Wheel and Tire upgrade help

[bluefish7]

Hello,

I have the 255 R19 stock wheels and runflats. As I need new tires I'm looking to do something bigger/wider that would lean more to the mountains than the track, but really mostly urban driving. I have never put on spacers or tried to change wheel/tire sizes so don't want to waste money or time with something that might rub, but I'm willing to pay for a lift or spacers for the right package. Looking to see if there are good posts or recommendations on how to approach this and where to shop for used stuff. Thanks

[JDStrickland]

Using your existing tire size as an example, if it was a 255/55x19, then a 255/50x20 would give you a functionally equivalent tire from the perspective of overall size and circumference. The idea is that for every inch change in the rim size, the aspect ratio needs to change by 5 to keep the same overall diameter.

It appears that you are looking to get more ground clearance, but the problem with doing this through larger tires is that the tires will have an adverse affect on the gearing, and your X5 will turn into a slug.

The numbers, 255/55x19 on a tire are, the 255 number is the tread width in millimeters, 55 is the aspect ratio, a percentage of the tread width, that represents the sidewall height (in this example the sidewall is 255 x .55 = 140.25 mm), and 19 is the rim diameter in inches. Depending on various factors, you could go from a 255 to a 275 and change the aspect ratio to 50, and get a tire that has a similar diameter. If you mounted the 275 to a 20 inch wheel, then your aspect ratio would become 45. I have no clue as to whether your tire plans will rub or not, I'm only trying to help you understand what you are asking. The tire specs might appear as 255/55R19, but the R only means that the tire is a radial ply. There is also a code, 94W for example, that denotes the weight that the tire can carry, and the speed that it can go. You can look up the codes on your tires to see what your tires can carry and how fast you can go. As a practical matter, you won't likely find tires in the size you need that carry less and go slower, but you need to know that the tires you get have the same specifications, or better, as the tires you have.

[bluefish7]

Using your existing tire size as an example, if it was a 255/55x19, then a 255/50x20 would give you a functionally equivalent tire from the perspective of overall size and circumference. The idea is that for every inch change in the rim size, the aspect ratio needs to change by 5 to keep the same overall diameter.

It appears that you are looking to get more ground clearance, but the problem with doing this through larger tires is that the tires will have an adverse affect on the gearing, and your X5 will turn into a slug.

The numbers, 255/55x19 on a tire are, the 255 number is the tread width in millimeters, 55 is the aspect ratio, a percentage of the tread width, that represents the sidewall height (in this example the sidewall is 255 x .55 = 140.25 mm), and 19 is the rim diameter in inches. Depending on various factors, you could go from a 255 to a 275 and change the aspect ratio to 50, and get a tire that has a similar diameter. If you mounted the 275 to a 20 inch wheel, then your aspect ratio would become 45. I have no clue as to whether your tire plans will rub or not, I'm only trying to help you understand what you are asking. The tire specs might appear as 255/55R19, but the R only means that the tire is a radial ply. There is also a code, 94W for example, that denotes the weight that the tire can carry, and the speed that it can go. You can look up the codes on your tires to see what your tires can carry and how fast you can go. As a practical matter, you won't likely find tires in the size you need that carry less and go slower, but you need to know that the tires you get have the same specifications, or better, as the tires you have.

Thanks so going to a 275R19 with maybe just a more of a A/T tread is not good idea for the gearing?

[JDStrickland]

When giving your tire specs, you should give all of the numbers, 275R19 is meaningless. A 275/45x19 is a completely different tire than a 275/70x19, especially from the perspective of the gears.

275/45 has a 123.75 mm. sidewall, the 275/70 has a 192.5 mm sidewall, that's a nearly 2.75 inch difference in sidewall height. The point is, you need to give all of the numbers else there is no answer to your question.

[bluefish7]

The current tires are 255/50R19, so if the car is stock I'm looking to find out what larger size to shop for without adverse effects, if possible to do. Thanks

[JDStrickland]

Okay, so a 255/50x19 (R just means the tire is a radial ply, so saying x19 instead of R19 is the same thing) will be 29.039 inches in diameter. If you wanted to buy new wheels, then the equivalent tire size would be 255/45x20.

You are thinking pf a 275, so to use the same wheels with a 275 tire, you would need a 275/50x19, the diameter would be 29.826 inches, or about 0.75 inches bigger. This would not affect the gearing adversely, and the tires should fit inside of the fenders. My X5 has a 245/65x17, which is 29.539 inches in diameter. Your idea to use a 275/50x19 is 0.25 inches bigger than my tires, I see no reason you cannot do this.

275 is the width. 50 is the aspect ratio, this is the width X the percentage, that equals the sidewall height. If you measure outward from the center of the tire, you measure one sidewall, but in the overall diameter of the tire, there are two sidewalls. The formula is 255 * .50 = 127.5, multiply this by 2 so there are two sidewalls, this gives you 255. This figure is in millimeters, so divide by 25.4 to convert to inches, this gives you 10.039, now add the wheel diameter, to get the total diameter of the tire, 29.039.


(width) x (ratio) = (sidewall) x 2 = (both sidewalls) / 25.4 = (inches) = (rim diameter) = overall tire diameter. You can multiply the diameter by pi, 3.14159, to arrive at the circumference and divide this number into 5,280 to see how many times the tire goes around per mile, when the revolutions per mile go up, the tire turns faster and the speedometer reads fast (it might read 70 when going 65) and when the revolutions per miles goes down then the speedometer might read 65 when going 70. You want any tire you select to be within about 5% larger (slower) than the original tire, this should give you a speedometer reading that is more accurate than the tires you have now. Get it?


You can go to tires.com and they have a tire size calculator where you can enter your tire size then see what tires can be interchanged. Tirerack.com is an online tire store, you can also shop tire sizes at America's Tire at tires.com. Both of these sites have tire calculators. The width of your tires is driven by the width of the rims, but you are going from 255 to 275, this is 20 millimeters wider, that's 10 millimeters off of center, and your factory rims should be able to carry this easily. Don't forget your spare. Since your car is AWD, if you must use the spare then you want it to be the same size as the other tires

[Z3MUSN]

Great info - as I’m shopping around looking to have a more aggressive tires BFG…

[Rich8]

X5 @ Chase cropped.jpg Just FYI, I didn't like the big gap above the tires on my 2002 E53 4.6iS. The car came with 20" wheels, so I put bigger tires on, and then lowered the car with the H&R spring kit. I went to 275/45-20 fronts and 305/40-20 rears which give 29.74" & 29.61" diameters