I had a chance to do another highway mpg check yesterday, on I-5. We were at 65mph. The cold early AM drive North wouldnt let the coolant go above 155 or 160 degrees. Hot temps on the return resulted in coolant temp being like it should at 180. 100% gas in tank. Oil was heavier, being 10/40.
It came out 18.5 mpg. I just cant break into the 19's . The last time for the check was on the same run, same temps, but a lighter wife than yesterday's passenger, less weight in the bed, and with 10/30, with 18.8 mpg. Crazy? Yeah, but I just want to get there.

If the current jets arent too small, a drop in size by one might do it. I forget what is in there now.

 

Maybe try 55 instead of 65. Probably see a big difference in MPG.

 

The traffic almost runs us over at 65 . I prefer 70 but the friend I run with doesnt like going that fast in his high performance Packard .

 

What gears?

 

I would say even at 60 mph you could see a jump in your mpg's.

My Mit. Galant at the 65-70 mph I only get around 30 mpg, but at 55-60 mph I get 34-35 mpg I had as high as 36.5 mpg before. I always try better the mpg's in my car all the time. I just drove a 5 hour trip 3 weeks ago and at a little slower pace I bettered the mpg's and only tacked on only 20-30 mins in 5 hours.

I would say slow down a little it's not like you will see a significant time difference anyways. Plus you might see a significant spike in mpg's (you might be able to brake 20mpg?). Be real easy on the gas and brake thats how you get the best mpg.

 

The rear is 3.00. Didnt pay attention to rpms but they were probably around 2300 to 2400. Put on some slightly taller rear tires last year but dont want to go taller with the 3.00.

 

Kinda sad to see such low milage with such a power robbing rear gear. I pull 14-15 with 3.89 gears and 235-75R15 tires.

 

Make a belly pan. That'll be a job.

Smooth the grill so there are no openings, angle the bottom of the radiator toward the engine and build a duct and air dam to draw air through the radiator. They tape up the grills at Bonneville for aerodynamics.

Did the mileage improve with the taller tires? You can get too tall with gears and tires.

 

The tires only are about 1 inch taller, maybe even a little less. Couldnt do more with the high rear end. It basically didnt change the mileage at all.

 

Mike, 18.5 isn't all that bad considering that I-5 up your way has some pretty gnarly hills. My '06 F-150 won't even get that...

Wonder what mileage you could get on I-5 south of you (on my side of the Siskiyous) where it's flat.

 

That's a valid point. Mine averages about 16.5 but during my trip to Columbus, Ohio last September, it was getting around 12 through the mountains but after my last fill up, toward the north end of Kentucky, where it's relatively flat, it was in the neighborhood of 19. I was looking at my fuel gauge and the number of miles traveled and thinking "that can't be right" but it was.

 

Good points. I-5 doesnt flatten out in the area until N. of Roseburg, or S. of Siskiyou Pass into CA. Sometime this summer I will get serious and head out with temp at least 80, with 10/30 oil, no passenger and little in bed, into a flat region. I will also drop the accelerator pump actuating rod one hole, and may be able to drop one jet size. If that doesnt do it nothing will.

As to flipklos comment. Back 2 or 3 years ago when I was getting this engine together, the hope was for 20-22 mpg. Probably could have done it with a stock cam, but the Isky E-4 appears to have knocked it down a little.

 

I don't know if I agree. Mine is by no means meant to be an economy build and uses a more aggressive profile, weighs more, and is probably not as aerodynamic and it gets 16.5.
That's not to say that a smaller cam won't help. I just don't think it takes a significant chunk out of the mileage until you get really big. I think that, to a point, how you drive is more important than the cam profile when we're talking about mileage.

Did you actually do a comparison where the only thing changed was the cam?

 

A larger cam profile may get better milage with low gearing. A stock cam should be ample for the 3.00 rear you spin in my mind. The cam profiles are generaly designed to build the power up higher. Not down low where your gears put you. The higher lift is theroeticly better but it is vaccum that one needs. High vaccum gives the best signal to the carb. Out of curiousity what is your mercury reading at cruise? I pull about 19.5 at 55 when I checked two years ago.

 

That is why I dont want to go lower than 65, figuring the higher speeds are liked more by the cam. The later the intake valve closes ABDC the less the MPG by my reckoning. The E-4 has a later closing than stock, but dont have the numbers in front of me.

This is all just figuring, no science or hard facts. What is available is what was built. E-4 was a second choice. Tried to find something a little milder, slightly above stock but it wasnt available. I dont remember what cruise vacuum runs, but think it is around 17. Will check next time out, as there is a vacuum gauge installed.

The 8 in. 3.00 rear was added way back in 1993 when the truck was going together. Back then there wasnt any internet to help a bare bones newbie to cars/trucks. A large number of the hot rods featured in the mags had 8in. 3.00 rears, so that is the story.