Ideal engine speed for highway FE?

[Backtobasics]

Originally Posted by comptiger5000

True. Whether or not the TCC allows it to lug depends on hwo aggressive the lockup programming is. In my Jeep's case, it's RPM dependent, not mph dependent, but it will let it lug down to 1050 rpm in OD with the TC locked.

Regarding Backtobasics' comment, my Jeep seems to do best around 45 - 55 mph, with only a slight drop up to 60 or down to 40. The TCC locks around 36 - 40 mph. For reference, I've got a 360 (although with a slightly different cam and more advanced timing than his), 46RE auto and 3.73s.

I should clarify that I have the 5 speed manual. 5th at 50-60 is under the happy power band, and it takes more throttle to maintain speed or accelerate. 62-65 is the magic place for my particular application.

[comptiger5000]

Assuming you have 31" tires, you're geared a hair taller than me. That would have you turning about 1690 rpm at 60 mph. With the auto, different axle gearing, and smaller tires, I turn about 1760 at 60mph. I've found the engine to be quite happy down to 1500 or so (51mph for me), and acceptable down to about 1300 (45 mph). Below that, it's definitely below its power band and mpg drops a little, as it's struggling a bit. Your RPM at 62 - 63 mph is the same as mine at 60, so your comfortable spot being right in that range seems about right. Then again, if you have larger or smaller tires than the 31s I'm calculating with, my numbers will be off a bit.

That struggling range is what the OP doesn't want to reach, as there is definitely a such thing as being below the sweet spot for mpg.

[zonker]

Originally Posted by cfg83

zonker -

I like to use this to play with gearing :

Fatboy Raceworks » Graphical Transmission Gear Ratio Calculator

CarloSW2

Love that - THANKS

As for how the car currently responds to the TC and lugging the motor, the 1-2 and 2-3 shifts in the trans are vacuum modulator signal dependent, and the 3-4 and OD lockup are ECU dependent, which looks at load and speed related variables to make it's choice.

there are a fair amount of hilly roads out here in so cal wine country, and right now, in the car's present state of tune, can climb the freeway grades at 65mph without unlocking the TC or downshifting into 3rd, so I know from a power perspective, the present gearing is plenty good.

I'm going to play with that fatboy gear calculator and see what my options are. who knows, with the 3.08's and a taller tire, i might have 3rd gear match what 4th gear was before, and have a REALLY tall 4th for flat road economy.

time to play

oh, and as far as the measure of difficulty, it will involve a complete drum to drum rear axle swap, which is not a lot of bolts, but i think my back and my girlfriend are going to hate me...

[zonker]

well... assuming a tire change to LRR 185/70-14's, 65mph is 2500 rpm in OD on the 3.73's. In 3.08's it will be 2750 rpm in 3rd! OD will come in at a lazy 2100 rpm.

hmm... flat road cruising at 2100 rpm with a LRR tire/wheel combo that will shed 12lbs a corner. sounds like a winner (providing 2100rpm is indeed more economical to run than 2400 rpm now).

[mcrews]

I think zonker is a keeper.........

only fifteen posts and he's off to the races.
Hey, zonkers, there is a guy named frank lee who posts here, great guy, real handy with tools. He'll swing down and do some of the heavy lifting on that swap.

just feed him lunch.....

[Frank Lee]

How did you know I could be bought for a lunch?

Seriously, I had a *friend* whom I did loads and loads of repairs for, often in exchange for a hamburger, cuz I figured the poor guy could use a break. But I eventually got tired of his using my nearly free mechanic work for his flipping side business. It got to where I'd tell him "if you're going to keep this I'll fix it, but not if you're going to flip it". Then he'd say he's keeping it; I'd fix it; and lo and behold, I'd see it advertised some time later. His last job cost him full market rate... and that was the last I heard from him.

[MetroMPG]

Originally Posted by zonker

who knows, with the 3.08's and a taller tire, i might have 3rd gear match what 4th gear was before, and have a REALLY tall 4th for flat road economy.

That's close to what I ultimately did:

Tallest final drive available for my car's transaxle made my "new" 4th gear just slightly shorter than my "old" 5th gear.

I started with "A" below, finished with "D" and "E".



I'd still go with a taller 5th gear if I could.

[MGB=MPG]

do you have a power chart for your power plant ?
i think you need one for decision
if not search the mustang enthusiast forums for one and/or data charts
for example i find this one for my engine on vehicle specif forum

[MetroMPG]

If you do have a BSFC chart, please also post in this thread:

http://ecomodder.com/forum/showthrea...-got-1466.html

[arcosine]

The 3.08 gear would give the best highway mpg at 65, 1980 rpm is not too low. My saturn 5 speed is at 2110 rpm at 65 mph and my crx HF is at 1830 RPM. Yes, I do more down shifting in the HF.