2000 f250 ext cab 4x4 v10!!
Have to wait till next week to pick it up. But I know it's gonna suck gas right?

 

Expect about 10 mpg

 

unless you have proof of all itemized services.....change all the fluids....and filters

leave the spark plugs alone...

V-10 consumes gas and hard to say what you will get since we do not know what rear gear ratio you have, howmany miles on engine, condition of exhaust system, etc.

 

A lot of it will depend on how you drive it. Drive it like you stole it and expect single digits. Be easy on it and you can see 14-15. 55-60 mph on freeways instead of 70-75 mph can save you 3 mpg easily.

If installed incorrectly, yes, the plugs can cause big problems. The easiest fix? Install them properly. It has aluminum heads on it, so "doing it like you have for the last 30 years" is going to cause problems. Forcing it out if stuck, using breaker bars, tightening it until it "feels tight" or won't go any farther work on cast iron heads but not these. Work them out slowly, start them by hand to prevent cross threading, use the recommended plug and a torque wrench and you will be fine.