I hate to comment on an old thread, but do NOT put your car in neutral going downhill in a 5 spd. the ECU has to keep sending fuel into the engine since none of the kinetic energy of your car is being used to keep the engine going. Keep it in gear, and the momentum of your car will keep the engine going without using fuel. And really, you should be getting at least 30 mpg +. I get 36-38 through hills and curves and driving fairly fast

 

I used to think this was good advice, but I have been convinced otherwise and feel your advice needs to be qualified:
With the transmission in gear, yes, the fuel is cut off and the wheels turn the engine. However, there is a cost. The engine is hard to turn, so what is called "engine braking" occurs. You will lose velocity/momentum and will not be able to coast as far as if you shifted to neutral, resulting in the need to use extra gas to regain the velocity/momentum lost to engine braking.

So I would modify your advice to read:
If the downgrade is sufficiently steep to require braking to avoid going too fast, keep the car in gear and allow engine braking to help avoid using the brakes. Otherwise, switch to neutral and coast longer. The small amount of gas burnt keeping the engine idling is well offset by the less that is need to be burnt after the coast is completed.

I just got my Civic yesterday so I have not been all the way through the manual yet, so if it advises against shifting to neutral while the car is in motion, due to potential damage to the automatic transmission, then certainly that advice should be followed.

 

My Si gets about 25-30 with some gear skipping,coasting and of course some hard accelerating at times. It should get low 30s on easy hwy driving. It has 0W-20 Mobil 1 in it now.

I had a '04 Civic EX auto that got a low of 29 and a high of 38 with ac off in winter. it had 120+k when I traded it in.

I had a 93 LX 5-spd that got as high as 43 hwy w/ac off...no 10% ethanol back then; it got stolen in 2006.

 

I get 30 or so and I drive way to hard for that car. Honda civic ex2008

 

Mythbusters did some research about clean or dirty cars do better mileage
Don't remember the result :S

 

The result was that clean cars do slip through the air more efficiently therefore get better fuel mileage. Polished and waxed will do even better. As for throwing it in neutral going down hill.....even in gear while going downhill with your foot off the throttle the ECU still fires the injectors each rotation just like any other time. It doesn't stop giving the engine fuel all together. Coasting downhill is more efficient. I get 28-33 mpg in my 99 SiR depending on how I drive. I have a K&N intake, DC header, magnaflow cat, yonaka ss catback with a phearable chip. The best I've ever gotten was 37.5 mpg on the highway. The OP should be getting beter than he is however I think he already knows how to get better mileage. Stop honking on it everywhere you go and put a muffler back on it.

 

This thread is over a year old. Why are people bumping old threads?