Right now I have 255/45/17 tires on my 97 x300 xjr with original 17 inch wheels. Will 255/55/17 tires fit properly? What would be the real difference? Any advice is appreciated. Thank you

 

How much gap do you have between the tire and the fender wheel? You should make sure that the new tires won't rub when going over bumps. I can understand why you would want taller tires. I had a "45" profile tire and i felt everything on the road

 

You'd be increasing the height of the sidewall by over 20%, and that will wreak havoc on your speedometer measurements; use the tire size calculator at Tire size calculator to figure out exactly how bad it'll be. I'm about to mount 225-50/17 snow tires on my R, which will be about the correct height thanks to the reduced width vs. the aspect ratio. That size should be a good fit and would also fit on your stock wheels.

 

it can get very tight.
I have some 18" asteroids on an X300, and 235/50s wouldn't clear the plastic wheel arch inners when the steering was turned.
However 245/45s are perfect on it.
I know this doesn't answer your question w.r.t 17" wheels.
However look up the rolling diameter. In the example above, the 235/50s were about 8 mm more on the radius (ie: 16mm on the overall diameter), and that was too much, even with the narrower width (235 vs 245)

 

I found the following the best online tire size calculator. You can enter your original size, then the proposed sizes and will see the impact.

Tire Size Calculator - Compare Tire Sizes

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This is pretty simple to figure out using basic arithmetic.

Wheel rim diameter is stated in inches.
Tire width is stated in mm.
Tire profile is stated as a percentage of the tire width and provides the height of the side-wall, also in mm.
To calculate tire outside diameter, figure out sidewall height (from width x profile percentage), then double that value and add those millimeters to the rim diameter (converted to mm)

For example:

Sidewall height (255 x 45%) = 114.75
Rim diameter (17 x 25.4) = 431.8
Total (114.75 x 2) + 431.8 = 661.3

So, using this calculation backwards, and knowing that anything other than the same rolling diameter will jack up the odometer/speedo, you can calculate the tire width/profile that will work with your rims to keep that diameter the same. The tire diameter does vary slightly with tread wear and inflation pressure, but significant profile changes will make a BIG difference

So in general, any increase of rim diameter would require a decrease of tire profile to remain close to original diameter, as would any increase of tire width.

Hope this helps.

BTW, changing from 255/45 x 17 tires to 255/55 x 17 tires will increase your rolling diameter by over 2" so choose wisely.