What's with the wall?

[QWaller]

I don't understand these 3G's. If I run empty 55-63mph I bet I could average 20+mpg. Start asking her to go over 70 and the mileage drops rapidly. Is there something different with respect to areodynamics vs. the 1st and 2nd gens? My previous 4 CTDs never had this abrupt wall for economy. And yes they were all pretty similar. I've never owned a 4:10 and always run stock tire sizes.

This truck definitely has a Sweet Spot.

 

[bpenrod]

In another thread a guy mentioned how the drag went up exponetialy (sp?) above like 68 mph, might try a search on aerodynamics to find it. My 03 is the same way, I once set the cruise on 55 mph on a backcountry secondry road for around 100+ miles and was near 25 mpg. Set the cruise at 75 and it drops to 20 or so.

 

[mconstable]

I've been told that it's more of an engine/RPM issue, don't know firsthand, but I know if I keep it below 1800 RPM's, I can get great mileage (around 24 Highway), but goes down drastically when hitting 2000 RPM's and over.

I know this doesn't answer your question at all... just an observation

 

[mberry]

I would think the fact of no 3. 55 gears has a lot to do with it also. If I could, I would get 3. 0x gears since I don't tow very often at all and when I do it's light.

 

[RMarkey]

I thought I read somewhere that the optimum speed for aerodynamics on these 3gen is 71. 4mph. And I know the feeling you're having. I have the 4. 10s and if I go above 1800/2000 rpm I can watch the mpg drop. I'm getting bigger tires next week so hopefully I can get a little better hwy mpg. I know it's a stretch, but I'm hopefull.

 

[mberry]

I forgot to mention that a box that alters the timing makes a difference in MPG. I run my TST on 1/1 most of the time and I now have a little over 17K on it. MPG has gone up about 1mpg since towing a load 2700 miles. I know the TST helps on that setting also.

 

[Fast 1]

It takes alot more fuel to maintain a higher speed at a higher rpm when your out of the sweet spot for hp/tq. 3. 73s don't help !!! I want 3. 54s back plus an extra overdrive like us gear.

 

[bpenrod]

It takes alot more fuel to maintain a higher speed at a higher rpm when your out of the sweet spot for hp/tq. 3. 73s don't help !!! I want 3. 54s back plus an extra overdrive like us gear.

That's why I'm installing a Gear Vendors OD unit on May 2nd. I took Klengers gear speed calculator and plugged in the overdrive ratios and 1700 rpm is 72 mph, 1800 rpm is 76 mph. Since I do alot of empty highway driving I'm hoping for good things cruising around 1750 rpm. Thats with my 3. 73's, even with 4. 10's it would be below 2000 rpm at 72 mph.