I ran some numbers, and I'm confused a bit and was wondering if someone could help me understand this. The gear ratios are public knowledge at this point.

At 60 mph in 5th gear with the manual tranny, the engine will be turning 2797 rpm.

At 60 mph in 5th gear with the automatic tranny, the engine will be turning 2159 rpm.

To me, it seems obvious that the auto tranny would be getting better gas mileage since it is turning at a lower rpm.

Am I crazy here?

 

Where/how did you get these numbers?

 

Manuals generally give you better mileage than automatic because, to grossly oversimplify, it takes energy for the car to change gears for you (instead of you doing the work using a mechanically connected clutch, the automatic tranny does the work use fluid coupling). Also, a good manual driver gets better milage by anticipating road conditions. Nevertheless at constant highway speeds you are correct that the automatic will get better mileage because of the different gear ratio. That's why in modern cards like the FIT it really doesn't matter, the mpg rates are very similar except for the lower city end.

 

basic math.

for example:

The Fit Sport comes with 195/55-15 tires.
(195 / 25.4 mm/inch * 55/100 aspect ratio) * 2 + 15 = 23.44" tire diameter
base is 24.47 tire diameter

Gear ratios on the MT:
3.462 1.870 1.321 0.970 0.757
Final drive on MT: 4.294

Gear ratios on the AT:
2.995 1.678 1.066 0.756 0.550
Final drive on AT: 4.562

Now just take a given rpm, I'm going to use 2500 in this case, and multiply it out as follows (hint - I use Excel):

Wheel diameter (23.44) * rpm (2500) / inches per foot (12) / feet per mile (5280) * minutes per hour (60) * pi (3.1415927) / final drive (4.924 on MT) / gear ratio (0.757 on MT in 5th) = 53.64 mph

Doing the same on the AT = 69.5 mph.

 

Wow those numbers are right , why would Honda do that? Just imagine what you would get for gas mileage on the manual if it had the same final gearing as the automatic. I wonder if the speedo conversion is done with software or gears ? Larger wheels anyone - change the ratio that way, and adjust the speedo and your all set.

 

https://www.fitfreak.net/forums/general-fit-talk/3975-auto-vs-manual-mileage-debate.html

I started a thread about this a while ago. hope the info is helpful. i concluded (from the info on the threaed as well as other things) that i'm getting the manual now

 

Autos may be much slower. A smaller gear ratio for 5th (in the auto) may get better cruise RPM, but the acceleration power of the 5th gear is hindered. Also, remmeber the math you calculated is ideal physics conditions, assuming no friction. Remember that autos also use a torque converter, meaning the link that connects the engine to wheel is actually a liquid instead of a metal stick. The rpm of the engine may not match the approprate calculated RPM of the wheels.

 

There are more factors at play here. First of all is what point the shifts happen at. Good drivers are better at picking their shift points for varied situations than any automatic ever will be. Next, an automatic transmission effectively becomes a viscous coupling around shift time, and a viscous coupling will always be less efficient at transferring energy than a non-viscous one. Finally, drivetrain power loss is higher for an automatic even at constant speed.

 

The automatic does get better gas mileage on the highway, but the manual gets better mileage in the city.

 

I've had two CR-V, and at http://www.hondasuv.com, there was a "CR-V engineer" on the board.

He said that the gearing on the transmissions was based on vehicle weight, engine, etc., due to one simple fact - when you have a manual tranny, and you are in the top gear, with the cruise control engaged, there isn't a method to downshift it automatically. So, the top gear had to be at a place where it would have sufficient power to pull itself up a typical hill on the interstate.

With the automatic, they were able to set it up more for efficiency because it will downshift when the vehicle starts loosing speed.

I know there are losses with auto tranmissions; however, there are fewer and fewer losses as the years go by. In addition, the tranny in the Fit is supposed to be able to lock the torque converter in any gear now, where just a few years ago it was on the top gear that would lock up.

You can read about this at: http://www.hondanews.com/CatID2150?m...55013&mime=asc

 

I would gladly give up loosing speed with cruise going up the occasional hill to gain the efficiency, quietness, and longevity of lower revs.

Besides the base Fit does not even have cruise.

 

A torque converter is a loss no matter how you slice it. That's why there is so much interest in SMG and DSG systems.

Oh, and anyone who can't figure out if they should shift or not while going up a hill in cruise control doesn't deserve a clutch.

 

I don't think anybody like that would want a clutch.

 

Sure you could shift in a clutch but that defeats the purpose of "cruise" control. Also it usually isn't a matter of shifting but more so a matter of just being using more gas in the same gear. When I am on the highway for extended periods of time in my 03 Civic 5 Speed I use cruise. While using cruise I never shift gears because again that the point of "cruise".

 

Speed limit is 75 where I live and normal speed is 80+. High revving engines like the Fit (and my Integra) are up between 3,500 and 4,000 rpm at these speeds - very few hills require even the slightest thought of a shift. I have never needed to downshift on the highway - and even running several hundred rpms less I still would not need to. They are just overcompensating because of the extremely rare case when a downshift is needed. If it is needed there is always the resume button - not too inconvenient.