Which speed/gear is the most efficient in my car?

[OfficeLinebacker]

Hey, been gone a while but I finally was not in the middle of something when I got the "hey OLB! we miss you!" email.

So I now have a 1999 Mazda Protege 1.6 MT that I am driving and it's the most fuel efficient car I've owned so far. I love it, except when it doesn't want to go into first when it's cold.

Anyway I understand that ~35MPH is where aero drag starts to dominate other factors as far as efficiency is concerned. Also, after reading a treatise on efficiency from one of the diesel big rig manus (I think it was Freightliner or Volvo), one of the takeaways was the goal was to minimize RPMs per mile.

On this particular car, the highest gear I can get away with at 35 is 4th, and I'm running about 2k RPM. The lowest speed at which I can get away with cruising in 5th is about 43 MPH, also at 2k.

So which is the more efficient speed, assuming I can choose?

TIA!

[IsaacCarlson]

dang...get a low-end cam. That can give you a little more torque on the bottom end. It might be enough to let you use 5th at 35mph. You could also pick up a small industrial turbodiesel and put it in there. Those things make some torque!! I have been looking at some for my pickup and even at full load running against the governor, they only use 1-2 gal per hour. And we don't need to run them at full load and full rev going down the road. I did the math and I gould get 50+ mpg with a small industrial turbodiesel. The reason I saw industrial is because they are more efficient and are designed to WORK all day on a tank of fuel. My dad burns about 1.5 gal/hr in his bobcat.

[OfficeLinebacker]

LOL I'm not going to do any cam or engine swaps dude. Given the parameters that the car will remain unmodified, what's the better option?

Thanks

[IsaacCarlson]

I would run with 45mph....35 is too slow if you are already turning 2k. At 45 you still don't have a heap of drag.

[PaleMelanesian]

I heartily disagree. Go for the lowest rpm you can that will keep you rolling. It's lower than you think.

My 90's Japanese 1.6L 4-cylinder with similar gear ratios:
Runs steady speed just fine down to 25 in 5th gear (1000 rpm).
P&G driving, where I need a little more power, I limit 5th to 30 mph and above.

In your situation above, 35 in 4th or 43 in 5th, my answer would be 35 in 5th.

[OfficeLinebacker]

The motor has 153k on it and is notoriously low on power/torque. It literally is lugging the engine at below 2k under any kind of significant load.

[PaleMelanesian]

Yours:
105 hp @5500 rpm
107 tq @4000 rpm

Mine:
106 hp @6200 rpm
103 tq @4600 rpm

Not much different. Looks like you have (slightly) more low-end than I do, since my torque and hp peaks are both higher than yours.

Trust me, it'll do just fine at lower rpm, unless there's something wrong with your engine. It's a poor day for my mileage if I ever even reach 2,000 rpm.

Mine has 177,000 miles.

[OfficeLinebacker]

Wow not to mention my peaks are at lower RPM. Funny, my cousins have the same year and it's gas pedal also feels like mashed potatoes. I bought mine with 153k on the odo (probably 155 now) so maybe it's lost significant compression....IDK

All I know is that the thing tries to buck and stall if I go any lower.

[Billy_BAD_Boy]

Originally Posted by PaleMelanesian

Trust me, it'll do just fine at lower rpm, unless there's something wrong with your engine. It's a poor day for my mileage if I ever even reach 2,000 rpm.

You are not overtaking, are you?

[OfficeLinebacker]

Based on your info though, I will try again to roll around at lower RPMs in gear.