...... So if you want to get the best mileage maybe you might consider installing a vaccum gauge.
I have asked this a couple times in other threads, but haven't seen the question answered: do vacuum gauges work on diesels?
__________________ "If a man does not have an ideal and try to live up to it, then he becomes a mean, base and sordid creature, no matter how successful." Letter to his son Kermit, quoted in Theodore Roosevelt by Joseph Bucklin Bishop, 1915
I have asked this a couple times in other threads, but haven't seen the question answered: do vacuum gauges work on diesels?
Nope. There is no butterfly to vary the vacuum. The only thing you will see is a slight increase as the RPM goes up due to airfilter/manifold restrictions. A boost gauge will give some indication of the amount of fuel you are burning
__________________ Hi Ho Silver RV!
Vernon, Sarah, Mac the Border Collie - 'Epiphany' the 29' Airstream and a honkin' long 34' named AlumaTherapy
Seems to me on this trip wind and hills were the factor for fuel consumption. Overall avg on 7.2k and going is 12.3. Immediate checks vary from 14 down to 7. I find now running on e85 and quick checks at 65mph and a tail wind is 12.6. Not bad for 28 safari and 2 buck a gallon corn fuel.
I use the cruise because my leg feels better at the end of a long day on interstates or straight two lanes (I prefer to make my own decisions on curves). I think it gives me better mileage, but never did any real comparison. But, sometimes going up a moderate grade to maintain 65 towing, it downshifts (sounds like 3rd or 2nd out of 6) and if I were in control, I'd have let it slow down a bit. It doesn't know enough (no eyes) to upshift near the top of the grade. I figure I can do that better, but can't keep the speed perfectly even on the flats. Everything's a tradeoff, but I wish cruise had a better brain and eyes too. At least my partially repaired right knee feels better. I have surrendered to cruise.
My truck tells me how many mpg I am getting at any one point. I like it when I rolling downhill and it reads "99" mpg. I wish it went higher, but there's no third digit. Going up a pass it reads 3 or 4, so I don't like looking at it then. Between feeling bad and watching the numbers change constantly, I turn the readout back to the temperature so I don't run into anything. I don't know how accurate those average mileage readings really are. In two other trucks they were 1 to 2 mpg high—I figured Toyota wanted us to feel good. The new Tundra seems to be really close to the old reliable: figuring it out at the gas station.
Days when we're traveling on back roads and going 40 to 45 we get the best mileage. We don't get very far. Another trade off. Boring interstates get us there, fewer stops in campgrounds make up for the increased fuel cost. What I try to control is accelerating too fast. I think that's where the most fuel is burned for us guys who love our trucks, but wish we still had those sports cars we had in our 20's. My Tundra would leave my TR4A or MGB in the dust off the line, but those cars were a lot of fun to drive and gas was cheap.
I think you are looking for a curve called the BSFC, or Brake specific fuel consumption. It maps the point where an engine is making the most power relative to fuel consumption. Most modern engine can vary enough factors via the computer to give a fairly flat curve at most throttle and RPM settings. The big factor that over rides the small gains is drag. We ran curves for some older diesel engines and they had a pretty specific BSFC peak that was near the torque peak. Then came computers, variable valve timing, fuel injection...
I think for a gas burner the best way for a common Joe to get a feel is to pick a reasonable speed that fast enough where you don't get run over. If the question is OD or No OD, lock your right ankle at your favorite speed in OD, let everything stabilize, then pull it out of OD. If it speeds up - don't use OD.
__________________ Hi Ho Silver RV!
Vernon, Sarah, Mac the Border Collie - 'Epiphany' the 29' Airstream and a honkin' long 34' named AlumaTherapy
Most modern diesels are turbocharged, so there is no vacuum. There is manifold pressure. Duramax diesels can run up to 30 PSI, but typically run about 22 psi max.
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CP 9 miles off Exit 399, I75.
2003 GMC 2500HD 4X4 D/A Ext. Cab
Propane Powered Honda EU2000i
Lots of Hot Sauce! Air # 283
The vacuum gauge does offer some level of feedback to the driver on how far they have the foot stuck into the floormat, which opens the airflow butterfly, which is measured by the CPU, that measures the mass airflow to the intake and the rpm from the crank, and tells the fuel injector when and how long to squirt fuel.
The three areas a Vacuum gauge will help increase fuel economy are
1) teaching the driver to hold a steady throttle position on flat land
2) learning it is possible to climb small inclines with minimal addition of throttle
3) Assist in learning how early one can roll of the throttle when topping a rise
Simply using it as a feedback tool can help us correct some undiscovered poor driving habits. (although I personally have NO bad driving habits...)
__________________ Hi Ho Silver RV!
Vernon, Sarah, Mac the Border Collie - 'Epiphany' the 29' Airstream and a honkin' long 34' named AlumaTherapy
Unless your intersection is at a speed between 40-45 mph the answer is NO. Here's why:
Your engine may be working at its most efficient at the intersection however wind resistance is a drag (pun intended) that doesn't increase 40,41,42,43 where each mph represents a 2.5-2.0 increase in resistance between 40 and 50 mph. Rather 41 has perhaps 9% more resistance than 40 and 42 has 10% more than 41; in short drag increases dramatically out of proportion to a minimal increase in speed.
Engine efficiency is only one factor but by far the most important factor and it becomes a bigger and bigger factor for every mph faster is wind resistance (drag). If the intersection is much over 45 mph then the slightly less efficient engine at 45 mph should use less fuel than the optimum engine at 55 mph with 125% (and this figure is probably low) more drag
In short, once your running in 5th or 6th gear, whatever your truck has, lower wind resistance (drag) trumps absolute engine efficiency.
General info...... The fuel economy sticker on new vehicles is less than it has been in the past (ie more accurate) because of the change in standards thus reflecting more of the actual economy.
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