Most gas engines likely create more hp per unit of fuel as top gear (highway) speeds increase and the engine is run closer to its BSFC peak, but that's not quite the same as saying "your engine will use less fuel at 80 mph than at 60 mph".

Modded V8's / drivetrains are going to have very different BSFC maps.

I still find it hard to believe that a 5-speed 4-cyl Accord uses less fuel at 60+ mph than 50, steady state.

Quite Possible
 

Hi Folks,

Although I'm new here, I do believe that what the original poster is saying is quite possible. Let's look at a simple scenario - How do we get 30 MPG:

1) Travel 30 MPH at a flow rate of 1 GPH
2) Travel 60 MPH at a flow rate or 2 GPH
3) Travel 80 MPH at a flow rate of 2.67 GPH

So, from case 2 to case 3 the fuel flow increases 33%, but he's gotten there 33% faster, because he's going 33% faster. That's the simple case. Now let's complicate it a bit.

When you increase the velocity, the drag increases as a square of that. However, maybe the engine and tranny also runs more efficiently at the higher RPM, so that it counteracts the increased wind resistance. The end result may be that the increased efficiency overcomes the increased drag (i.e. the flow rate doesn't increase linearly with the speed of the vehicle).

It's not that he's using less fuel at 60+ mph. It's that he's getting more distance out of the increased amount of fuel he's using, which changes the MPG ratio.

How can you scientifically test this?

Start at point A, drive to point B at an average speed of 75 MPH, then drive back. Measure the fuel economy. Then start at point A, drive to point B at 55 MPG, then drive back. Do both trials under the same conditions. Post your results.

You can also use a ScanGuage to watch your fuel efficiency, but that's already been said, and IIRC you have OBD1, not OBD2, so that won't work for you.

Cheers,

Joe

I think the main thing to consider is the trip...
My best tank ever was on a full highway trip drafting a bus...

If you do the same exact trip slower than you could compare your mpg. But comparing my highway trip where there is no stop and go traffic, not having to warm up the engine etc, to my daily commute is no comparison.

You didn't say anything about the automatic tanny: Does it have a lock up torque converter? If it doesn't, there could be a lot of torque converter slip at lower speeds, and this would torpedo your mileage at low speeds.

A similar thing happens in my F350. The torque converter will simply not lock up under about 45 mph, no matter how much I baby the throttle.

I've been looking for a chip to reprogram the tranny, but no luck so far.

It can, but this tends not to happen with a couple exceptions. Big vehicles with big engines and auto trannys, since getting the tcc to lock up can really increase efficiency, and may not happen at lower loads. And... relatively aerodynamic cars with really big engines and manual transmissions, since the change in BSFC compared to load can match or outpace the change in energy compared to speed. However, just about every other car/engine combo out there can and probably does have a drop in efficiency wrt over most of it's OD/top gear range.

My CRX is getting the same FE at an average of 75mph on the fwy as it did when I was averaging 65mph. This is averaged out on multiple fillups, so I can only conclude that the HF engine is more efficient at 2500 rpm then it is at 2000 rpm to make up for the increased drag.

It's not related to engine speed much, if at all. It's related to engine load. Provided your statement is in fact correct, since I'm not there to verify it and I have no way of knowing what you are and aren't controlling for. ;)

Edit- I'ma callin' BS. A stock 49 or CA HX should have near ideal gearing. Imo there's no way it'll get exactly the same FE at an average of 75mph compared to 65mph provided a controlled test.

It's not inconceivable at all. My Dodge van with 360 got its best FE at 70mph. I could never break 15.5 @ 55, 60, or 65mph, but at 70mph I was pulling nearly 17mpg.

I think it has alot to do with rpms as the cam timing and intake manifold resonances are rpm related. At 2500 rpm, it may be running more efficiently because of this.

Could be... But not according to every singe BSFC map I've ever seen. I've never seen an engine geared properly that will increase efficiency by 30% if it's already near or greater than half load and minimal BSFC. Course, it could be that Honda designed a weird engine unlike any other engine, but if I had to pick between that and a lack of consistent testing on your behalf, I'm sure you can guess which one I'd pick. ;)