Ideal engine speed for highway FE?
 

Greetings,
I'm embarking on a project to make my 1992 Ford Mustang Convertible (2.3 four cylinder) achieve 40mpg, and I have a collection of rear differential / axle assemblies at my disposal... a 3.73 (in the car now), a 3.45, and a 3.08:1 ratio.

Now I know in theory that the taller the numerical ratio, the better FE you will achieve, but taking into consideration usable engine torque and peak engine efficiency, what rpm should I shoot for when travelling at a 65mph freeway cruise speed.

Currently, with the stock automatic trans, while cruising in OD with the torque converter locked I am seeing around 2400rpm with the current 3.73:1 ratio. If i change the ratios to one of the other two selections, I will see 2220 or 1980 rpm.

Would the car be more fuel efficient at the lower rpms, or am i looking at a scenario where the car will be downshifting more, therefore using more gas, since the engine's peak torque (135) occurs at 2600 rpm.

I have numerous posts concerning my personal experience. My 2002 Q45 has a 4.5L/ w 360hp. THe rear ratio is 2.76. Stock tires were 245/45/18 on 18x7.5 fsctory alloy rims.
I run a 255/45/18. So i have effectively lowered the factory ratio with slightly taller tires.
I am 1825rpm at 63mph. I can get 27mpg all day long EPA is 16/22.
So it works for me. Plust the ecm for the transmission 'learns' how you drive and adjusts.
But with so much hp and torque idling along at 1825rpm is ok.
If you have access to some tires that are taller, that might be the way to also get what you want.
also, how much time are you on the freeway? for me its 60% so the deal works
Corevettes run 6speed tranny and 5 and 6 are a less than 1. Vettes get 28mpg on the freeway but the tight gearing accesses the torque as needed.

good luck!

Without knowing the load conditions under which the transmission will downshift, you won't be able to predict whether this will become a problem.

Which leads me to wonder... how difficult is that final drive swap?

If it's not a huge job, I'd simply aim for the lowest cruising RPM you can get, and try it.

having worked on many 2.3L (mainly turbo ones ) , I would be concerned on using the 3.08 .these motors with there 3" stroke don't have a lot of TQ below 2000 .
If your at 65mph and its at 1980 , that would be ok but what happens when your at 50-55, you still be in OD AFAIK and I think she would lug at that speed .
If you had the T5 MT then it wouldn't matter , you just choose your gears when you want it .

I'd give the 3.45 a shot and see what happens. With the low powered little 4 cylinder, 3.08s will likely lug it down too low. If it doesn't feel like it's close to lugging at times with the 3.45s, then maybe try the 3.08s.

I agree with the general opinions, that the 3.08 might be too low.

I am running into a similiar situation with my 97 Ram. I frequently read that the magic target number is 55mpg, but my 360 truck with 5 speed and 3.55 gears doesn't like 55, and the scangauge backs it up. I consistently get better instant and avg MPG at 62-65 then I do at 55.

I'd figure out what your new hwy rpm would be at 65 mph, and try driving at the same rpm and see how much the tranny downshifts. It will down shift more after the swap because of greater wind resistance.

I would imagine the torque converter would unlock if the engine were lugging. It can't only be speed (MPH) regulated.

Again, I'd base the decision on how difficult the job is.

If it's not a big job, go for the mod with the biggest change first and try it out.

zonker -

I like to use this to play with gearing :

Fatboy Raceworks » Graphical Transmission Gear Ratio Calculator

CarloSW2

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.

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.

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...

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).

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.....

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

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... :mad: and that was the last I heard from him.

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".

http://www.metrompg.com/posts/photos...pare-RPM-z.gif

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

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

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

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

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.

Millions of dollars were spent developing the double nickle (55 MPH) signs in the seventies.

From what I have read it is the magic number for FE. I am sure there are different gear ratios to place the number a bit higher or lower.

nothing the government ever does is purely scientific. throw a few special interest lobbyists in the mix and then you have the real reason on the 55mph selection.

i'm sure the best vehicle speed has to do when the aerodynamic drag starts to contribute substantially to the speed equation.

as for engine speed, i'm going to guess it's on the idle speed side of the peak torque curve, so in my engine's case, it produces peak tq numbers at 2500 rpm, so somewhere between 1000 rpm and 2500 rpm would be ideal.

55 is generally the point at which aero loads overtake other resistances. Due to the nature of aero loads- that they increase exponentially rather than linearly- 55 seems a sensible place to assign max velocity in order to get maximum benefit.

zonker -

I think basjoos likes to maintain a higher average MPH than most of us because of his aeromods. For the majority of cars, however, this is the way things play out :

http://ecomodder.com/forum/member-cf...her-speeds.jpg

In all cases the trend is downward, and it was going down *before* 55 MPH. The only special car in all of this is the Mercedes C 180 K. It "bucks the trend", but it is still going down.

CarloSW2

thank you for the chart!

from what info i have been picking up your chart and from other websites, it seems 37-62 mph is "the zone" you don't want to leave, and where it falls after that depends more on the car's own engine/trans/aero specs.

so... i know the car will shift into it's tallest gear (0.75:1 with TQ lockup) at about 46mph (1700 rpm approx).

so i'm going to guess for me it's somewhere between 46 and 57mph. I'd say 62mph but my car is NOT aerodynamic

i think i need to take my lil 90 mile loop and slow my speed from 65-70mph to 50-55mph and see what happens.

p.s. i did donate for a mpguino so hopefully i'll have that in my measuring arsenal real soon.

Engine tune has a great deal to do with optimal engine speed on the highway. This includes volumetric efficiency of the engine, as considered to be an air pump. If low end on a given engine is questionable to begin with because that engine can't really breathe at low RPMs, then odds are it will get worse FE on the highway with taller gearing, than with lower gearing.

On the other board I sometimes visit, there was at least one example of somebody swapping in a transmission computer to get an extra overdrive gear for their electronically controlled transmission, and they actually ended up hurting their FE as a result. When I looked at the other engine mods this guy had, I could see why.

a couple other good graphs...

http://www.atmosphere.mpg.de/media/archive/7516.gif

http://www.atmosphere.mpg.de/media/archive/7519.gif

well besides the 90 mile trip at 50-55mph, i have another thought... there is a substantial uphill grade for approx 1 mile on the I-15 near my home. I will see how slow i can lug the car before it wants to shift out of TC lockup, so basically i'll try it at 50mph (1850 rpm) and then try at subsequently higher speeds until the trans will not unlock the TC. That should give me an idea about the motor's torque efficiency at lower rpms.

UPDATE: did the I-15 hill climb. Set the cruise control for 50mph. The white rag would lose speed and downshift from TC lockup. At 55mph, she climbed without a speed drop and kept in TC lockup. So... I think my magical hwy speed rpm is greater than 1850 and less than 2050 rpm.

+1!

I understand that it's the sum of the whole. I've already made air/fuel/ignition modifications to the motor and thru testing have found more mpg thru smaller injectors, a smaller diameter intake tube (creates higher air velocity thru the maf for increased ignition timing/fuel to offset smaller injectors), and a full time warm air intake by removing the flap valve that directed only cold air into the tube at operating temps.

That and some under-driving of the accessory speeds have gotten me a personal best of 31 mpg so far (it was 26 when all was stock, same as the epa hwy specs). That was at speeds between 65-70 mph constant. Now with the new info, I'm going to attempt the same trip, but 50-55mph speeds and see if I pick up any fuel economy.

http://ecomodder.com/forum/showthrea...peed-1477.html

There seems to be a point of lower rpms at which engines don't get more efficient.

55 was chosen because thats the slowest they thought people would drive. Speed limit was 45 mph during the war.

phew, now thats getting way up into that engine cavity in the search for better mpg.

i'll bet that at a certain point, the speed in which the combustion explosion propels the piston down it's bore has an ideal.

I'd guess that's probably related to why under-square motors (greater stroke than piston bore) are more efficient at lower rpms than over-square motors (larger bore than stroke).

another update...

did my 90 mile loop tonight, but with lowered speeds.

92.6 miles, 50F, 50-55 mph, 2.57 gallons used.
36.03 mpg!

the previous run, same 92.6 miles, 55F, 65-70 mph, 2.90 gallons used.
31.93 mpg...

that is a 4 mpg difference between 52 mph and 67 mph. pretty substantial.

Good results zonker! I found out my GP does much better in the 50 - 55 region last year. Of course it makes my 50 mile trip to and from work about 10 mins longer, but oh well, at least I am saving some money in gas. Or at least I am not burning that money uselessly.