I got a 1990 f150 4x4 with a 5.0 and I want to get a little better gas mileage out of it is there anything I can do to accomplish that

 

100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com

65+ Vehicle modifications for better fuel economy - EcoModder.com

 

What milage are you gettng now?

Basic driving habits will have the largest impact on overall milage, if you drive 80mph on the highway slow down to 70mph, if you do a lot of city driving plan your route to avoid traffic lights and left hand turns. Don't warm up the car in the morning for 15+ minutes, idling = 0mpg so minimize idling at all costs. Avoid driving short distances, if you only need to go a mile walk instead of driving.

Specifoc mods that will help your particular truck include a high voltage ignition upgrade often called the 6Liter tunup around here. If the truck is lifted with oversized tires remove the lift and put stockish tires on. Change the axle gearing to an optimum ratio for your truck, despite the suggestions in the hyper miling link above some of these trucks will benefit from lower numerically gearing, and similarly these early V8's benefit from bigger cams and better flowing exhaust systems, a 6Liter tunup, longtube headers, and a cam change can net 20+ mpg hwy and mid teens city from these trucks in some cases.

 

it's bone stock right now but I'm getting around 14-15 highway. I'm wanting to get better gas mileage because I am turning it into a pilot truck so most of my speeds will be at or under the speed limits. What size cam should I go with and if I change the cam won't I have to change my lifters rocker arms and valves

 

try the air dam.that will likely provide the largest gain.nothing short of a little 4 cyl diesel swap will yield gains of aero mods.

 

Since you have a hydrualic flat tappet cam in that motor (most likely) then yes you will need new lifters with a new cam. Springs? Most likely not if you get a cam that Paul usually recommends. Rocker arms and valves? heck no!

Proven combos on this forum: long tubes headers, y-pipe into a single converter and a single single 2.5" to 3" pipe to a muffler of your choice. Then finish it with a tailpipe. Nothing fancy, but it works and saves you $$$

Add some of the aero items from the links above you should pick up several MPGs.And recoup your investment in a short period.