Just as a heads up when changing tire size, especially with a 4cylinder engine:
Assuming you have 225/70R15 tires now they have a 27.4" diameter
Other option on 2WD in '99 were 205/75R14 is 26" diameter
Assuming you have 3.73 rear differential gears, it's the most popular.
Look here to see what you have: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
30" tires would change the rear gearing to 3.41
The lower the rear gear number the slower the acceleration, its a loss of torque at low speeds, but MPG at highway speeds would be better, unfortunately the extra weight of the tires is pretty much a break even, so...........just notice less acceleration.
Weight of tires is not just the extra LBS, it requires more torque to get heavier tires to spin, so along with gearing ratio change some torque is lost in getting heavier tires to spin.
If you have 26" tires now then 30" would change gearing to 3.23, a real hit on acceleration.
To gain the acceleration back requires you to change the differential gears.
4.10 would get it back to "as is" assuming 27.4" to start with.
You could also get Limited Slip(L/S), both wheels are powered instead of just one as with OPEN differentials.
Speedometer/odometer is also effected since the electronic system is programmed for the rear gearing and tire size at the factory, Ford Dealer can reprogram it for new tire size.
If you have an automatic trans then speedometer being off will effect when trans shifts, manual off course would be unaffected.
Switching rear gearing would get speedo back to "normal", or close to it.