Hello,

Just spent the last two years doing tons of work on the truck (head gasket, cam, suspension, hoses, A/C, cruise, body work, fluid flushes, tires, etc., etc.) and now my mileage is down about 1/2!!!!!

I'm sure I used to get around 13-15 mpg, 300 miles per tank.

Now I'm getting:

186.2 miles 81.5 L = 21.5 gallons gives 8.66 mpg

159 miles 75.7 L = 20 gallons gives 7.95 mpg

(I always run the A/C in econ to conserve fuel - that's right isn't it?)

Any suggestions...thanks!

 

Who's cam did you install? Who's wires and plugs? Do you have a real time scanner that will show you your timing as you drive?

 

In addition to above, most scanners will show either a symbol or a phrase for closed loop fueling. If the computer is maintaining open loop only (not using all the sensors is concert with each other) then mpg will be in the range you suggest. So O2s can also be an issue, etc.

 

Cam: 4.6 Motronics camshaft from PT Schram (told would increase power without effect on mileage, as recommended)
Wires: STI721 Spark Plug wires from PT Schram (as recommended)
Plugs: 4418 Bosch Platinum +4 (as recommended)

I don't have a scanner but I can ask my mechanic if he does...? What are we looking for?

 

Does it still shift into 4th gear? Can you tell if the torque converter ever locks up? Any codes stored?

 

I'll find out.

FYI:

-I don't use Sport mode
-I have a lift and slightly larger tires (as recommended)
-I use premium gas only

 

How would I tell if the torque converter ever locks up?

 

The converter lock up is controlled by an electric valve inside the tranny. That valve (MV-3) is energized by the tranny computer when conditions are correct for lock up. The tranny computer would supply + voltage on connector 193 pin 32 a slate with orange wire. That would be under the seat with the computer. So you could set up a meter before test driving. To see if the computer actually signals for lock up. Which may not be happening if tranny computer and engine computer think your are driving in a manner that does not need lock up.

As for scanner, your mechanic's scanner will have the fields for monitoring open loop/closed loop fueling.

 

If you're at around 2500 RPMs at around 65mph then you're in 4th gear.

That being said a bigger cam, bigger tires, and/or more aggressive tires will decrease fuel economy.

 

Bigger tires will also makes your odometer read low(less miles than actually traveled). I checked mine against my GPS over a long trip. Figured out for every 10 miles of driving the odometer would register 9. Not huge but enough to account for at least a 1mpg difference.
Also, my gas mileage sucks right now because its winter and that means letting the truck warm up for about 10-15 minutes.