Transmission/gearing question
 

I have a couple of questions about converting my 2005 Honda Civic with a 4 speed auto to a 5 speed manual. My first question is what would the overall effect on gas mileage be if I install an EX 5 speed trans? I initially thought that highway MPG would stay approximately the same and city mileage would improve.

However, I found out that the trans that I am looking to install (An EX 5 speed, which has more aggressive gearing than an LX or HX trans) would cruise at a high RPM. With my auto, the engine runs at around 2300 RPM going 70 with the converter locked, but with the EX 5 speed, the RPM would be more like 3000 going 60. I am not sure if that would mean lower highway MPG. You would think it would, but the manual transmission has much less losses, even cruising with the converter locked, so the mileage may not drop.

The other option I have is to get an EX 5 speed trans and swap the 5th gear to a taller gear from the LX trans before I install it, which would drop cruising RPM at 60 to around 2800 IIRC. Would that help gas mileage measurably?

Thanks in advance for your input

Also, I forgot to mention that I am also wondering about the effects of a higher cruising RPM on engine longevity. I can't imagine that cruising at 4000 RPM all day going 80 (like you might on a road trip) would be great on the engine. However, the load on the engine would be lower too, so for all I know the higher RPMs might not cause any more wear.

The manual transmission will provide better fuel economy around town, but the automatic will likely get better fuel economy on the highway, due to its taller gearing, unless you regularly do engine off coasting in the manual.

With many ecomods and slow, careful driving, my '04 auto Civic had a best highway trip of a hair over 60 MPG for over 300 miles. To get that kind of mileage in the city required extreme hypermiling techniques.

To beat the auto's highway fuel economy you would need to use a taller geared transmission.

Thank you, that's pretty much what I thought. Oh and btw 60 MPG highway is seriously amazing, the most I ever got on the highway was around 40.

I am not going to go with the LX or HX manual trans, so I guess the best I can do is get the EX trans and swap to an HX 5th gear.

The major difference between EX and HX transmissions is in the final drive. Swapping the 5th gear would help but I think you're still going to lose fuel economy on the highway.

*However* pulse and glide can largely eliminate losses from too-short gearing, if you're willing to use it.

I'd personally use an HX transmission if I had your car. If you need more power you always have a lower gear, with the exception of first.

I agree with these posts. I'm not sure what the specific gearing on the automatic transmission is. But if your swap is going to make your car hum along like a buzzsaw on the freeway, you are going to see a decrease in fuel economy. And someone is really, however, the main advantage of a manual transmission is the ability the cut the fuel injectors and Coast, to do the so-called pulse and glide technique. So it's true that in the city with a manual transmission you'll be able to get better gas mileage in an automatic almost no matter what manual transmission you have. It's just nothing better than for gas mileage averages than Infinity.

Great point! I know it would be hard to say, but what do you think the difference in mileage would be between an EX and HX trans? The EPA says that there is a 1 MPG loss on the highway with the manual vs the auto with a Civic EX, but that doesn't really mean much to me.

Also, that's a great point that you can always downshift for more power unless you are already in first gear, but when you are in 5th gear you can't upshift. However, I do like having good off the line power. Even though I have substantially improved the mid range and top end power from stock, the low end torque hasn't improved a whole lot. And besides, I would like the clutch to last as long as possible.

As for pulse and glide, sorry, but am not willing to do that. I understand that pulse and glide is efficient because it basically eliminates pumping losses, but it is inconvenient, illegal, causes extra wear, and it is (by my standards) not safe.

Thanks. With the gearing of the EX 5 speed, the RPM would be 3000 going 60, 3500 going 70, and 4000 going 80. However, my buddy Rudy has one of these cars with similar mods and he gets 38 MPG on the highway going 70, so that's pretty good IMO. He said that these engines like running at a higher RPM, especially with the cam that I have.

EPA figures I found say 34 vs 39mpg EX vs HX, and most of that is going to be in the transmission. Lean burn helps a bit but I don't think it's the holy grail some others make it out to be. Now, that's an average fuel economy mind you, and the highway test has mixed driving, so I'm going to guess at 15-20% increased economy during steady-state driving.


The off-the-line power isn't too substantially different, but pick the gearbox that's right for you. Don't let me tell you what to go with.

:thumbup:

Final drives:
EX: ~4.41
DX/LX: ~4.11
HX: ~3.84
Hybrid: ~3.60

For what it's worth the HX actually has a shorter first gear (10%) which offsets half the difference of the taller final drive.

https://ecomodder.com/forum/showthre...ios-26279.html

^ Looking at the ratios here, a pretty good frankentransmission might be a DX/LX transmission with the HX final drive - you get a shorter 1st/2nd/3rd/4th and keep the tall tall tall 5th. Either that, or put the 1st/2nd/3rd from a DX/LX into the HX box.

I know it's not remotely apples-to-apples but in my Insight I'm still on the original clutch at ~250k miles and it still feels like a healthy clutch to me. It has an even taller 3.23 final drive, and power is "fine" in 1st gear even without the hybrid system enabled (accelerating with only the 66HP 3-cylinder). It's only the very tall 2nd gear that I take issue with; I'd prefer a shorter 2nd gear when lacking the torque from the hybrid system.

They produce good power at high RPM and run smooth as butter, but ultimately it won't be good for the lifespan of the engine. You'll probably still get 250k out of it, sure, but at that point I'd bet it will be drinking a lot of oil. Get the RPMs down and you'll keep your cylinder walls and piston rings much longer.

Thank you for all the great information! My plan was to get the EX 5 speed and swap to the DX/LX 1st gear for even better off the line power, as well as swap to the HX 5th gear for less noise and more MPG on the highway.

Also, it's not just in the numbers for me. I personally don't think I'd like the way the car feels when it is cruising at a high RPM for a sustained period of time because it would sound noisy and to me it would feel like it is working harder than it needs to. I also imagine that when cruising at 4000 RPM at 80, releasing the throttle in gear would feel like hitting the brakes.

As for engine longevity, unnecessarily high cruising RPM probably wouldn't be great. Even though the load on the bottom end would be lower, a lot of parts that are unaffected by load would be moving a lot more than necessary, which would have to cause more wear. Parts like the water pump, tensioner pulley bearings, camshaft, rocker arms, valve stems and guides, timing belt, and maybe piston rings would wear out more quickly without a doubt.

However, I am not as concerned with engine wear as I am with performance and how the car feels driving. My thought is I needed to replace the engine after 200K hard miles because of a water pump failure. I was really upset and disappointed because I did EVERYTHING I could to take care of it to the absolute best of my ability and make it last as long as possible, and everything I did ended up being for nothing because the water pump failed, despite being an OEM part that I installed 20K miles ago. :(

So nothing I did ended up mattering anyways. It\'s possible that I could install the HX trans to try to get the engine to last as long as possible and then something stupid happens and I could lose this engine too for some stupid reason anyways.