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A friend and I just finished taking out a blown original automatic transmission out of my 04 Honda Civic LX and replaced it with a JDM lower mileage automatic transmission. We put the original starter on the new JDM transmission. The issue I am having is that during acceleration the transmission seems to be starting in second gear so the acceleration is very slow, but if I move the gear shifter down just a bit from "D" to 3 you can feel the tranny shift to 1st and them you have to almost manually shift the transmission (by moving the gear shift back and forth a bit) in to 2nd and 3rd then after that the transmission with auto shift. I do have the check engine light still on and the entire time I have the car turned on the "D" indication light is flashing. I really have no idea what is causing this or what is the fix, and would love advice and help with this issue! Thanks

The codes I get are: P0705, P2770, P0700,

 
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adjust the range sensor with the shift on neutral

 
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adjust the range sensor with the shift on neutral

 
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I have same jdm transmission and what's bothering me is the higher rpm than usual, at 70mph I get 3050rpm or so and 80mph almost 3500rpm.

 
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That's normal for these JDM transmission, it is geared shorter than the USDM ones. Mine is the same way.

USDM DX/LX/VP auto ratios
1st: 2.722
2nd: 1.469
3rd: 0.975
4th: 0.674
Final drive: 4.07

JDM SLXA ratios:
1st: 2.722
2nd: 1.516
3rd: 1.078
4th: 0.711
Final drive: 4.36

Green=JDM, blue=USDM

 
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Did you ever get the transmission to shift correctly? I’m having the same exact issue

 
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Perfect, you are in the right place. I will take a photo and upload to show you what I did with mine and is very simple. You need to add a stopper so it doesn't move down. Find the green light stays D and secure it with a stopper. I will up load what i did with a piece of plastic and 2 screws.

 
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That's a cleaver solution. When I had a JDM auto I just put a piece of electrical tape across the base of the shifter where drive was so I could easily see where drive was until I got used to it and then took the tape off. If I remember correctly, drive was about half way between D and D3 on mine once I adjusted the cable but I swapped to a 5 speed almost a year ago so I don't remember for sure.

 
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Cool, so you have a 5speed transmission automatic now? On a 01 to 05 civic?
Mine shift very good but I dislike the high rpm on highway. Got to stay between 60mph to 65mph to be tightly under 3000rpm which bothers me a lot cause that's too much and 3000rpm and up don't average a good gas mileage. I wish I had a magic gear to set it on low and do 70mph steady at low mid 2000rpm

 
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Nope I have a 5 speed manual in my 05 Civic 1.7 VTEC. The JDM auto was trouble free for the ~2 years I used it, I just wanted a manual.

Mine did the same thing, that's just how the JDM transmissions are geared. 70 MPH was a tad over 3K and 80 was around 3500. The EX VTEC 5 speed manual is even higher. In my case the higher RPM didn't hurt the gas mileage, I was typically in the 31-34 MPG range with both the USDM and JDM autos. I think the reason for that is because I live in a hilly area and unlike the USDM trans, the JDM trans could easily climb the hills in 4th gear without downshifting or unlocking the converter.

Of course the 5 speed manual I have now is way more efficient than either auto, I normally average 46-49 MPG and sometimes get 50+ MPG with the 5 speed and I have not yet averaged below 38 MPG with this trans although I am a somewhat hardcore hypermiler.

 
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Wow, impressive mpg and good to know this. You're exactly right same rpm here, more power for sure and it gets past 100mph quickly easily very responsive with JDM transmission.

I do food delivery in the city and is constantly stop and go with all traffic and stop signs. I am averaging now 320miles a tank to about 11/5 to 12gallons. When the car was fairy new i once measured 420miles a tank plus, i have yet to try a fully highway trip on this JDM and after I did the head gasket this year. Which everything resulted good so far.

 
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In the type of stop and go driving you describe, I could see the shorter geared, closer ratio JDM trans actually being more fuel efficient since it will accelerate easier at those low speeds and it can get into 4th gear and lock the torque converter sooner. I also preferred the shorter gearing since it noticeably improved the acceleration and responsiveness with no measurable MPG loss compared to the stock transmission. For only being a 4 speed, I think the JDM ratios were about as good as it gets in my opinion. It was a really great transmission, I definitely wouldn't hesitate to buy one again if I wanted an auto.

But either way, the way you drive in traffic will affect your MPG way more than the gearing differences. The biggest tip I can give you to improve your MPG is to drive like you don't have brakes. So in traffic, leave plenty of room ahead of you and try to maintain a reasonably constant speed as much as possible instead of accelerating and braking. And when there's a stop sign ahead, let off the gas early so you have just enough momentum to roll to the stop sign. Same with turns. Remember that every time you press the brake pedal you are turning your fuel into brake dust and heat. It takes a bit of practice to get the timing right, but it's well worth it. Out of all the hypermiling techniques I use, driving without brakes has by far shown the biggest gains in traffic.

 
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Thank you very much. Very good technical information.
I wondered if with a manual transmission and taller set of wheels but light weight, the mileage would improve? I'm sure in highway it will
I will try the best I can, another problem driving in this city people want to run you over if I do much of that, normal driving became a thing of the past here. Crazy place

 
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A manual transmission definitely has the potential to be much more fuel efficient if driven efficiently since it gives you much more control, is way more mechanically efficient, and is 60+ pounds lighter. If highway MPG is your main goal, the HX 5 speed is what you want (if you can find one) since it's geared the tallest. It's 5th gear is slightly taller than the US auto's 4th gear, which is great for efficient highway cruising on reasonably flat ground. The HX (VTEC E), DX/LX (non VTEC), and EX (VTEC) transmissions all have different ratios and you can even mix and match the ratios if you want to make something custom.

Bigger wheels will throw off your speedometer/odometer and probably hurt your city MPG, but could help on the highway since it effectively makes every gear taller. Years ago I got lightweight alloy 15's (stock size on mine) to replace the heavy steel wheels for an estimated 30-40 pound savings. I wasn't tracking gas mileage as closely back then but I did notice the ride was smoother from the reduced unsprung weight and I felt like the acceleration was a little better spinning up the lighter wheels but that could have been in my head. Lighter is always better though. Did you notice any drop in MPG with the JDM transmission? If not, bigger wheels probably won't help.

Understood, that's definitely a consideration. Fortunately where I currently live there usually isn't much traffic since I do a lot of driving on country backroads, but growing up in Chicago I definitely understand the frustration.