Well i recently got bored and installed a cheapo Harbor Freight vac gauge in the old truck. And i noticed that it didnt pull as much vacuum at speed as my Nova, which yes can be a direct cause of cam, intake design differences etc etc.
The main difference was the truck had OD and the car does not. So, yank it out of 5th and into 4th and voila, vac jumps up a bit. So this gets me thinking. Which is better lower rpm/low vacuum or higher rpm/higher vacuum? You would think higher vacuum would mean lower load therefore less fuel consumption because the engine is working easier. But the high rpms require more fuel to sustain that rpm.

Well my truck is my work driving truck. It sees the same road 6 days a week and same driving conditions every day. Short of having to make an extra run to work for being called in, it uses the same amount of fuel (weather conditions not withstanding) every week. Summer it gets 16.5 to 17mpg and winter it gets 14.5 to 15.5 depending on the temperature swing that week.
So here is the truck running 60 in 5th gear cruise set to make sure speed is steady

Approximately 7ish in hg.
Now here is the same section of road same speed in 4th

Right about 13in hg.
So not a small difference.
This last week i did my regular trip to and from work as usual except i never used 5th gear.
My results were a wash.

15.0 mpg right in the middle of the range i expect for winter driving. May try the experiment again in the summer when i typically get better mileage and see what the results are then.
So, i guess this could be another thing to tell 5.0 owners when they ask about regearing. MY experience is overdrive or drive doesn't seem to make a difference other than rpm and ring wear up and down the bore.

What are you guys thoughts?

 

With EFI engine load is the most significant factory determining how much fuel is injected and Load is inversely proportional to vacuum... low vacuum = high load, high vacuum = low load. Engine rpm does factor in of course but a relatively small increase in RPM can be offset by a significant decrease in load and result in an improvement in fuel milage. So what was the engine speed in your truck during those tests in both 4th and 5th gear? If I had to guess I'd say 1500rpm and 2300rpm. Based on my experience 1900-2000rpm is the perfect hwy cruise rpm for a 5.0 truck, lower rpm hurts performance and fuel milage, higher rpm performs better but while fuel consumption is better than it is at 1500rpm it's not as good as it could be.

 

If my tach is right 4th 2300ish and 5th 17-1800ish. Both at 60. ( dont quote me right now, i dont remember for sure. I will double check on my way home after work and update)
If i had fewer miles on the truck, i would probably just use 4th from now on, but im sure that overdrive is why its lived as long as it has. However, i do like the snappy response the throttle has at the higher revs without needing to downshift.
Ive been wanting to do a compression test but haven't had time. She has been seeming a bit sluggish lately, but its not like i expect it to run like the newer trucks i have. (But it i my favorite)

 

Thought about grabbing one of those, but couldn't do it. Did ride around a few days with my Mityvac's gauge on a hose through the hood gap.

Wasn't enough time & driving conditions to get any firm numbers. Other than it is damn hard to maintain high constant vac with an auto.

 

I have it on the dash, so it is just a quick glance over to see it. I think it would be interesting to see if i could find a set of 27 inch tall tires to throw on to simulate a 4.10 gear swap. Then drive for a week and do mileage. Like i said before its the same stretch of road over and over so i know its the same distance every time. So i wouldnt need the odometer to be accurate that week. Just divide the gallons by the same miles i always do lol. I know rolling resistance and weight should come into play, but there really isnt any stop and go, so once moving the weight shouldn't be much of an issue. Rolling resistance on the other hand would change some. But i also think the added mechanical advantage from a real gear swap would offset that some so call it a close wash.

 

I just installed this gauge. I sprung for the Made in USA AutoMeter gauge, but was disappointed in the quality. Had to return the first two before I got a good one. I considered switching to a different gauge, but I like the look of this one better than any other one on the market. In reference to your question, I don't have much input, but I'm surprised 5th gear doesn't produce the lower mpg.

 

Yeah, keeping the RPMs help considerably with fuel mileage. I have an Autometer Air Fuel Ratio gauge installed in my rebuilt 302, with AOD transmission.
When I cruise beyond 60, kicking the transmission out of overdrive, I get better mileage. When the vacuum drops below 7", my gauge stays in the RICH/Green side. Keeping the needle above 7", my O2 sensor shows mostly STOICH, as its constantly sweeping. As I learned from Conaski, this helps with complete burn, you're not pushing the injectors at full cycle.

At some point I would really like to install a WideBand O2 sensor so I could see the fuel trim in real time.
If it were a 351, chances are Overdrive would be fine. Both trucks have 3.55 gears and auto trans, and I can tell the 351 does WAY better in Overdrive than my 302.

 

Conanski, i double checked the rpm at 60 mph 4th pretty close to 2400 and 5th looks to be 1800ish. As close as a 26 year old tach can be haha.

 

What was the problem? We're talking about possibly the simplest gauge there is.

 

x2 yeah, pretty simple. Arguably more simple than mine since it is a vac/boost gauge.

 

Reminds me of those old fuel economy gauges that some 60s/70s vehicles have. I wonder is premium fuel will increase fuel economy, since the 5.0 has a knock sensor? I know using premium will net greater MPG in modern cars. Better yet, premium ethanol-free.

 

I dont think in our old trucks the nonethanol would be enough to offset the higher price of the 91 fuel. Ive been looking for non e 87 in my area, but been less than successful.

 

Yeah, it really surprised me. The first one had an off center dial so that when the backlight was on, you got a moon shape along one edge of the dial face. The second one was reading low when connected to the intake, and wouldn't hold a vacuum when I applied vacuum with a hand vacuum pump. I reconnected the vacuum line, but couldn't get it to hold a vacuum. Thankfully, the third one seems fine. I'm somewhat turned off by AutoMeter now though.

 

It won't because of a fundamental difference in the way the PCMs handle ignition timing. With these old trucks the computer adds advance based on preset tables, and however it is implemented the knock sensor doesn't seem to have much effect because it is possible to put too much or too little base timing in and no matter how long you give it the PCM will not fix it. The system works for the most part independent of the engine just like the old mechanical ignition systems, it doesn't have enough sophistication to really dial it in.
Newer vehicles use some form of distributorless ignition, with these the only reference setting is TDC on piston #1, computer has complete control and is constantly pushing ignition timing to the edge of detonation so it does respond to fuel octane.

 

That's really unusual for sure. One that wouldn't hold vac shouldn't have made it past QC. Last I looked, a Phantom like I use was $70+.