Need some help identifying what's killing my mpg

[Ecky]

I'm trying to figure out what's hurting my fuel economy most. I get ~30mpg city, ~30mpg highway. I realize that the stock cd of 0.35 isn't very impressive, but I think at highway speeds it's mostly the gearing that's killing me. I've been doing 99% of my driving in the city lately though, so it's not an immediate concern.

I've done 2 tanks with pulse and glide (engine on), and I've seen no discernible change from my old mileage. In the past 8 months, I don't think I've ever had a tank below about 29mpg, and not one over 33mpg, regardless of whether or not I play stoplight hero or drop it in 5th at 15mph (which I can do, because of the silly gearing). I expected P&G to help significantly by keeping the load high while at midrange rpms and letting the engine slow down between pulses, but it has been within margin of error thus far. Does this sound odd at all? Will I need to do engine-off P&G to see any real improvement? I'm not comfortable shutting it off in city driving.

If I'm going to be travelling at ~45mph between stoplights, meaning low aero drag at my typical speeds and needing to avoid holding up traffic too much, what should I be looking at to improve my mileage?

I'm already very adept at predicting lights and minimizing using the brake, and I keep my tires topped off. I've been wanting to coroplast the bottom, but I don't think aero is my biggest enemy at 0-45mph... what do you guys think?

[BHarvey]

Aero comes into play anything above 30 mph. If you don't HAVE to go 45 between lights, don't. If 45 is the speed limit in those zones, this means you don't HAVE to go 45.

In town, if you can get by doing 30-35, then by all means do it.
This means you will spend less time accelerating and more time at P&G speeds.

Keep revs as low as possible but high enough the car does not lug and struggle to pull you along.

[hamsterpower]

Judging by the pictures in your garage, having an engine that was not a factory option is a big part of the problem. If you do have the SI model Del Sol, you probably have extremely sporty gearing. Mix that with the B18 non VTEC engine I see in those pictures and the short ram intake, and the cards are stacked against you. All that considered and those MPG numbers are not bad. I wonder what other mods are hiding in that engine bay.

You might try starting in second or even third gear for slow city driving.
Can you describe your P&G technique? Your fifth gear is nearly the third gear in most eco cars. Also these Honda engines go into fuel enrichment fairly low on % of throttle. I have the B16 engine and my SGII shows 70% LOD at 25% throttle or less during P&G.

[Ecky]

I may well be over-throttling it, I don't have a scangauge yet (backlog and I'm not confident in making my own without a good guide), I'll try backing off.

The tranny IS short, the car was stock with a B16 - though I do see better mileage with the LS motor than I did with the old B16, probably due to a leaner burn and better stroke ratio. It also has an aftermarket big exhaust pipe - 2.5" I believe, and I've been tempted to look for a junker and put a ~2" pipe back on. It's very likely robbing me of some low-end torque.

I'm not even sure what timing I'm running, but it's not too far off what it should be. I reused the distributor from my B16, and at it's most advanced setting I think it's close to 16° BTDC, though I don't yet have a timing light to check with.

My tires are still the sticky tires that were brand new when I got the car, so that might be hurting me a bit. I keep them inflated at around 40psi - max sidewall on the fronts is 50 and 44 in the rear.

----

I can drop it in 5th at 15mph, as that puts me at just over 800rpm, about where the engine idles, but I'd have thought that would be less of an issue if I use P&G, and at city speeds.

Typically I skip 4th gear, and sometimes I skip 2nd (only use it in situations like when I'm rolling up to a light really slow and it changes). I use 1st to get me rolling, 3rd to ~20mph and then drop it in 5th. I P&G with about half throttle, clutch in and let it drop back down to 800rpm. In typical city driving here I can P&G about 75% of the time, if traffic isn't bad and the speed limit is, say, 45, I might P&G between 47 and 37, and if there's more traffic, I narrow it to 5mph differences and try not to annoy anyone.

Attached is a diagram with my gear ratios.

[IsaacCarlson]

It takes more gas to get a vehicle moving faster. We all know this. One thing I have done is get up to about 25 and then speed up slowly the rest of the way.

Another thing to do is not get up to full speed on short roads or between lights. If it is only a mile to the next stop, get up to 35 and hold it. Then coast to the stop. Getting up to 25 takes about the same amount of gas either way you do it. 25+ is what matters the most. Just remember to take it easy and not drive like you are in a hurry.

[hamsterpower]

As you have a '94, unless you converted it to OBDII scan guage will not work. You will need an MPGuino or similar.

Try to find a stock intake pipe and filter box. That will, likely help more than the smaller exhaust. Consider an LS transmission, I got mine off ebay very cheap. The ratios are better matched to your engine and better for economy, but sporty enough to still enjoy the car you have.

There is a difference from loading the engine and going for peak power. Like I said earlier. Mine only needs 20 -25 % throttle to see 70%+ LOD at low RPMs. Many say this is ideal for Hondas of this era. 50% throttle is a bit high and probably deep in fuel enrichment. Ease off a bit and try P&G from 40 to 30 when you can.

[Cassem]

I agree with IsaacCarlson. You really should get a mpg gauge to learn the idiosyncrasies of your car, though.

I noticed that in my car I tend to get worse numbers when I do straight P&G. Once I am up to speed I will apply the lightest pressure to the pedal to maintain speed. The instant consumption gauge shows between 48 to 99 mpg then. If I pulse and glide, that time spent accelerating at about 22mpg ruins the >99mpg I get on the glide.

[slowmover]

Gear choice tells me what rpm range I'll be working in (diesels have a very short range). The upper speed limit tells me the do not exceed. As an easy rule of thumb I chose 5-mph under that limit to be the top cruise speed.

And the trick has been to accelerate to 5-mph under that. In a 40-mph zone I'll be runnning 35, so acceleration tapers off as I near 30-mph . . . from there I'll drift on up if when it's a good choice. I may never actually reach 35-mph. And so much the better that others flow around me that I may glide towards and through the next stoplight. No lane changes, no extraneous steering input, etc.

For me, all else proceeds from this.

.

[Ecky]

Originally Posted by hamsterpower

As you have a '94, unless you converted it to OBDII scan guage will not work. You will need an MPGuino or similar.

Try to find a stock intake pipe and filter box. That will, likely help more than the smaller exhaust. Consider an LS transmission, I got mine off ebay very cheap. The ratios are better matched to your engine and better for economy, but sporty enough to still enjoy the car you have.

Sorry, I meant MPGuino. They're backlogged right now so there's no way to know when I'd get one if I donated today, and I don't know of any guides to building one.

I'd like to swap the tranny but it's not work I've ever done before. I'd need some more jacks. I'd like to get any LS tranny though.

As for intake vs exhaust, I was under the impression that restricting the intake was at best neutral on efficiency (and possibly bad), while a correctly sized exhaust could actually net efficiency gains. This is incorrect?

Thanks all!

[user removed]

Focus on your driving environment. Drive like you have no brakes. Coast as much as possible in neutral, engine on. Learn the timing of traffic lights. Accelerate in each gear at 1300 to 21-2300, 800 is to low.

When you start off in traffic after the light changes green accelerate with the normal flow of traffic while using the highest gear that provides that acceleration without any more than half throttle.

Any time you can coast your getting literally over 100 MPG at any speed over 30 MPH. Don't obstruct the flow of traffic, but don't be aggressive around others. The most important thing is to never sacrifice your inertia unless you have no other choice. Coasting through a light as the traffic ahead of you gets up to speed is your ideal situation.

Unless your daily drive is thought really nasty traffic you might get to 40 MPG by simply adjusting the nut behind the wheel. Once you get there then think of modifications to further improve your mileage.

regards
Mech