I thought I had my MPG issues taken care of. I just bought this truck last month it's a 03 quad cab 3.9 4 wheel drive with 265/60/16 tires, auto trans and 63,000 miles. Best I can get around town is 10 mpg and that's babying it. It has new plugs,wires,dist cap,rotor and a cold air intake. What am I missing?
Shouldn't it be a little better than that? At my wits end and so is my wallet. Is there other stuff I need to check? Thanks Kelly

 

LOL! Welcome to my world. 12-14mpg with the 5.9 V8. 10 if I'm towing or heavy on the throttle.

But for a V6 that is bad.

What size tires came on it stock, check your door sticker. If they are larger then stock, then your speedo and odometer is off. Which would mean your MPG calculations are off. Double check your speed and mileage against a GPS. Most modern phones and smart phones has apps you can view speed ect.

 

Sadly you wont be getting much better 12-13 in town. My 5.2 used to get 10mpg. Im up in the 13-15 range depending on how I drive. Check throttle body make sure its clean you can try a new map sensor,Check to make sure your wheels spin freely,Exhaust system is not clogged.

 

The MAP sensor and O2 sensors can both have an adverse effect on MPG. O2's can get lazy and cause bad MPG's but never throw a code. I replaced both of my upstream O2's at about 80,000 miles just as preventative maintenance. There is a good thread about the MPA sensor and MPG's here:

https://dodgeforum.com/forum/2nd-gen...er-solved.html

Jimmy

 

Having the old Automatic not a 5 manual I think is the problem second is the O2 and a few other of those engine self-adjusting sensors they just get old and worn out.

Best change I did was replacing the upstream O2 the TPS and the intake oxygen temp sensor all at one time my Dakota has run so much better after that.

Can't you go to a shop have it checked out by the computer to see if it’s running at its best that may zero you into the exact problem this should show a poorly performing O2 or other sensor in the system?

 

The tires on it now are 265/70/16 and the door sticker says it was born with 245/70/16. So the tires are one size wider. Can't justify $600 on new tires at this point. Yesterday I noticed the bore of the throttle body was very dirty. Going to pick up a can of carb cleaner and a new MAP sensor.Give the TB a good cleaning and install a new map sensor. I am hoping that is the majority of my problem. I removed the clutch fan yesterday. WOW what an improvement. A little better throttle response and it is so quiet you can barely hear the engine running. Ran the truck in town for about 1/2 hr and the temp gauge barely moved up a hole line. Did not hear the electric fan even turn on. I know the fan is functional because when I run the A/C it will turn on. Maybe with these changes I will see a little improvement. Thanks Kelly

 

Got the throttle body cleaned and a new MAP sensor and TB gasket. I guess I wont know much about fuel mileage until I drive it some. I drove it about a mile and it runs pretty smooth and I noticed the idle RPM's is about 100 lower than it was. Used to be about 750 now 650.

 

Im getting 14 - 15 mpg and i did notice a differance when the O2 sensors were changed out along with new front wheel bearings/hubs made a slight differance too thinking the noise i was hearing was the aggressive tire tread with my 31/10.50/15's it was about 11 or 12 mpg before the new parts,what pisses me off is when i bought the truck in 2000 i got the six (3.9) thinking that it would be better on fuel but i was wrong the V-8 gets the same fuel consumption,if i knew that i would have bought the v-8.

 

Every little bit helps for better gas mileage


Look at your belt tensioner wheels if old they could be dragging your engine down most look just fine but if they are old the bearings could be dry they are deceiving they look ok and spin ok but may be dry inside dry bearing cause engine drag a very small amount but with gas at record highs every little bit helps.

Most are only $15 at your local auto store.

Pic below shows what I’m talking about

 

Actually it is just over 1in taller and wider. Using a simple tire calculator I got these numbers. But I doubt it is enough to get you speedometer and odometer that far off. If I were you I double check to see if 1 mile is 1mile on the gauge. Use mile marker on the highway to check.

245/70R16
Width 9.64 in
Height 29.50 in

265/70R16
Width 10.43 in
Height 30.60 in