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I ask you, if this truck is being custom built to my specs, why can't i have these rims and tires?
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Different trim levels have different options.
You ordered XL trim level, so the only wheels available are the standard 17" steel wheels.
Move up to the XLT trim level, and you can have 18" alloy wheels, and if your XLT is a 4x4 SuperCab or CrewCab, you can have 20" alloy wheels.
The FX4 trim level comes with 18" steel wheels, and 20" alloy wheels are optional.
The Lariat trim level comes with 18" alloy wheels, and the 20" jobbies are optional on 4x4s.
If you haven't already done it, download the 2008 F-250 Order guide from www.sunrisefordfleet.com
The Order Guide has all the ifs, ands and buts of what is optional and what each option requires.
As to the "Why", it's simply cost. Every option Ford offers costs them money in assembly line complexity, and publishing what's required with what, and in maintaining the automated databases used by dealers, customers, and others such as the car-price websites. Not many folks that want a "work truck" with XL trim will be wanting fancy wheels. So there wouldn't be many takers for such an option. So to reduce costs, Ford doesn't offer such an option with XL trim.
Most folks that want the fancy wheels will also want chrome bumpers, nice seat covers, and other fancy stuff, so they'll want at least the XLT trim pkg. So Ford makes the fancy wheels available with the XLT tim pkg. They expect enough orders for fancy wheels on XLT trucks that it makes the cost of having the option worthwhile.