Well my truck had a lift put on it so they put bigger tires on it that what were originally on there. My dad said that my speed will be different than what the speedometer says because they are bigger tires. Would the speedometer read faster or slower than what i am actually going?

 

Bigger tires will go farther each revolution, so you'll go fargher than your odometer reads and will be going faster than your spedometer reads. Another way of saying that is that your speedometer will read low.

 

The speedo can be recalibrated.. what year is your truck?

 

if you are talking about the 91 in your profile, you will need to chance the gear on the end of the speedometer cable where it goes into the transmission.

 

Ok well thanks. Is it easy to chance the gear?

 

yup. remove the screw holding the speedometer housing into the trans or transfer case tailshaft and pop the housing out of th etailshaft. remove the "C" clip and slide gear off, slide the new gear on, pop the "C" clip back on and put it back in.
the hardest part is figuring out what gear you need.
i tooth is good for about a 2.5 mph