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Hey, Lately I've been hypermiling with great success. Gets like 7.5-8.0 L/100 km in my modified Montana. However when maintaining a constant speed. I've noticed a trend in RPMs to Speed

45 km/h - lower then 1500 RPM
55 km/h - 1500 RPM
60 km/h - 1600 RPM
70 km/h - ~ same as 45 km/h
80 km/h - 1500 rpm
90 km/h - 1600 rpm
100 km/h - 1900 rpm

Does a lower RPM mean better FE? It is logical to say that driving at 80 km/h is better then driving at 55 km/h? One of you people out there know what I'm really trying to ask here.

Happy Motorin'

Thanks,
Currens

Not necessarlily. I had a conversion van that seemingly defied physics and got 1.5mpg BETTER at 70mph than at 60. I wouldn't have ever guessed it except for the fact that these results repeated themselves consistently in my MPG log.

Theoretically, your best economy comes at the lowest RPM at which your engine is pulling its peak vacuum. If you have the inclination, install a vacuum gauge in your van and see where this occurs. In GM's pushrod V6's (I presume yours has the 3400, but it should be similar for the 3800), I would guesstimate this to be at 1700-2000rpm.

There's also other factors to consider. If, for instance, your van is running at a point on your power curve where it's just not making a whole lot of torque (that is, really low rpm), you'll probably notice that you'll get into the throttle more. Certain points on the RPM curve may also have an unusual spark retard programmed into the computer. That means reduced fuel economy, once again. Other times, there are even some odd harmonics that can increase friction in your engine at a certain RPM, though this is pretty much un-diagnosible and getting into the realm of physics and engine theory.

This is the long way to say that no, your best fuel economy is not necessarily at the lowest RPM.

Hi Currens:

___It appears your GM tranny is performing as it should from this end. A lock up of the TC between 70 and 80 kph appears normal. What you will more then likely find is max FE at or around 75 - 85 kph while on level road with a very light foot. Increasing load makes for a more fuel efficient engine but the mechanical losses as well as aero and RR drag increase in a logarithmic fashion reducing the systems FE even though you are approaching an ICE’s peak thermal efficiencies. If you were SG equipped, you would find a relatively nice area in or around 75 – 85 kph in which max FE on the iFCD will probably appear. With smooth flat roads, warmer temps and little to no weather (no winds).

___Good Luck

___Wayne

Not necessarlily. I had a conversion van that seemingly defied physics and got 1.5mpg BETTER at 70mph than at 60. I wouldn't have ever guessed it except for the fact that these results repeated themselves consistently in my MPG log.

Theoretically, your best economy comes at the lowest RPM at which your engine is pulling its peak vacuum. If you have the inclination, install a vacuum gauge in your van and see where this occurs. In GM's pushrod V6's (I presume yours has the 3400, but it should be similar for the 3800), I would guesstimate this to be at 1700-2000rpm.

Whats a vacuum gauge? Where can I get one?

I'm new to the group, but thought I'd add a comment.

The best ever recorded MPG I ever got was from a DC to Philadelphia sprint in my Mazda3 5mt on a scorching summer day with 3 passengers, A/C running, and speed well in excess of the law.

Whats a vacuum gauge? Where can I get one?

An engine is basically a big air pump. The speed of it is regulated by a butterfly valve; this is the throttle. Because the engine is always wanting to pull more air in than the butterfly valve is letting in, the engine creates more vacuum in its air inlet tract. The vacuum gauge measures this vacuum. They can be purchased at most reputable auto parts houses (AutoZone, Kragen/Checker/Schucks, O'Reilly, etc). Not sure on the cost, but I'd be surprised if it ran more than $40.

A variation on this theme is the ScanGauge II. This is a much more expensive piece, but it essentially monitors the same thing and digitally displays for you what I've been told is a pretty accurate representation of your instantaneous MPG. Your vehicle is new enough to take advantage of the unit as well, but at $160-$180 depending on the retailer, it's probably only worth it if you can use some of its other functions as well to help maintain the vehicle, etc.

Hi Jcp123:

___A SG-II is infinitely more valuable then a Vacuum gauge including real time close to actual FE vs. a vacuum gauge’s what am I doing now. As far as value, it will pay itself in very short order for anybody serious about reducing their consumption. With a VAC gauge or nothing, you are literally flying blind.

___Good Luck

___Wayne