I've been debating with myself about changing gears for better MPG. Right now I have a 85 300/6 with 4od trans and (I think) 3.55 gears running on 31x10.50's. It originally came with 2.47's and 215r15 tires. I've been thinking about going to maybe 3.08's but kind of worry it will be a real dog to get moving. Right now I need to be at 1000+ rpm to take off, at least that's what my tach said before it broke. And I'm worried I'll have to really wind it up just to get going, especially if I have any kind of load. So, what's ya'll thoughts?

 

If it's MPG you're after a numerically lower set of gears will certainly help you out. I'm going to be pulling the 3.08's out of the rear of my truck soon so I'll have set available. The 4.9L coupled to a C6 and 35" tires with the 3.08's don't make my engine happy.

 

I might be interested in those if they're for an 8.8"

 

Yep. Let me know through PM's.

 

There are 3:27's available for the 8.8. They came in some production T-birds and auto Mustangs in the 90's.

 

You really need to know for sure what gearing you have now, to be able to judge a change.

A 3.55 axle, even on 31" tires, shouldn't give you any trouble starting out (on the flat anyway).......maybe you have a higher (faster) ratio axle already.

I don't know what speed you mainly drive at but .....
- with a 3.55 axle, you would be doing around 1950 rpm @ 65mph in OD.
- that would drop down to 1690 rpm @ 65 mph with 3.08 gearing in OD.

Higher (numerically lower) gearing & lower rpm doesn't always mean better mpg. You reach a point where the engine becomes loaded, which means the throttle has to be open slightly more, vacuum drops, & so does mileage.

 

Just for reference, my son's 1984 F150 and stock 4.9L with SROD manual and 2.47 gears WAS a "dog" regarding pulling, accelerating, etc., BUT would get 24-26 mpg @ 60-65mph. The rear gears were changed to 3.55. The truck was much more fun to drive, but dropped 2-3 mpg under the same conditions.

My 1986 F150 with same driveline except for duraspark conversion and 3.08 rear gears drives well and gets 21-22 mpg @ 55-60 mph.

Based on the above combinations, I believe that a 3.27, as mentioned by Terpfords, would be an ideal combination as long as one is using stock sized tires and is looking for all around driving pleasure and fuel efficiency.

 

It really doesn't have any trouble taking off it just seems like it takes more RPM's than it should to do it smoothly ( ~1000rpm). And just wondering if it would be up to around 1200-1500 if I drop the gears to 3.08's or even 3.27's. It cost around $500 to do a gear change or about $400 for a used axle around here so don't want to spend that kind of money just to find out I'm not happy with the results. Guess before someone ask I should also say it has a new clutch, slave and master as well as the flywheel being surfaced.

1986F150six, I'd be ecstatic if I could get 20mpg, heck I'd be happy if I could do 17-18. Running ~65 mph I do 13-14 with an all time high of 15.5. That's figured on GPS miles/gallons used. I do still have the TFI and feedback setup but I have it running great. Gone thru replacing bad parts, fixing vacuum leaks and replacing the intake/exhaust gasket and carb. EGR valve is the only known bad thing left to replace but it doesn't leak it just does not activate. It starts first or second turn and idles smooth. That why I've been wondering if I had to do gears next. Just running out of ideas.

 

kedwinh,

I've been doing some figuring to help put this in perspective, so here goes...

I went to a tire calculator and compared the diameter of the 215/75 X 15 to the 31 X 10.5. This calculator did not have the diameter of the latter, so I am going to take liberties and guess that it is 31" compared to the original equipment tire @ 27.7". That would mean that tire diameter has increased by ~11.9%. That would mean that your 3.55 gears are effectively decreased by 11.9% = 3.13 [you should now be close to what 3.08 gears would be like with the 215/75 X 15 tires]. It also means your speedometer and odometer are off by the same 11.9% [is this close?].

With everything being equal, going from 3.55 to 3.27 will effectively reduce RPMs by 7.9%; going from 3.55 to 3.08 will reduce RPMs by 13.2%; going from 3.55 gears to 2.47 will reduce RPMs by 30.4%.

I can tell you from experience that changing the gearing by that last number [30.4%] creates challenges regarding getting the speedometer to read correctly.

 

It started off as 215/75r-15, or 27.7" tires and was swapped to 31X10.5-15, about 10% larger. But at the same time, I'm guessing, it was also changed from 2.47's to 3.55's, about a 30% change? But anyways, it reads ~75 on the speedo when the GPS is showing 65. According to the mile marks compared to the speedo it's going about 11% less than shown, 11 miles on the speedo compared to 10 miles traveled. And to add to the confusion, went down to get the tires rotated and balanced today because have been feeling some bounce. Well come to find out the bounce wasn't because of balance it was the 2 front tires starting to separate, guess that's what you get for running 7 years and 14000 miles. So I got to talking to the man there and he convinced me to go with a narrower, more street friendly tire tread now that it's not used much off road anymore. So wound up putting 235/75R-15's on it dropping it about 5%. Love the difference in the ride compared to the old tires that were getting a little hard. Interested to see what the smaller footprint, lighter weight and street tread will do to MPG but think it's going to make it even more important to do gears.

 

Changing gears is a complete waste of money if you are doing it for MPG. It typically costs 5-800 to do a gear change. How long will it take to recouple those costs

 

Good point.

 

kedwinh,

May I suggest that since you are seeking an improvement in the MPG department and as Diesel_Brad wisely pointed out, it would take quite awhile to recoup your proposed investment, consider the following as a partial list of things generally accepted to assit in gaining the best MPG...

1. Check all tires for proper inflation
2. Good clean air filter
3. Good tune-up
4. Make SURE the choke is functioning correctly
5. Remove all unecessary weight
6. Reduce wind drag [remove roof mounted carriers or lights, etc.]
7. Some swear by having a tonneau covering the bed]
8. Narrow tires [i.e OEM size]
9. 195 or 205 degree thermostat [original]
10. Synthethic oil as thin as practical [see owner's manual]
11. Synthetic fluids in transmission and differential
12. Tubular style rear bumper
13. Drive with windows up, when comfortable.
14. Drive like you have an egg between your foot and gas pedal.
15. Short shift [if manual transmission], but do not lug.
16. Make sure brakes are not dragging.

The above suggestions cost very little.

Some have experimented with adding a larger air dam beneath the front bumper or fabricating a "skin" to cover the bottom of the truck, as much as possible [watch out for exhaust and drive shaft locations].

Food for thought!

 

Thank for the ideas. They're all good ideas and I do, and have done, most of them except the oil and tonneau. Got to finish stopping the oil leaks before I switch over to synthetic. I tinker with it all the time so will keep trying to "fine tune" it for max fuel mileage. Just keep hearing people saying they're getting 18+ out of these 300's and lucky if I can do 15.
Also keep meaning to ask what tire size are you and your son running?

 

My son's truck has 235/75 X 15 and mine rides on 215/75 X 15.

One thing I failed to emphasize in my previous post is speed. It definately affects the mileage figures. Our trucks are shaped like bricks and do not flow smoothly through the air. The faster one goes, the less the mileage, especially speeds above 60 MPH.

If I remember correctly, the resistance doubles with the square of the speed, so @ 55 MPH you have a figure of 55 X 55 = 3025; @ 60 MPH, that figure increases to 3600; @ 65 MPH = 4225; @ 70 MPH = 4900. As the resistance increases, the engine works that much harder, so mileage plummets.

So, drive like an old man and smile as everyone passes you!