Will these tires fit on my rim? 99 mustang?
What kind of Mustang is this? Base? GT? Cobra?
When plus-sizing a tire you want to match the speed rating and load index of the tires originally on the car. You also want to maintain as closely as possible the same overall diameter +/-3%.
The Mustang S tire size P205/65R15 92T has an overall diameter of approximately 25.5 inches rotating 815 times per mile. At the recommended inflation pressure of 35 psi the load capacity is 1,400 pounds.
A GT with P225/55R16 94H tires will have an overall diameter of 25.7 inches/811 revs/mile at at the recommended 30 psi the load capacity is 1,290 pounds.
The GT or Cobra 245/45ZR17 (equivilant to 245/45R17 95W) tires has an OD of 25.7 inches/811 revs/mile and at 30 psi of 1,380 pounds.
Your projected 215/35R18 XL 84H tires have an OD of 23.9 inches and rotate 872 times per mile. This is a significantly smaller overall diameter than any of the tires originally fitted to a Mustang and will result in large errors in speedometer and odometer reading, reduced top speed and fuel economy. They also have a lower speed rating and dramatically smaller contact patch (less traction) than the 17" tire so if your car originally had 17" tires I can not recommend them.
In terms of load carrying capacity this size you propse is normally used to upgrade compact cars like Honda Civic's that are 1/3 lighter than your Mustang and originally came with 14 or 15" wheels. Notice the Load Index of 84 is significantly lower than the 92 to 95 of the stock tires. What does that mean? At 30 psi inflation this tire can only carry a load of 849 pounds. Even maxed out at 42 psi the load is an inadequate 1,209 pounds. This is why this tire size is used on riceburners, not muscle cars. No reputable tire shop would mount this size tire on this car.
The correct tire size to use on this car if it originally came with 16 or 17" wheels is a 245/40R18 93W (or 93Y) which has an OD of a perfect 25.7 inches/811 rotations per mile, inflated to 35 psi for a load of 1,389 pounds. Minimum rim width for this size tire is 8.0 inches so 8.5 inches is perfect.
If the car originally had 15" wheels a slightly less radical 235/40R18 XL 95W will do, inflated to 38 psi. Or you could use a 215/45R18 XL 93W at 41 psi, although I would use a 8.0" wheel with a tire this narrow.