Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Manual vs. automatic

I am shopping for a new Civic. Honda specifies better highway mileage for automatic transmission. Is it really so? If I commute 80+ miles per day, 95% of which is interstate without traffic, how can automatic transmission beat carefully driven manual?

This difference in gas mileage is not unique to Honda, and in fact, you will find this on some other new cars also. For instance, the new-design 2010-11 Subaru Outback gets ~2 mpg better gas mileage with its automatic (CVT) version, as compared to the manual trans version with the same engine.

Newer automatic transmissions are far more efficient, with far less slippage, than older designs. While it may still be possible for some drivers of manual shift Civics and Outbacks to beat the mileage figures for the automatic versions of those models, apparently the majority of drivers will get slightly better mileage from the automatic version.

Things change!

As efficient as modern automatic transmissions are, if there is any slippage, even minimal, it is energy loss, isn’t it? Doesn’t it mean that at constant (high) speed direct connection between engine and wheels (manual transmission) would be more efficient?
Another advantage of manual transmission is $$ difference…

I wonder if the final drive ratio of an automatic transmission vs a manual transmission might be different, and that the automatic transmission might actually have higher gearing. The automatic can quickly downshift if needed to avoid lugging the engine, so the gearing can be higher. On the other hand, there are drivers of manual transmission cars that don’t downshift soon enough and lug the engine. Maybe the manufacturers get around this problem by gearing the manual transmission lower.

That direct connection is generally achieved by a torque converter clutch. Pretty much all automatic transmissions these days lock that up once you’re cruising so it then amounts to about the same thing.

It just sounds like you just want a manual. The difference in gas mileage will be so small (and mostly affected by other things anyway) that I think you should just get whatever transmission design you want.

There is something more to this than just resolving converter slippage.
The website shows the 2012 Honda Civic mpg at:

28 city, 39 hwy for the 1.8L 4-cyl, 5-speed Automatic
28 city, 36 hwy for the 1.8L 4-cyl, 5-speec Manual

Both take regular gas.
Given the 3 mpg hwy advantage the automatic has over the manual, I can’t help but think there’s a final drive-ratio difference involved.

Consider also how an automatic beats the manual, in resale. If an auto has been well cared for, it also beats the manual as a buffer for poor driving habits as “Triedag” indicated. Not one can be perfect when shifting all of the time; today’s computer controlled autos come as close as possible. When one of the few true off road SUVs for years can only be had with an auto and the auto is recommended for for plow truck use, the economy extends beyond just the miles per gallon on the interstate.

Torque converter lockup, such that even in an automatic you have a direct connection from the wheels tomthe engine, and different gearing ratios explain why autos can get better interstate milage.

One minor annoyance is that this means the manual ends up being louder on the interstate because the engine is revving higher. Combined with honda’s typical loud road noise, it can be negative factor. It’s something to consider on a test drive.

In addition to torque converter slip, an automatic also has a very parasitic high pressure oil pump to drive. This is usually a fixed displacement pump with the excess oil being dumped with a blow off valve.
Perhaps future automatics will have variable displacement pumps that regulate pressure by reducing piston stroke, as is already being done in commercial hydraulic pumps for presses etc. When oil flow is reduced to zero, the pump becomes very easy to turn in such a design. This design would also reduce the parasitc loss of engine lube oil pumps.

I would think that the final drive ratio would be the determining factor although I don’t know what the ratios are on new manual and auto Civics.

It’s quite common to use a lower ratio on manual transmission cars so that the clutch will take a little less abuse when taking off from a dead stop. Otherwise, with a high ratio many drivers may have a tendency to ride the clutch a bit.
This problem would be less noticeable on a larger displacement engine with a lot of torque.

All you need to make life easy on the clutch is a low ratio for first gear. You can still have a tall high gear for highway economy.
I believe the reason manuals are frequently geared lower in top gear is because manuals are often bought by people who want sporty performance and these people want “no downshift” acceleration so they get cars that are permanently downshifted.
I have heard more than one person complain “fifth gear is usesless” or “this car is totally gutless in fifth gear” back in the days when manual economy cars did have deep overdrive fifth gears.
I have always countered with “what do you think those other four perfectly good gears are for?”

From the late 1930s through the mid 1960s, Borg Warner offered an automatic overdrive unit that went behind the manual transmission. Many manufacturers offered this as an option. Cars equipped with the overdrive came with a numerically higher (lower gearing) than did cars without this option. I assumed that this was so that overdrive equipped vehicles would have better acceleration when the overdrive was engaged. I also think that the automatic overdrive was used rather than an overdrive gear incorporated into the transmission was that some drivers would lug the transmission rather than to downshift. The automatic overdrive automatically disengaged when the speed dropped below about 30 mph. It was engaged by releasing the accelertor above 30 mph.

5-speed manual:

Gear Ratios: 1st: 3.143, 2nd: 1.870, 3rd: 1.235, 4th: 0.949, 5th: 0.727, Reverse: 3.308, Final Drive: 4.29

5-speed automatic:

Gear Ratios: 1st: 2.666, 2nd: 1.534, 3rd: 1.022, 4th: 0.721, 5th: 0.525, Reverse: 1.957, Final Drive: 4.44

If you multiply 5th gear by the final drive ratio, the manual is 3.12 and the auto is 2.33. For every wheel revolution, the engine with the manual rotates 3.12 times and for the auto, it rotates 2.33 times. This information is for the 2012 Civic. I doubt that it changed much if at all in one year.

Even with the 3mpg Hwy mileage difference for the auto, I’d still get the manual trans. In the real world I don’t think the difference would be as great. The manual trans will need less service and less expensive servicing compared to the auto trans which will recover whatever is lost by spending a bit more on gas.

Toyota Yaris 3 door liftback.
5 speed manual
1st 3.545
2nd 1.904
3rd 1.310
4th 0.916
5th 0.815
final 3.722
5th gear overall 3.033
EPA mileage 29/36

Same car with 4 speed auto
1st 2.847
2nd 1.552
3rd 1.000
4th .735
final 4.237
4th overall 3.114
EPA mileage 29/35

So, autos aren’t always geared higher than manuals nor do they always get better mpg.

“So, autos aren’t always geared higher than manuals nor do they always get better mpg.”

Cars are geared to provide the driving experience that the manufacturer thinks their prospective customers want. The engineers provide what the marketing departments thinks will sell cars.

Mfrs. can claim any fuel mileage number with the EPA as long as the fuel mileage does not exceed what it will be if the EPA should happen to spot check a vehicle. The reason for this is because automatics are more profitable than manual transmission cars so a lower mpg number can be claimed for a manual. Note that the 2011 Cruze eco figures are 28/42 for manual and 26/37 for the automatic. For that particular car, the maker wants to advertise the maximum possible mileage number which overrides the desire to sell a more profitable automatic.

Even though my manual transmission Yaris is rated 29/36, I usually get over 40 mpg, sometimes as high as 44-45 when the weather allows me to not use AC.

I think most manual cars could use another gear (usually 6th) and those postings of gear ratios support that. Manufacturers would like to make only one version, so they are trying to make the manual’s advantage in fuel economy go away, thus eliminating consumer’s desire for them, and allowing further economy of scale profits for themselves.

Or so the conspiracy theorist inside my head would say.

My Yaris has P185/60R15 tires on it and according to the tire manufacturer’s website, this tire has an overall diameter of 23.9 inches and after crunching the numbers that translates to 844 revolutions per mile or 844 wheel rpm @ 60 mph.
This would put my manual Yaris engine RPM at 2560RPM @ 60 mph. If it had the Civic automatic’s 2.33 ratio in a 6th gear that would take the engine rpm down to 1967 rpm @60 mph. A gear ratio that tall would likely only be useful for coasting down hills or towards red lights or in situations where it takes very little power to maintain cruising speed. I would bet if you watch the Honda Civic’s tach while you drive, you will see that the automatic transmission uses its 4th gear as much as it uses its 5th gear on the highway.
On a hill and dale road, the automatic with its deep overdrive 5th will be in 4th on every upgrade and in 5th on every downgrade.
With my 5speed manual, in that same scenario, I would be in 5th on every upgrade and in neutral on every downgrade, getting virtually the same fuel efficiency.