Little help here please?

[sodomoto]

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Hi guys,

I'm proud to say I just bought my first CTD. I had a 1996 GMC crew cab, dwr,4x4 with the 6. 5 diesel that I loved but she hated. After several years of listening to her gripe I let her talk me into a 2003 Chevy 3/4 ton x-cab with the 6. 0 gas motor. Hated it!

A couple friends have the CTD's and love them, always bragging about 20+ mpg,longgevity etc. . So I went out and bought a 1998 3/4 ton 4x4, x-cab, auto. LOVE IT! But after 2000 mostly highway miles (70mph) I'm only getting 15. 86 mpg. What is going on here? The truck is as follows;

1) 1998 3/4 ton 4x4 with 24v cummins, auto with O/D

2) DRE rear axle spicer 70, anti spin (gear ratio?)

3) 3" exhaust

4) K&N air filter

5) HX35w turbo

6) 285/75/R16 "wild country" m&s tires

7) 104,000 miles

Any ideas and comments greatly appriciated and respectfully considered



Thanks-Mark

 

[Steve St.Laurent]

A few things come to mind. One is I would be willing to bet that your speedometer was never recalibrated for the larger tires and your actually getting 10% better than that. The 24v engines don't get as good mileage as the 12v's. Which gears do you have in the rear end - 4. 10's or 3. 54's? Have you checked your fuel pressure? You may want to take out the intake air temp sensor (do a search on IAT) and clean it - that has been known to affect mileage. My truck get's right around 16 mpg and that's with a 5 speed and 4. 10's. 4. 10's are rough on mileage because the engine is spinning so high all the time - 3. 54's are better for mileage. Optimal RPM for best mileage is right around 2,000 rpm.

 

[NMansfield]

Don't be alarmed!!

A few things come to mind. One is I would be willing to bet that your speedometer was never recalibrated for the larger tires and your actually getting 10% better than that. The 24v engines don't get as good mileage as the 12v's. Which gears do you have in the rear end - 4. 10's or 3. 54's? Have you checked your fuel pressure? You may want to take out the intake air temp sensor (do a search on IAT) and clean it - that has been known to affect mileage. My truck get's right around 16 mpg and that's with a 5 speed and 4. 10's. 4. 10's are rough on mileage because the engine is spinning so high all the time - 3. 54's are better for mileage. Optimal RPM for best mileage is right around 2,000 rpm.

I agree with Steve St. Laurent on this one, I had an 01' with 3. 54's the engine wound about 2000@65mph if my memory serves me correctly and was getting 19. 3-20mpg no matter where and how I drove it. I'll bet your speedo hasn't been recalibrated for the larger tires. I even had 315hp injectors a 4in exaust and a power puck installed, and still got this mileage. Wish I could say the same for my 04. 5 :{

 

[JLEONARD]

Also don't forget that if you are in a locale that serves up winter blend fuel you will lose about 2 mpg.

Jay

 

[KD7GYJ]

Your mileage sounds the same as my mileage when I had a 98 24V Auto CTD. It would consistently get 16-16. 5 miles per gallon. Traded it for a 2002 HO with 6 Speed. Got consistently around 18-20 miles per gallon.



So it looks like you are getting about the normal MPG based upon my experience.

 

[WBusa]

All the responses you've gotten here are good advice, and cover the major issues, with maybe two exceptions:



1. You already know this, but its worth repeating: 70 MPH kills your fuel economy. If you can stick with 60-65 you'll see a noticeable improvement.

2. A tonneau cover will prolly buy you 1-2 MPG

And as jleonard pointed out, if you're in a cold climate you'll see better MPG this spring, once you stop burning winter fuel. [Tip: complete your user info w/ your location, and add a sig file]

 

[rkressg]

I am assuming you have a 24v '98, correct? If so, changing the IAT (Intake Air Temp Sensor) can provide for a decent improvement in fuel economy. Changing your MAP (Manifold Atmosphere {Absolute in newer trucks} Pressure sensor) can also help with improving mileage. I would also get a different air filter than a K&N.

 

[DieselLady]

I agree with slowing down a little. I have a 99 and the difference between doing 65 mph and 70 mph is remarkable. A consistant 65 will get me 21 mpg fwy, and 70 will knock it down to 18 mpg. I have 3. 54s in the rear end and tach 1800 rpm at 65. Around town I can't stop playing around so I usually get aroun 15 to 16 depending how many fast vehicles I come across.