Why don't more cars have taller final gears?

[LTC8K6]

This thread is bringing back the line about "no replacement for displacement"...

 

[HarryLui]

I got 36mpg in my 3000rpm at 70mph Saturn my last road trip...

It only has 4 forward gears...

 

[Jzero]

In some ways, I think that the newer trend of automatics getting better mileage is a result of the fact that they have come down in price enough and are considered standard enough that most manuals are now designed to give sporty feel rather than for an economical alternative.

Spot-on. Manual in the US is now mostly for enthusiast drivers who just plain like it better. Perhaps not always for "sporty feel," but always personal preference.

 

[Zap]

I wish my srt4 had a 6th gear cus if I cruise at 80 (which isn't that fast for empty LA area fwys) the engine is @ ~3100rpm.

Isn't that a misnomer? 😀

 

[SooperDave]

This must be killing ol fleabag. I can picture him beet red screaming something about peak effiency while reading this.😀

 

[patronanejo]

So most people travel at interstate speeds everyday? Uggh Nope.

He's talking about mileage, not time. Probably.

 

[jlee]

lmao, two posts, two years apart...and two necros.

That's impressive.

 

[waltchan]

When driving at the tallest 6th overdrive gear (37, 43, 47, or 53 mph), and RPM runs at 1500 RPM minimum, you can get up to 50 MPG on highway easily for the new 4-cylinders. My 2015 Malibu overdrive gear is at 43 MPH and runs only 1200 RPM, and it gets up to 65 MPG on flat-road. 2014 Chevrolet Cruze I rented before is at 37 MPH.

I've heard the 2015 Chrysler 200 with standard 9-speed has the most generous tallest-gear around 72 mph area I read in other forum.

 

[JulesMaximus]

When driving at the tallest 6th overdrive gear (37, 43, 47, or 53 mph), and RPM runs at 1500 RPM minimum, you can get up to 50 MPG on highway easily for the new 4-cylinders. My 2015 Malibu overdrive gear is at 43 MPH and runs only 1200 RPM, and it gets up to 65 MPG on flat-road. 2014 Chevrolet Cruze I rented before is at 37 MPH.

I've heard the 2015 Chrysler 200 with standard 9-speed has the most generous tallest-gear around 72 mph area I read in other forum.

Nice necro.

BTW-The 2015 Malibu is rated at 25 city/36 highway for fuel economy. I seriously doubt you'd ever see anything over 36mpg over the course of one tank full of gas and probably more like 26-27mpg average.

I rented a Chevy Tahoe recently and averaged just under 16mpg out of that and it is rated at 16 city/22 highway.

I wouldn't touch a Chrysler 200. Rental car? Yes. Own? No freaking way.

 

[tortillasoup]

I have a Honda Fit and in 5th gear I'm doing about 3.5K RPM at 70mph. My old Toyota Echo does 3K RPM at 70mph. Assuming that a lower RPM will use less gas, why don't more manufacturers put a six speed gearbox in their econo cars, with the final drive able to do something like 70mph at 2.5K or 2K RPM - intended solely for cruising? Wouldn't that save gas? At that gear ratio with a 120hp motor, the air resistance wouldn't stress the engine too bad (resulting in less economy), would it?

The simple answer is that a lot of people (though not necessarily you) find that small engines in cars like yours a little too pokey. So they gear the car into its powerband while cruising on the freeway because then when people want to pass, they just slam the accelerator without downshifting. Honda used to make cars with taller top gears like the Civic CX/VX/HX/HF and by tall I mean tall enough so that cruising RPM was like less than 2000 RPM @ 60mph on a 1.5L motor! My Civic with the standard transmission and 106HP motor cruises at 2500RPM @ 60mph so I get how much of a buzzbomb your car is with it cruising at 3500RPM.

 

[Paratus]

Nice necro.

BTW-The 2015 Malibu is rated at 25 city/36 highway for fuel economy. I seriously doubt you'd ever see anything over 36mpg over the course of one tank full of gas and probably more like 26-27mpg average.

I rented a Chevy Tahoe recently and averaged just under 16mpg out of that and it is rated at 16 city/22 highway.

I wouldn't touch a Chrysler 200. Rental car? Yes. Own? No freaking way.

We've got a 2015 Suburban with the same estimated mileage. We've seen a lifetime average fuel economy of 19.6mpg with mostly mixed driving. The trip computer also keeps track of your best 50 miles fuel economy. Ours was 23.9mpg which outstanding. Especially coming from an 04 Suburban that averaged 13.8mpg over its life.

You pay for the fuel economy in throttle response. While the engine has plenty of power you have to really get on it to feel it.

 

[yottabit]

I didn't read anything else besides this, but that's a false assumption. You really need to plot the brake specific fuel consumption to see what RPM/Load combination is ideal for fuel economy.

Thought I would take the chance of this necro to elaborate on my 5 year old response

Here's a BSFC chart for a Saturn 1.9L 4 cylinder:

The "islands" on the chart are isolines of a certain rate of fuel consumption

You can see 2500 RPM lines up with the center of the island of least fuel consumption at 250. In this case this also corresponds to a torque peak which also helps with passing. This seems to be a true for a lot of gas ICEs (I haven't looked at charts for direct injection and VVT to see how that's changed things)

To set this up for optimum fuel economy you would need to choose your cruising speed (say, 75 mph) and then figure out how much horsepower is required to overcome drag, etc at that speed. Then you can follow the blue lines on the right that represent HP back up the chart to determine optimum RPM for fuel economy.

For instance if you needed 30 hp that may end up being 1750 RPM, but if you needed 40 hp (just a little extra on hand for passing) that would be more like 2500 RPM

It's too bad these charts aren't readily available for all cars but you can find more here:
http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

 

[Fenixgoon]

Thought I would take the chance of this necro to elaborate on my 5 year old response

Here's a BSFC chart for a Saturn 1.9L 4 cylinder:

The "islands" on the chart are isolines of a certain rate of fuel consumption

You can see 2500 RPM lines up with the center of the island of least fuel consumption at 250. In this case this also corresponds to a torque peak which also helps with passing. This seems to be a true for a lot of gas ICEs (I haven't looked at charts for direct injection and VVT to see how that's changed things)

To set this up for optimum fuel economy you would need to choose your cruising speed (say, 75 mph) and then figure out how much horsepower is required to overcome drag, etc at that speed. Then you can follow the blue lines on the right that represent HP back up the chart to determine optimum RPM for fuel economy.

For instance if you needed 30 hp that may end up being 1750 RPM, but if you needed 40 hp (just a little extra on hand for passing) that would be more like 2500 RPM

It's too bad these charts aren't readily available for all cars but you can find more here:
http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

pretty cool :thumbsup:

 

[Ferzerp]

II think my car is doing about 8,000 rpm at 70mph...

 

[Brian Stirling]

Modern engines, particularly the GDI engines, have a damn near flat torque curve from about 2000-5000 or even higher.

And, running an engine at lower rpm but higher load factor IMPROVES efficiency, not lower it.

My car, a 2012 FOrd Focus with the 2.0L GDI engine and 6 speed DCT gets its best gas mileage at about the lowest speed in top gear -- 36mph. At that speed the engine is turning about 1300rpm and I can get more than 60mpg. Going any slower and the car will drop down into 5th but even if you could run slower in 6th the curve starts going the other way even though the power required to push a car at lower speeds is less owing to the losses in the engine itself (pumping loss, etc).

As far as Honda goes, yes, they tend to gear for higher rpm operation as that does improve 0-60 times and other performance benchmarks though that does lower mileage. My 1998 Acura Integra GSR would cruise at 80mph at 4000rpm whereas my Focus will be more like 2900rpm at 80mph.

If I were designing a conventional sporty car I'd use a dual clutch transmission of 8 or 9 speed with gears 1-5 or 6 being close ratio for performance with the top two gears as cruise overdrive. Figure what the top speed of the car would be in normal conditions then gear 5th or 6th to be at or near redline at that speed.


Brian

 

[ultimatebob]

Because then people complain that the car is "gutless" when they can't pass in top gear. My Miata suffered from this low top gear affliction.

Yeah, pretty much. My 2012 Cruze is practically idling at under 2,000 RPM when I'm doing 65 on the highway in 6th gear, but it needs to downshift before I can get any decent acceleration.

Funny... that car probably didn't even exist when this thread was new. Looks like Fuzzy got his wish with all of these new 6, 7, and even 8 speed automatic transmissions.