Need Ideas. Truck is an 89' F150 2WD w/ EFI. Truck has the Mazda 5sp and 3.08 gears. I'm not the most happy with mileage. Right now I'm lucky to get 15MPG. The other 300-6 I had was an 83 and it got 18. Is the worse mileage just the way it is? (dang emissions) Would different gears help enough to jusify the cost? What would a different gear set cost?

TIA,
ORB

 

How much highway driving to generate those numbers?
My approach is to determine what the truck achieved when new and work down from there. You can find mpg stats at FUELECONOMY.GOV. There are those who say the EPA stats are garbage and for some vehicles I tend to agree. For the Ford pickups I think theyre valid. My 83 achieved the published numbers from day one, and I'm carrying the weight of a Supercab and long bed combination.
Anyway, I suspect your highway mpg started out around 22, so work your way down from there? Carrying excess weight? Got oversize tires that are heavier and poor rolling resistance? Got tires that will carry 40 psi? In 83 some models carried 41. Does the truck run good? Does it have high mileage?

 

a quality tune up, new, premium plugs and wires, filters (oil, air, fuel, tranny if its on there), and a good carb cleaner run through a tank always help. be sure to read the directions, some only treat 10-15 gallons, so you may need two containers, and you need to run the truck down to under a quarter tank in most cases. those are my personal standbys that kept my truck running well.

 

The 3.08s and EFI are killing your milage.
I run 3 carbed trucks, 2 YFs 1feedback YFA, all run 3:50s/3:55s and all 3 are in the 18-22mpg range. I've owned several EFI 300s and none of them got good milage.

 

I was about to say with 3.08's and an OD things aren't going to be good. Your next highway trip cruise in 4th instead of 5th, see if that helps.

Of all the iterations of 300 powered trucks i've sported the best mileage has been with a non-feedback YFA, 3.0's and a 4 speed cruising at 65-70. It would consistently bring down 22mpg.

Second best was the same truck but with 3.5's. It brought the crusie down to 60-65, but it would still get 20 on the highway.

Try to get your cruise RPM right around 2200 RPM and report back.

Justin

 

I have the exact same setup and I make about 14-15 mpg, but why would the O/D and high gear ratio hurt the mileage if its runnin at a lower rpm?

 

It doesn't hurt it. Why on Earth would it be offered if it didn't give better highway mileage. Lower rpms at highway speed = better fuel mileage. What is this, revisionist history?
Check fueleconomy.gov and do some research.

 

Welllllllllllllllllllllllll


You know more than I do, but I have 3.08's and EFI and I get decent gas milage. I started driving more sane-like and I began using FI cleaners and gas treatment, and I've seen a big difference. I'm sure my tire setup helps too.

 

and what would that be???

 

I usually average right around 16 and I drive like I stole it. On the highway I get about 17 or 18 as long as I keep it at or below 70. Over 70 it does downhill in a hurry. Truck has just under 320k on it. I've had it since 86k and it gets the same now as it did then.

My last truck was an 86 GMC with a 4.3, 3 on the tree, and 2.73 gears. It got 12 mpg. When the rear end broke we replaced it with one that had 3.08 gears and it got 18. Taller gears and overdrives won't get you better mileage if you have to keep your foot on the floor to pull the gear. That was the problem with the GMC. It isn't the problem with 3.08's in an F-150. The 300 has more than enough torque to pull that down the road easily.

 

so what are you saying, that you will get better millage with 308 gears than 273 gears???

 

All depends on driving habits and cruising speed - I've got the Carter YF and the 3.00 rear end, and my highway mileage is a decent 17 MPG, cruising at 60-70 mph in 4th gear, and my truck is a long bed Supercab, too.

Lower rpms don't neccessarily mean better gas mileage - if you're driving the 5 speed tranny with 3.08's, but your pedal has to be pinned 3/4 of the way down to keep the truck going 60 mph, then your efficiency could be worse than if you upped the rpms a bit and cruised in 4th gear instead. It's a balance between air/fuel ratio combustion and engine speed.

 

I don't think I see that anywhere in my post. I just said that taller gears and overdrive won't always get you better mileage.

On a lot of vehicles the mileage won't change a bit with gearing changes. My uncle's 88 F-150 X cab with 300, 5 speed and 4.10 gears got the same mileage as my truck. I've swapped from 2.73 to 3.73 or even 4.10 in several f-bodies with no change in mileage.

 

What Silver Streak said about tall gears and overdrive...especially running on cruise control where there is a constant change in grade up and down. Works that way on Big trucks too because one is running too much in the torque range rather than in the HP range.

 

Ok, with all this being said about gears this and that, what about if you've got an auto like myself? I don't have a tach so I can't say for sure what rpm's I'm running, but I do know that it will drop into overdrive at 40mph. and if I keep my foot in it from 40, it will pull hard and strong all the way to about 80, then it kinda falls on it's face. I, too, drive like I stole my truck, and all I'm getting is about 12mpg. The vin says I've got 3.08's, but my step dad is convinced that the construction company (county) that owned the truck before me put some retarded low gears in it, and that's why it will accellerate like it does (for what it is). I'd like to get better than 12mpg.