MPH Vs. RPM

[Jay FX4]

The only thing I don't like about my auto trans is that the OD doesn't kick in until about 42-43 mph. There's one particular road I like that's very scenic with long stretches without any stops, and a 40 mph speed limit. So it has me wondering what would be better for fuel economy - going exactly the speed limit in 4th, or going 45 mph in 5th at about 500 rpm less? I think about this whenever I'm on a road with a 40 mph speed limit, which is quite often.
I'm able to consistently get 18-20 city mpg as it is, but for every tank of gas I like to match or exceed the mileage of the tank before it. I know it's not that big of a deal, but I figure over the course of an entire tank of gas (I can get up 350 miles on a tank) it would add up. Anyone have any thoughts on this?

 

[martin]

how many rpm's are you turning at 45 in fifth?

 

[Jay FX4]

Roughly 2000 rpm in 4th at 40, 1500 at 45 in 5th.

 

[Beanmachine7000]

Roughly 2000 rpm in 4th at 40, 1500 at 45 in 5th.

1500 rpm is a little low for your truck... To be completely honest, 2000 rpm is still low... I'm still in 3rd gear at 40 mph and that gets me to 2500 rpm, and that is where I get best fuel mileage...

 

[rmcnelly]

According to my scanguage, gas mileage definitely goes up when the truck shifts to 5th at 45. I'll speed up to get it in 5th then slow to 40mph or so to stay out of trouble.

 

[Beanmachine7000]

According to my scanguage, gas mileage definitely goes up when the truck shifts to 5th at 45. I'll speed up to get it in 5th then slow to 40mph or so to stay out of trouble.

Oh boy... Not another one of these... Instant fuel mileage readings are inaccurate, the only way to correctly and accurately gauge fuel mileage is average...

 

[darkspork]

If I were to try to shift as a speed lower than that I would bog. Sounds like you're doing fine to me.

 

[rmcnelly]

Oh boy... Not another one of these... Instant fuel mileage readings are inaccurate, the only way to correctly and accurately gauge fuel mileage is average...

My experience has been when I keep the instant mpg readings up, my avg mpg increases too. Your mileage may vary

 

[Will]

You guys didn't read the forum rules very well. So I'll go over it again for those of you that were in the cheap seats:

Anyone that wants to post their MPG, HP or RTI results has to have their data collection overseen by at least 2 members of the IEEE or SAE. No exceptions.

Seriously, it seems like that thing should just go right up into OD. The only thing I have to compare it to is our Pilot and that thing goes right up into OD and cruises at about 1,700rpm on the interstate. It's 1,000# heavier than a Ranger and has a half liter smaller engine so a 4.0 should easily cruise below 2,000.

On the other hand, it doesn't hurt anything to drive it forever at 2,000 or 2,500rpm. I think the mileage is better at lower rpms provided the engine doesn't have to open the throttle very much to keep the speed up. The reason cars started getting overdrive added to them in the late 70's, early 80's was for MPG. Our Pilot gets 22mpg loafing along at 1,700. I'd want that Ranger to gets its ass in OD too.

 

[Jay FX4]

1500 rpm is a little low for your truck... To be completely honest, 2000 rpm is still low... I'm still in 3rd gear at 40 mph and that gets me to 2500 rpm, and that is where I get best fuel mileage...

Why would you get better mileage at that gear/rpm than to upshift and drop your revs even more?

I'll speed up to get it in 5th then slow to 40mph or so to stay out of trouble.

I can pull that off every once in a while, but it's rare and usually only for a minute. The problem I have is if I can even manage to get it into 5th and bring it back down to 40 mph, the slightest twitch of my foot on the pedal or the slightest hint of an uphill grade will make it downshift. For me, to consistently stay in 5th, I have to be doing at least 45. I've even tried playing with the cruise control, but that just makes matters worse when setting it at the 40-45 mph range. The OD develops a mind of it's own and just can't decide what it wants to do and stay there. At that speed its in a constant state of upshiftdownshiftupshiftdownshift......

 

[Jay FX4]

Will, I guess I was typing out my reply when you posted yours.

You guys didn't read the forum rules very well. So I'll go over it again for those of you that were in the cheap seats:

Anyone that wants to post their MPG, HP or RTI results has to have their data collection overseen by at least 2 members of the IEEE or SAE. No exceptions.

^ I don't understand.....

 

[fastpakr]

It was a joke.

1700-2000rpm with a 4.0 automatic is fine as long as it's not under a load. I doubt you'd see any fuel economy improvement, however. With a 5 speed I'd shoot for 2000-2400rpm to protect the trans and still ensure optimum fuel economy.

 

[kansas 4x4]

You guys didn't read the forum rules very well. So I'll go over it again for those of you that were in the cheap seats:

Anyone that wants to post their MPG, HP or RTI results has to have their data collection overseen by at least 2 members of the IEEE or SAE. No exceptions.

Seriously, it seems like that thing should just go right up into OD. The only thing I have to compare it to is our Pilot and that thing goes right up into OD and cruises at about 1,700rpm on the interstate. It's 1,000# heavier than a Ranger and has a half liter smaller engine so a 4.0 should easily cruise below 2,000.

On the other hand, it doesn't hurt anything to drive it forever at 2,000 or 2,500rpm. I think the mileage is better at lower rpms provided the engine doesn't have to open the throttle very much to keep the speed up. The reason cars started getting overdrive added to them in the late 70's, early 80's was for MPG. Our Pilot gets 22mpg loafing along at 1,700. I'd want that Ranger to gets its ass in OD too.

you must have a lot of time on ur hands to be this bored!!

NOW watch me get banned for speaking my opinion

 

[4x4junkie]

Why would you get better mileage at that gear/rpm than to upshift and drop your revs even more?

Because when you drop the revs too much, the engine will instead be lugging (which also consumes more fuel, not to mention is brutal on the engine's bottom end).

Anything below 2200 RPM or so is very unlikely to save anything in fuel (2500 if you're cruising at interstate speeds).

Yes, Ford didn't do so well on gearing these trucks from the factory. Even stock, many are far from optimum (3.27s in a 4WD is a classic example). Even 3.73 is a tad tallish in many cases.

 

[Will]

you must have a lot of time on ur hands to be this bored!!

NOW watch me get banned for speaking my opinion

Writing is what I do. You have no idea how fast I can pump out 1,000 words. Or 10,000.

I don't see any thread jacking in my post. I'll be happy to ban myself for it though.