speedometer.
however, i have seen caprices with much bigger rear tires and they look pretty good imo.

With the factory 6" wheels, 225 tops. If you want you can use 7" rims and have up to 265.

Tire Size:
If you do a little checking, you may find a tire in your size range that looks wider than it is. Look for tires in the same size range with a wider tread. (usually a little more expensive because they have a bigger 'foot print.") Find out just how wide your rims are. Are they 6", 6.5" 7" or 8"? Most tire stores have information on how wide the rim needs to be for the tire. Its usually a range where a 235/75R15 will have a rim width spec of between 5.5" and 7" and a 245/70R15 will have a with spec between 6.5 and 8 inches. (these numbers are for illistration purposes only, check with the tire infomation for exact ranges). Take a tape measure with you. Measure the tread width on the tires you are interested in. It is usually narrower on the cheaper tires. The tire has a 'bulge' built into the sidewall and the tread is almost always narrower than the section width (at the "bulge").

A "P" series tire usually has a skinner tread than a "LT" tire in the same size.

A round shouldered tire looks skinny on the rim. a more square shouldered tire looks fatter. Once about 6 years ago I was looking to replace tires on a Big 71 Chrysler. The tires all looked too small for the car. I mentioned it to the salesman and he suggested I go up to a load range "C" tire from the "B" because it would have a little wider tread for the same tire. He got them down and showed me. I measured the tread on the "B" and then on the "C" tire. The tread on the "C" tire was a full inch wider than on the "B" tire (235/75R15). It was enough to change the whole look of the tire on the car without going to a bigger size tire. The stiffer sidewall for the higher weight rating made the car handle better with almost no decrease in ride quality. (it actually rode and steered better, because that car did not have a "radial tuned" suspension, so it was set up to work better with the stiffer tire.

Another thing to consider is how the tire looks on the rim. Right now most after market wheels are 8" to 12" wide verses 5.5 to 8 inches on stockers. The result is the tread width on the installed tires is narrower than the rim width . Example, A tread of only 6" wide on a 8' wide rim. It makes the tires look skinnier. If that same tire is on a 6 or 7 inch rim, it looks fatter. I prefer to to make sure the tread is about the same width as the rim or a bit wider. It makes the side wall bulge out further than the width of the rim. This does 2 things. It gives the illusion of a fatter tire on the car, and it also protects the rims from curb scrapes. (Or the hubcaps.) They will be recessed as little below the section width of the tire and will have a little more protection. The down side is a wider tread may want to follow the groves in the road a little more, maybe slight decrease in fuel economy due to increased rolling resistance of the wider tread. The more square shoulder of the tire may wear a little faster in cornering manuvers.

Don't forget, you have the "plus 0" (zero) option. This is where you go wider with a lower profile. You start with a 225/75R15, you go to a 235/70R 15, or if you have the clearance, a 245/65R15 or 250/60R15 and be close to your current diameter.

There are a lot of tires out there in your size and price range. you just have to take the time to find the 'one' for you.

Ron

With the factory 6" wheels, 225 tops. If you want you can use 7" rims and have up to 265.
hmmm. i thought the stock wheels were 7" wide.
however you are most likely correct.
:banghead:




Don't forget, you have the "plus 0" (zero) option. This is where you go wider with a lower profile. You start with a 225/75R15, you go to a 235/70R 15, or if you have the clearance, a 245/65R15 or 250/60R15 and be close to your current diameter.
of course i was talking about keeping the actual OD of the tires about the same, i just didnt feel like doing the math.
if you do the math, it doesnt usually work out that +1 tread width = -1 profile width.

i have found that +3 tread = -2profile in most cases.

if you are simply going up one tread width, i would say keep your profile the same. if you are going up 2 or more, do the math and figure it out.

Ok I am getting some idea from all the good post here, I am on the fence though with the decision of big in back VS all four wide. I now have the factory rims with very rare SS Super Sport Classic wheel covers (yes, cragar hubcaps) not a scratch on them. Anyone have any pics of an 86 with any of these two combinations, (wide all around . Wide on rear)? Gas miliage is not an issue as the car gets less then 2000 miles a year on it.
Thanks again , Ed.