Long term driving in 3rd Gear at 1:1 helpful or harmful?

Jun 15, 2021
Folks, I have been lurking a long time... this is a great resource.

I have a 2006 Mercury Grand Marquis (Panther) with the 4R75E four speed transmission.

The ratios on the transmission are: 1:1 in 3rd gear and 0.7:1 in 4th (Overdrive).

I drive mostly at 50mph on hilly terrain, and like to turn off the OD, meaning I am driving in 3rd at 1:1.

I have heard that 1:1 is good for the transmission because everything is turning at the same rate, and you sacrifice some MPG, but the transmission will last longer.

I have also heard that anytime you are at less than top gear, your are prematurely wearing your engine as it has to turn at higher RPM, and you have the additional issue that at 1:1 each tooth on the drive gear contracts the same tooth on the driven gear and that can cause problems of its own.

So, with respect to Transmission Wear is 1:1 better or worse?
With respect to engine wear?

Any insight would be appreciated.

Thanks in advance!
Sounds like you need to ignore about half of what you hear. Dropping down a gear to tackle hills will keep transmission wear & heat down, because it doesn't have to constantly shift between 3 & 4. Drive & driven gears matching up tooth for tooth has nothing to do with anything.

Yes, it will impact gas mileage, and yes, technically, engine wear will go up, but it's not like it will take years off its life. I'd gladly sacrifice a few mpg to keep my transmission from shifting repeatedly. This is a pretty common practice amongst people who actually think about their transmission's health, or people who tow.
I'd install a trans temp gauge to keep an eye on the fluid temp. Keeping your fluid at a reasonable temp will keep your transmission healthy more than anything.

If you have OD locked out and see your temp rising, then that is your sign to lock it back in so you're not burning your torque converter up and glazing your clutch bands.
Depending on the rpms, as long as its not extraordinarily high, lets say 4 to 5k and up, I wouldn't really worry about the engine wearing out, you are doing good for the transmission.
Civilian panthers have ridiculous gearing. 2.73:1. Especially behind a 2 valve modular V8 which needs revs. You won't hurt anything by not using overdrive and would probably see a bit better fuel economy in real world driving. The stupid tall gearing on these cars was for the EPA test cycle anyway. Which does not reflect real world driving.

Hilly terrain with a 2 valve modular and stupid gearing is deifnitely a case for turning off overdrive.

Depending on the rpms, as long as its not extraordinarily high, lets say 4 to 5k and up, I wouldn't really worry about the engine wearing out, you are doing good for the transmission.

Those cars are so ridiculously geared that you can run 70 in 2nd gear and only be at 2000 RPM.
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Prob doesn't go into overdrive until about 45 only, so 5 mph won't make a big difference in fuel economy. In fact, like Miller88 says, it may even improve.
Try it for a few tankfuls and see what happens.
Thanks for all your responses...

@WylieCoyote I was not sure about the 1:1 tooth to tooth issue, I tend to overthink things and read something about hunting tooth frequency, so I appreciate your response.

If I may, let me refine the question...

Say you are traveling a long distance (20+ miles) over moderately hilly terrain at 50 mph with cruise control set... in OD the transmission shifts every time you start up a hill, and sometimes "hunts" for the right gear. As @JohnnyG points out 45-50 is really a tough speed for that transmission moving between 3rd the 4th, especially with the hills.

Do you use

1) 3rd gear @ 1:1 about 1900 RPM


2) 4th Gear (OD) @ 0.7:1 about 1200 RPM (allowing the transmission to shift into 3rd for hills and sometimes seeming when it feels like it).

Ignoring fuel economy, which gear do you choose and why?

I don't think 3rd gear is doing any real harm, but would it be better to leave it in 4th (set it and forget it) and just let the computer and transmission do its thing...

It is a better driving experience in 3rd, as the transmission does not shift or hunt as much.

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In rolling hills I will shift down to 3rd to minimize shifting and increase throttle responsiveness. It's not really about trans. temps unless you ae towing a heavy load.

I lived most of the last 10 years in the Cascade foothills east of Seattle. Lots of rolling hills, ridges, etc. Some areas around home or on my commute I just left the truck in 3rd for a while for that reason. Ditto in some areas of Eastern WA where there are rolling farmlands - 50 is about the max safe speed anyway because of sightlines, ag vehicles, etc. I'll leave it in 3rd for how many ever miles until the terrain changes. In my lighter car with more HP and torque, I'll leave it in D as the shifting isn't as obnoxious.

I do have an a-pillar mounted trans temp gauge in the truck, so know what speeds/temps/gears influence the temperature of the AT. Anything in 3rd up to 50 is not much different than 4th really. If I were to leave it in "D" and let it shift as it wants, it will not influence the temps much (they may drop a FEW degrees, but they will not be higher than normal).

So, I would say to do what you want that makes your driving experience better and then not worry about it. Personally, I wouldn't want to be cruising in hills at 1,200 rpm because then my responsiveness is compromised. So, if I were driving your car, I'd likely put it in 3rd and not worry at all.
I can see that you are practicing your mechanical empathy, thinking about how to optimize engine and transmission life. I think that what you propose, driving in 3rd gear in hilly country at about 50 miles per hour will give you the benefits that you have mentioned. According to SAE papers that I have read, engine wear is not proportional to engine speed but is more related to engine load as measured by brake mean effective pressure (BMEP) in the cylinders, with less wear at lower loads. I think that 1200 RPM with the engine under load is too slow of engine speed and could be considered lugging which is bad for the engine because high loading and slow rotation speed do not allow for optimum oil film development in the engine bearings. Also in the old days all manual transmissions operated at a 1:1 ratio in high gear and I never heard of any excessive gear wear, so I don't think that the gear ratio is an issue. A more lightly loaded engine running at 1900 RPM should last longer than I more heavily loaded engine running at 1200 RPM on a mileage basis.
I don't think I'd worry about engine wear, this is one of the longer lived engines, and known for being able to idle "forever".

What might be telling is to get a scanguage and to monitor instantaneous fuel consumption, and see if you burn more or less fuel if you try to pull top gear or 3rd on a hill. I swear, on my Tundra I used to get better mpg in 5th than in 6th on my rolling hills--driving along at 50mph or so, it just couldn't pull 6th on any hill but on the flats leaving in 5th didn't "cost" me much.

If it's hunting then I'd lock out OD. But you can engage in the flats, when it's not under load, for the least wear and best mpg. I used to force a downshift at the bottom of a hill so the trans would shift when not under a load, then the torque convertor would stay locked on the hill--I knew my route and knew what the trans would be doing.

Of course, the transmission is designed to do this, day in and day out, so I don't think I'd sweat too much over it. You can add some life but in the end, it's still a wear item.
I can get my best mileage on rolling hills. Leave it OD and gain speed going downhill, and roll out of the throttle going uphill. You must leave the cruise control off for it to work.
Just don't overthink things. Driving in 3rd for longer distances isn't harmful at all. If it's helpful that's up to you, but I'd prefer 1900 rm over 1200.
When people think of engine wear, they tend to over-emphasize RPM and under-emphasize BMEP. Each contributes to wear and they work in inverse. For a given power output, less RPM means more BMEP and vice versa. Within the normal operating ranges of the engine there should be no significant difference in longevity or wear. But making the transmission shift back and forth hunting for the right gear does increase wear. In that case, lock out the overdrive.
My Ford products (Ranger and a Lincoln Navigator) are the only vehicles I lock out overdrive on when not just cruising down the freeway. The final drive is geared so ridiculously low they hunt like crazy to maintain speed.