Optimal cruise speed will vary depending on gearing. I find 70 in my truck is not optimal. It is closer to 60.

I noticed something recently though.... If you are using the over head display for MPGs, it is a little false when the tank is full and false when it is under 1/2 a tank. What it reads when the tank is full is overstated (better) than you are actually getting and what it reads when when the tank is under 1/2 is worse than what you are actually getting. I can easily get 14.5 or better when the tank is over 3/4, after that I struggle to get 13 with the same driving habbits. Obviously if you run the tank from full to near emptythat would be a true average, but to run your tank to near empty is not a good idea.

So, for those of you who fill up when you hit 1/2 a tank and you read 14.5, 15MPG, you are not getting that good of milage. For those of you that fill up when empty and only fill 1/2 the tank you are getting better than what it reads.

If you want to see this for yourself just run a few tanks from full to 1/2 and get an average. Then run a few tanks from 1/2 to empty and get an average while keeping your driving habbits the same (within reason). You will see a considerable difference in the averages.

 

Yea, when I first got the truck I noticed the differences in console versus calculated, so I always do a reality check. Calculated mpg was 19.8 mpg. Console was actually reading 19.3 mpg. I rechecked my math about 5 times, haha. I couldn't believe it myself. Even called a friend to make sure. But I always reset my trip odometer when I get a new tank and use the gas reciept to find out how much I put in.

 

Yeah, it is interesting to see how big the change can be on fuel economy with a different driver. When I first got my truck, I used to get great gas mileage, about 16mpg in the city, and upwards of 19mpg on the highway, not it is about 14 in the city, and about 17 on the highway. I guess as I got used to the truck, I press the gas a little harder from stop light to stop light. So every now and then, I force myself to drive cautiously and see better results in mpg's..

My trucks best cruising speed is about 70mph.. If I am going 65, I get terrible mileage, if I go 70+ I begin to see better and better mileage, but If going through the hills on HWY 14 here is So Cal, 70 is optimum for cruising. When I go North, and the highway is all flat, 75ish is the best cruising speed for mpg's.. Weird..

 

BgBluRam:

I read your last post several times being sure that what I thought you were saying was what I was reading.

I give up, how do you get BETTER mileage at 70mph than at 65mph? This was the same argument of the semi-drivers of the '70's until tested under controlled conditions (which blew their argument out of the water). It makes no sense and I've found the opposite, a double digit difference between the two speeds, picking up a slightly greater proportion in mileage than that lost in speed (going from 70 to 65) on a 1600ish mile trip last month that I reported on here.

Sorry, I just don't believe you get better mileage going faster. Wish it were true! D**n if true at 225mph on the Salt Flats I ought to be getting 50mpg instead of 2!

 

Its all about the optimum engine speed -- finding that "sweet spot" where you are getting just the right amount of power output for the speed at the most efficiency.

My previous truck, a Sonoma with the 4.3v6 would get 19mpg all day long around home, work and back cruise sitting on 65mph. When I started going to NJ and back, rolling 85mph up and down the turnpike, I got huge spikes in mpg -- averaged at 24-25mpg for those trips.

My console gauge is consistently 1mpg above what I end up as calculated at each fill-up. I'm getting right at 15mpg from my 4.7 and I'm not running it hard at all -- wish it were better, but at 6'9", I'm just glad to finally have something comfortable to drive.

Israel

 

Has anyone run their Hemi without the viscous fan as suggested above? Any power/economy improvement? Any overheating?

 

I agree 70 is hard to swallow for the optimal speed. Like I said mine is close to 60, actually just under but I do have 3.92 gears.

 

Well, I am telling you the truth. Of course it depends on what kind of terrain you drive daily. Where I live anywhere I go is up and down hills, If I set cruise at 65, it downshifts constantly, which uses a lot of gas. If I set cruise at 70mph It is the right speed where the truck doesn't have to downshift, thus creating better mpg.. You must take all these factors into consideration.

 

Thats a key point -- finding the right speed where the engine isn't struggling to maintain speed within top gear. Any slower and you are expending excess energy to overcome extra leveraging effects of the overdrive gearing, any faster and you are wasting hp on fighting the air resistance of our wonderfully aerodynamic trucks.

Given that criteria, IMO your most efficient speed will be a combination of which engine you run (how much available HP on tap at a given RPM), rear gearing, and coefficient of drag (different for each style, 2 vs 4wd stance, lifted, lowered, tonneau cover, etc.).

Israel

 

I average 18mpg on the freeway with the cruise set at 75mph, and about 14 around town, if I'm not lead footing the pedal. I have a '05 hemi 4x4, with 3.55 gears, and the 17" rims. The 17" rims are gonna give a truck better fuel economy than the 20" rims because of the rolling resistance. The most important thing about getting good fuel economy is keeping your throttle steady at any speed. At 75mph, the engine revs at about 1800 RPM.I notice a HUGE difference if I set the cruise at 80mph. If set at 80, then I see about 15 to 16 mpg. Tonneau covers helps a lot also. I could talk about this topic forever, but don't let your transmission shift out of overdrive either cause that'll hurt also. Lastly. and the most interesting thing about the window sticker numbers for economy, is that I was reading an article about how the EPA measures those numbers. They get the highway number from speeds of less than 60 mph!!! I can't remember exactly what speed it was but 54.5 mph keeps coming to mind. You could probably do a google search and find the article... I thought it was pretty interesting, especially when I can get better than the sticker, at faster speeds.