You may be getting the best you can at 13 mpg. Thats an awful big engine and a heavily constructed vehicle. Given that your truck is a 2000 model, can we assume that you have a transmission with an overdrive gear? Over drive can help on the highway.
You may want to add a good dual system. A dual system usually will allow the engine to run cooler, which is good. But, with computor controlled vehicles it is best to get the advise of someone who knows about them. Any mechanical changes you make will have to be compatable with the computor or the change can work against you. Also, if the change is significant enough, other changes may have to be made to coincide. A good dual system will generally have a crossover pipe.
You also may want to add a bed cover. An open bed traps air and can assist in getting poor mileage. If you have a sliding rear window and leave it open with a side window, you will get a good idea about how the wind blows around in the bed. I had a 1-2 mpg increase on the hwy with a fiberglass bed cover on an F-150 I used to own.
Also, keep your tires properly inflated. You want the tires to role at their maximum efficiency. The friction created by a rolling tire is what keeps us "stuck" to the road. Too much is friction can work against you. Off-roaders commonly "deflate" their tires to give them traction. That technique allows the tire to flatten somewhat, increasing the tire contact to the road resulting in better traction. On a clear road (no ice of snow) that amount of traction (friction) can be a hinderance to a vehicle. Now, some folks will over-inflate their tires to eliminate even more friction. DO NOT DO THIS!!! Tires build heat due to the friction. As heat builds, the tire pressure increases. If the pressure exceeds the maximum safe pressure, a blowout can result. Over inflating the tires already puts them above the safe limit. Remember, the P.S.I. rating listed on the sidewall of your tires is usually the MAXIMUM tire pressure recommended by the TIRE MANUFACTURER, not the vehicle manufacturer. In your case, Chevrolet, has determined what they recommend the P.S.I should be for your truck. Always follow the vehicle manufacturer recommendations.
Make sure your tires are the correct size. Increasing or decreasing tire size sometimes by more than two sizes can have an effect of the gear ratio. That change is reflected by your RPM's and your indicated speed on the odometer will be incorrect. In other words you may think your doing 65 mph, when you may actually be doing 70 mph. The faster you drive, the more gas you use.
Finally, make sure your truck is properly tuned. Use good spark plugs, wires, etc. The longer your wires are, the more need for a quality set. That spark has to travel a long way. Change the oil reguarly and use good oil and filters. Keep your fuel filter up to date.
Hope this helps. Good luck.
EVOC ONE
