FE not so good for Civic
 

I was hoping for 35-40. I get 28.9. I drove it hard for the first tank full and got 28.9. Second round I did 110 miles mostly highway @ 65-75 and refilled and got 28.9. I was being easy on the throttle.

Here's the specs of the HB:
95 VX Shell
CX 5 speed trans (it looks like one)
D15B Vtech performance JDM
P08 ECU stock programing
1 wire O2
17" Rims 205/40/17 tires
Stock suspension
Stock bumpers
Stock air intakes
Stock exhaust
SI doors with power windows / power locks
I do have AC not being used right now.
CX seats

Car shouldn't be much heavier than the VX. I doubt weight is the problem. I check the air filter and plugs and both look clean.

Suggestions? taller tires? O2? P28 / 4 wire O2 and lean it out?

Drive 60 instead of 65-75. Coast more.

You will get better mpg with a four wire oxygen sensor since it has a built in heater to speed up its operation and allow the engine to go into closed loop operation quicker. I am surprised that 1 wire sensors are still in use.

And - your tires are too wide. Swap to a narrower size if possible.

Stop using AC if you can. If you need the AC defroster, you can see how well it does with the defroster on with the AC off - I believe you turn it off by pressing the circulate air and AC buttons with the key in the off position, and then in the on...they'll blink at some point. Not sure if applies to your civic though, it works on my 04.

i would say most OBD1 cars are single wire.

i would almost suspect somehtign ebign clogged on your car. my truck would routinely get 25mpg no matter what or were i drove or how i drove. i took the blasted egr valve off and mpg instantly shot up.

how well is your engine maintained?

First of all, the P08 (stock DX/LX ECU) uses a FOUR WIRE SENSOR. Using a single wire sensor is going to cause you to get a check engine light and a code 41. And this is going to make your engine run rich and KILL your mileage. Remedy this, and your mileage is sure to increase. Also, are you sure you are using the correct ECU for this motor? The JDM ECU for this motor would probably be best. Or you could use a P28. By the way, I have a Civic 4DR with a B20Z CR-V motor, and a CRX with a B18C5 ITR swap. Neither of these cars are really built for economy. And I drive these cars like jet fighters. Yet both will give me over 30MPG on the freeway. So something isREALLY, REALLY wrong here.

But there is something else to consider. A tall transmission may not be the best way to go with this motor. This motor was never made to pull a CX transmission. So it may not operate very efficiently cruising down the highway at under 2000RPM. Lower revs are not necessarily better. After all, I tried swapping a 2G HF transmission on my 1989 Civic. And I gained NOTHING in terms of fuel economy as a result.

What are your crusing RPMs at? I think you might not have a CX tranny.

But the other mentioned things are a great start to raising your MPG.

I would also do a complete tune-up on the d15 including plugs/wires.

It's the right ECU for the engine. The 1 wire O2 is what these use. I know it's not the VX tranny, it doesn't have the LSD stamp, so I doubt it's the D15B tranny. The engine timing belt cover has LSD written on it. I bet the tranny was sold seperate. Should be the CX tranny, since it was the donor car. I don't have a tach right now, but the RPM doesn't seem low to my ear. I'll get a tach soon.

Spark plugs looked good. Doubt that would improve anything. It's not setting any ECU codes. Runs great. Perhaps an O2 change. I'm sure it's the original one.

If tires don't make that much difference then I'd like to keep this set of rims. I have a set of stock steels I can use. But they need new tires.

There is no H in VTEC.

Looking for an LSD print is a very inaccurate way to tell if you have LSD, most of the time print is rubbed off. The only way to properly tell is by pulling an axle out of the tranny.

If there is a bar seperating the two sides you do not have LSD, if there is no bar you have LSD.

You have somthing seriously more wrong than a weight issue, or a tranny issue to cause your low MPG.

My B16a gets 33-34 MPG with lots of VTEC and poor fuel economy driving in a VX chassis.

This is very true, my B16 is very inefficent shifting at low rpms, I get better FE by shifting at 4000 rpm where the motor is more efficent.