Another Tire question for a newby

Eck · 09-15-2007, 11:06 AM

I need to get new tires for my 78 SC, right now it has 205-55-16 on the front and 225-50-16 on the rear. Although in reality it isn't but due to the sidewall heights it looks like the front tire is bigger than the back from the side. Any one else experienced this? I need to know if there is a reason the PO put this size of tire on, or if I can go with different sizes and if so what would anyone recommend? Is there an ideal size?

I tried to do a search but was a bit overwhelmed

so I am sorry if this has been answered an million times.

Kirk

Zeke · 09-15-2007, 11:21 AM

You don't mention the brand. If you go to tirerack.com , you can get the data on most tires including height. Also, your rears may just be so worn that they look shorter.

mkossler · 09-15-2007, 11:24 AM

The sidewall heights should close - within .25 millimeters. To break it down:

205/55r16 Indicates that the tire is 205mm wide, and the aspect ratio of the sidewall height to Tire width is 55%. The R16 indicates that the tire is of radial design for a 16" wheel.

225/50R16 similarly indicates that the tire is 225mm wide, but that the aspect ratio is 50%.

We can easily do the math to calculate the height of the sidewall, and of the tire overall:

205 * .55 = 112.75 millimeters

225 * .50 = 112.5 millimeters

This means that the overall height of the fronts should be larger than the rears by half a millimeter.

([2*112.75] + [16 * 25.4]) - ([2*112.5] + [16 * 25.4]).

You have good eyes!

mkossler · 09-15-2007, 11:28 AM

Of course different brands are made to varying tolerances, so the difference may be more or less than the nominal value. After having handled literally tens or hundreds of thousands of tires, I can say that the variances can be dramatic. The primary variance seems to be width/footprint followed by sidewall stiffness.

Eck · 09-15-2007, 11:49 AM

There are two different brands, the fronts are Bridgstone Potenza RE71 and the back BF Goodrich Comp T/A, all four tires are shot so I am going to replace all four with the same brand, just wasn't sure if I should keep the same size as I have now or try and up the size of the back, hate to lower the size of the front as I think they wouldn't visually look good, they might look a little small in the tire well. Just an FYI I work for an art museum so I am probably too visually inclined!

Thanks for the help.

Kirk

cattledog · cattledog's Garage

On factory 6" fronts and 7" rears, these are the correct sizes. Your car may still have a sticker in the drivers door sill that indicates this.

Zeke · 09-15-2007, 01:51 PM

Quote:

Originally Posted by cattledog

On factory 6" fronts and 7" rears, these are the correct sizes. Your car may still have a sticker in the drivers door sill that indicates this.

And, if you buy all four the same brand, they should relate properly to each other as kossler says.