Power/fuel usage at various speeds in an 80

[IdahoDoug]

Craig Vincent from New Zealand published these numbers on another forum. I'm assuming they're accurate but emailed him for a source to be sure. I'll let you know what he says.

Makes a case for slowing down a bit on the freeway as going 75 takes about 75% more power than going 62 does. I agree I'd need something to keep me awake too, but a recent trip of 900 miles I'd have saved around $80 in fuel costs. Basically, power output is a proxy for fuel use. I get a steady 13.5 MPG at 75mph, so I should get 18 or so at 62. Not sure I could have done that , but heck, just knock a 5mph off the freeway speed and I'll pay for my road food and a new CD enroute.

MPH HP
31 2.9
50 11.9
56 16.9
62 23.2
68 30.9
73 38.2
75 40.1
99 95.1
118 159.3
124 185.8
186 627

DougM

 

[NorCalDoug]

Assuming my math is correct...
900 mile trip @ 75 MPH would take 12 hours

900 mile trip @ 62 MPH would take 14.5 hours

Ruling out time for refueling, rest stops, etc.
Would $80 be worth 2.5 hours of time on the road?

 

[T Y L E R]

Can anyone then comment on how the unsprung rolling mass of a 35 MT will come into play; in general?

How hard can it be??


TY

 

[LandCruiserPhil]

I also found going slower will get you better mpg but sometimes conditions dictate going 80. Last year we traveled 4000 miles and the 55 mph secondary roads netted the best mpg the worst was 80mph and gaining altitude in 100 degree plus heat but our overall average was 13.5mpg. The 1 to 11/2 possible mileage increase is not worth me paying attention. I’m on vacation and want to relax not worry about maintaining any given speed.

Another interesting find looking back on the records was gas prices. Only a 10% increase on gas prices from June of last year. So not a lot to complain about in my book, right now.

 

[Gold Finger]

I think that driving at where the maximum torque of the engine produces the best fuel efficiancy and ideally this would be at a speed that does not require to much power to overcome drag. So it is a ballence. Torque bands are reasonably broad so finding a suitable turing speed should be easy enough. It may be easier on the diesles perhaps as the band peaks at about 1800 rpm with 266ft-lbs. The Uk official figures for 1993 HDJ80 with a 1HD-T engine which are government aproved are in imperial gallons for the


auto constant 75mph 19mpg constant 56mph 30.1mpg

manu constant 75mph 19.8mpg contant 56mpg 31mpg 31

 

[RavenTai]

going 75 takes about 75% more power than going 62 does.

DougM

but you are covering more ground at 75 that cuts into the that 78% gain. definaly will get better milage at 62 but I dont think you will get 18 MPG


would be interested to know how those #'s were come up with

 

[OZCAL]

Craig has a pretty fast truck!

 

[landtank]

900 / 13.5 = 66.67 gallons consumed
900 / 18.0 = 50 theoredical gallons comsumed

a savings of 16.67 gallons and at a total savings of $80.00, so you pay $4.799 a gallon for fuel. Here in MA we only pay 2.099 a gallon so my savings would only be $34.90. Still a good amount of cash but not worth the danger of being the only one on the crowded road doing 62.

 

[rgsiii]

Does anybody with an 80 series really get 18 mpg at 62 mph? My numbers when completely stock, were 16 mpg going at about 65 mph on an extended trip. I was trying to see what my best fuel economy would be. I assume that the 3 mph difference would account for some of the variance.

 

[landtank]

best I've recorded was a tad over 16. I think doug has something wrong with his trucks as they get far worse mileage than mine and i'm lifted, geared, sporting 315s and running at 80~85 with 14.2 mpg.

 

[IdahoDoug]

Yeah, I'm not sure on this data how linear the power usage is with gas usage, nor am I sure about it's predictive correlation (meaning at a certain speed would you really get 18mpg for instance). There has to be some factor that needs to be added that comprehends the basic limitation of actual MPG high and low.

When new, we used to regularly get 16-18mpg at steady 70-75 speeds.

 

[97 FZJ80]

Let's do our math here.

62 MPH = 23.2 HP
75 MPH = 40.1 HP

(40.1-23.2)/40.1 = 42% more efficient @ 62 MPH compared to 75 MPH AT ANY GIVE INSTANCE.

Assuming Doug's travel example as at 75 MPH, if he would have traveled at a steady 62 MPH, it would net him 42% more efficient fuel consumption at any given time and NOT 42% increase in MPG.

Now let's look at the efficiency over time.

(75-62)/62 = 21% more time (more engine running time) to cover the same distance when travelling at 62 MPH than 75MPH
At 62 MPH, it takes 21% more travel time (and fuel) than 75MPH to cover the same distance.

(40.1-(23.3*1.21))/40.1= 30% total efficiency over time, or MPG savings.

Again, taking Doug's 13.5 MPG @ 75 MPH example. If Doug were to drive at steady 62 MPH, he would net a 30% total savings or:

13.5 * 1.3 = 17.55 MPG

This is assuming all factors are constant.

 

[Cattledog]

With the original Mich. LTX I would get 17 to 18 mpg with 75 % highway driving. I replaced the tires with BFG TA KO 285/75-16 and have yet to see better than 14 mpg. The 14 mpg is not corrected for the increased tire diameter. My wife has the lx most of the time now and it sees only many short trips. I am getting about 11 to 12 mpg.

 

[turbocruiser]

Does anybody with an 80 series really get 18 mpg at 62 mph? My numbers when completely stock, were 16 mpg going at about 65 mph on an extended trip. I was trying to see what my best fuel economy would be. I assume that the 3 mph difference would account for some of the variance.

Yes 18 mpg even exceeding 75 up to 80mph! I've even gone through the trouble of gassing up, hitting the highway almost as soon as pulling out of the station, running a half tank or so and then gassing up again right after pulling off the highway and into the station. Granted, I had stayed at speed the whole time and was not passing or braking any at all so that helps too. HTH

 

[Riley]

Assuming my math is correct...
900 mile trip @ 75 MPH would take 12 hours

900 mile trip @ 62 MPH would take 14.5 hours

Ruling out time for refueling, rest stops, etc.
Would $80 be worth 2.5 hours of time on the road?

In my books this is a big fat no.

An extra 2.5 hours on the road per day is a long time, even to save $80.

On the other hand, you guys drive fast down there. Averaging about 65 maybe 70 miles/hour is pretty fast up here if you're on a long trip. I try to not go more than 10% over the speed limit (for ticketing reasons) which translates to about 68 miles/hour on most highways in BC.

Note that I exceed my rule of thumb lot's of times but only in certain well known routes.

 

[speyrod]

I just got back from a 3K trip , mileage varied from tank to tank.. A low of 10.88 MPG pushing 85+ to a high of 17.59MPG The trip averaged 14.49MPG, I went from Sea level to 8500 ft. down to 3,000 to 7,500 to 200 ft below death valley and then the nasty climb out to 395 back up to 8,000 in short order. Then up 395 which climb some altitude as well.

Stock 96 loaded at the family truckster, 265/75 BFG ats

I'm looking to do more of these road trip I wish the 80 would cruise comforably at 85, 80 is great 85 seems like its working. Time is something you can never get back.

 

[cruiserdan]

Does anybody with an 80 series really get 18 mpg at 62 mph? My numbers when completely stock, were 16 mpg going at about 65 mph on an extended trip. I was trying to see what my best fuel economy would be. I assume that the 3 mph difference would account for some of the variance.

Not 62MPH but at 55MPH. On a trip to SLC. 33 inch tires on OEM steel wheels and a supercharger. Pre sliders and Slee rear bumper.

This snip is from my fuel log:

8/9/2001 TEX MCELLO 87729 295.5 15.68 1.859 PREM $29.01 18.85 TOP SPEED 55MPH
8/10/2001 UNK SLC 88028.8 299.8 16.26 1.599 PREM $26.00 18.44 TOP SPEED 55MPH


From Albuquerque to Monticello UT 295.5 miles, 18.85 MPG

From Monticello to SLC 299.8 miles, 18.44 MPG.

I was flat-towing a Dodge crew cab pickup with my Dodge-Cummins. I could not go over 55 mph as it got very loose and unpredictable. My wife was following me in the 80 and that's what kept her speed down.

The return trip at "normal" speeds was back down to the mid 14's as usual.

 

[Riley]

Dan - I'm really impressed at your records/logs. It's like a freaking science experiment (probably not far off).

Top speed? In most cases I wouldn't want my wife to know my top speed (when I can get away with it).

I can rarely be bothered to track my mileage as of late. I will on big road trips but that's about it.

 

[cruiserdan]

I guess I shouldn't tell you that I've filled it 512 times for a total of 8,958 gallons then, should I?

 

[IdahoDoug]

Wow, exceeds even my anal logging detail. I can tell you that on 3-03-02 I noted "6 small faintly corroded circles inside the fuel tank near the fuel pump" but I've never logged fuel use. You're a freak.

DougM