Downshifting VS Coasting 5MT

skyline44512 · #1 09-05-2010, 06:31 PM

I've always just pushed in the clutch in my Civic and coasted towards lights. It's a 2003 5MT.

Someone recently told me that when you're decelerating a car burns no fuel. When getting off a freeway ramp I usually put it in neutral and brake a little when I have to. Same thing heading towards a red light. Would it be more beneficial to downshift and do some engine braking? I've always just figured to keep the RPMs as low as possible, so at 800 rpms coasting I figured the mileage was better than at 3000 RPMs decelerating.

hyperme · #2 09-06-2010, 09:45 AM

Quote:

Originally Posted by skyline44512

I've always just pushed in the clutch in my Civic and coasted towards lights. It's a 2003 5MT.

Someone recently told me that when you're decelerating a car burns no fuel. When getting off a freeway ramp I usually put it in neutral and brake a little when I have to. Same thing heading towards a red light. Would it be more beneficial to downshift and do some engine braking? I've always just figured to keep the RPMs as low as possible, so at 800 rpms coasting I figured the mileage was better than at 3000 RPMs decelerating.

Generally, if you leave it in gear, and your RPMs are above approx 1000 (this differs a little, car by car), and you're coasting, you will get DFCO (deceleration fuel cut-off), where no fuel is being fed to the injectors. So your mpg's during DFCO are infinity. However, you will also get some engine braking since it is in gear, which slows the car down considerably more that neutral coasting. So, which method is better depends on how long you need to coast. If you find yourself needing to brake during your coast (like you mention), then leaving it in gear would be better. If you have a very long coast, such that if you coasted in gear, only to have to re-accelerate, then a neutral glide would be better. But, keep in mind that once your RPMs get below ~1000, even in gear, you will start burning fuel again. Many people will downshift at this point just to keep their RPMs above 1000.

VegasDude · #3 09-07-2010, 05:14 AM

If you install an engine kill switch you can have the best of both worlds and go into DFCO at any RPM, although it gets bumpy if you don't push in the clutch before you come to a stop.

SentraSE-R · #4 09-07-2010, 12:21 PM

The only way to know is to monitor DFCO with an Ultragauge or Scangauge.

My SE-R hates to go into DFCO. It has to be warmed up and going over 2500 rpm to see DFCO. 2500 rpm is ~65 mph in 6th gear, so it rarely sees DFCO. If I FAS the car, it takes a full four minutes of driving to re-enter closed loop and be ready for DFCO (DFCO puts it into open loop, but it has to be in closed loop to enter DFCO).

My xB, OTOH, loves DFCO. It'll enter DFCO at 1500 rpm, and won't drop out of closed loop even after minutes of FASing, so it's almost always ready to enter DFCO. 1500 rpm in 5th gear is ~35 mph, so the xB is almost always ready to go into DFCO.

WriConsult · #5 09-07-2010, 02:14 PM

If you need to burn off speed anyway (coming to a red light or a corner) then keep it in gear, because your momentum can keep the engine spinning and DFCO is smart enough to cut off fuel in that situation.

If you're trying to coast as far as possible (trying to glide to the end of a long exit ramp) then take it out of gear and coast in neutral. In neutral you WILL burn some gas to keep the engine from stalling, but that will be more efficient than having the engine drag your speed down.

PaleMelanesian · #6 09-07-2010, 02:25 PM

^ what he said. FAS (engine off coasting) is best, then engine-on neutral coasting, then DFCO, except when you HAVE to stop anyway. In that case, you may as well use engine braking and save fuel and your brake pads.

On your civic, if it's anything like my two hondas, DFCO is always there if the rpm is above 1200. When it drops below that point, you can feel a slight surge as the fuel flow is restarted. Downshift to keep it above 1200 and you'll not burn any fuel.