3 ms·

A better comparison would be a 5 speed auto vs an 8 speed auto in the same car (not just similar but differing by year, you don't know what else they changed li

A better comparison would be a 5 speed auto vs an 8 speed auto in the same car (not just similar but differing by year, you don't know what else they changed like maybe ECU mapping). The problem is that the ECU itself is going to be much better at optimizing the engine load, rpm and gear than someone driving a manual even with a gear shift hint on something like the EPA tests. And AFAIK the epa highway test is more like a boulevard test than a test of efficiency at a constant say 65 MPH, so gear shifting itself will cause a loss that won't be made up by the slightly higher efficiency of the manual itself since locking torq converters/etc. Thats part of the reason why people have been picking automatics, they started getting better EPA ratings, what 20 years ago when they still had roughly the same number of gears, and not long after started posting better 0-60 numbers as well. Plus, car manufactures frequently do things like change the default tire with a given package (the car you linked as differing gas tanks too). So you get the sporty version and along with the larger engine you get fatter less efficient tires. What I'm trying to say, is that you can't claim the 3MPG diff on that car is entirely just the number of gears by itself.

windowsrookie 4y ago
"A better comparison would be a 5 speed auto vs an 8 speed auto in the same car (not just similar but differing by year, you don't know what else they changed like maybe ECU mapping)." Unfortunately I'm not aware of any car that this would be possible to test. I picked the Dodge Charger because it's the closest option we have, and very little changed between those two model years except the transmission. I participated in forums back in the ~2015 era because I had a Dodge Challenger at that time. And it was quite clear that the 8 speed made a huge improvement in fuel economy in the 2015+ cars. That matches the official EPA tests.
- StillBored 4y ago
  Just based on the usual engine efficiency curves it seems unlikely that 3 extra gears would have an average ~10% efficiency/MPG gain by themselves. For that to happen the engine itself must be falling off a lot more than 10% over a few hundred RPM outside of the efficiency peak. Which seems unlikely, look at this stack overflow question, about half way down where someone posts an efficiency map. https://mechanics.stackexchange.com/questions/2161/fuel-efficiency-and-shifting-rpm https://mechanics.stackexchange.com/questions/2161/fuel-effi... Note that the most efficient curve for a given load is frequently ~1k rpm. Which given the usual gear ratios a 5 speed transmission should be more than capable of keeping it a couple hundred RPM on one side or the other of the actual peak. Its more likely a long list of optimizations where the most noticeable was the gear count change on a car that probably wasn't particularly optimized to begin with. A good part of it might have been the transmission/torque converter, but even then it had to be a pretty crummy transmission for that large of an improvement. The usual things to look for are drag improvements, weight improvements, tire efficiency, oil/transmission viscosity changes, and an untold number of small transmission and engine tuning changes to the ECUs as well as any number of subtle mechanical changes to the intake and exhaust to ignition system changes. Most of those things aren't going to show up on a white paper at the dealership. So while the 0W-30 oils and the like have an effect, the tires things as I mentioned is huge on some cars. You can walk around in the truck area of many dealerships and see 10% deltas in MPG on some of the them that are identical except for things like offroad tires and 300Lbs of bed liner.

windowsrookie 4y ago

"A better comparison would be a 5 speed auto vs an 8 speed auto in the same car (not just similar but differing by year, you don't know what else they changed like maybe ECU mapping)." Unfortunately I'm not aware of any car that this would be possible to test. I picked the Dodge Charger because it's the closest option we have, and very little changed between those two model years except the transmission. I participated in forums back in the ~2015 era because I had a Dodge Challenger at that time. And it was quite clear that the 8 speed made a huge improvement in fuel economy in the 2015+ cars. That matches the official EPA tests.

StillBored 4y ago
Just based on the usual engine efficiency curves it seems unlikely that 3 extra gears would have an average ~10% efficiency/MPG gain by themselves. For that to happen the engine itself must be falling off a lot more than 10% over a few hundred RPM outside of the efficiency peak. Which seems unlikely, look at this stack overflow question, about half way down where someone posts an efficiency map. https://mechanics.stackexchange.com/questions/2161/fuel-efficiency-and-shifting-rpm https://mechanics.stackexchange.com/questions/2161/fuel-effi... Note that the most efficient curve for a given load is frequently ~1k rpm. Which given the usual gear ratios a 5 speed transmission should be more than capable of keeping it a couple hundred RPM on one side or the other of the actual peak. Its more likely a long list of optimizations where the most noticeable was the gear count change on a car that probably wasn't particularly optimized to begin with. A good part of it might have been the transmission/torque converter, but even then it had to be a pretty crummy transmission for that large of an improvement. The usual things to look for are drag improvements, weight improvements, tire efficiency, oil/transmission viscosity changes, and an untold number of small transmission and engine tuning changes to the ECUs as well as any number of subtle mechanical changes to the intake and exhaust to ignition system changes. Most of those things aren't going to show up on a white paper at the dealership. So while the 0W-30 oils and the like have an effect, the tires things as I mentioned is huge on some cars. You can walk around in the truck area of many dealerships and see 10% deltas in MPG on some of the them that are identical except for things like offroad tires and 300Lbs of bed liner.