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zonker 02-20-2011 11:59 PM

Ideal engine speed for highway FE?
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.

mcrews 02-21-2011 12:19 AM

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!

MetroMPG 02-21-2011 08:09 AM

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.

EdKiefer 02-21-2011 09:42 AM

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 .

comptiger5000 02-21-2011 10:30 AM

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.

Backtobasics 02-21-2011 10:42 AM

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.

roosterk0031 02-21-2011 11:09 AM

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.

MetroMPG 02-21-2011 11:16 AM


Originally Posted by EdKiefer (Post 221288)
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 .

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.

cfg83 02-21-2011 12:07 PM

zonker -

I like to use this to play with gearing :

Fatboy Raceworks Graphical Transmission Gear Ratio Calculator


comptiger5000 02-21-2011 12:19 PM

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.

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