ok... I got an 05 mack, 8 speed, flat top.

it'll run 72mph and get 6mpg

5.5 loaded and 6.5 empty on the govenor (hauling liquid chemicals)

for the kicks of it I wasted my friday and saturday driving just as good as I possibly could.

57mph loaded 1300 rpm, shifting low and lugging it up the hills...
6.6 mpg loaded !!!!

58mph 1325rpm empty... 8.7 mpg

roughly 7.6 mpg for the thousand mile test. a nice mix of interstate hills and back road fodder.

what a scary pain in the arse... and not my fuel so that's the end of that... but

you could definately put another 50-60 a day in your pocket with careful driving.

and...

this may sound hard to believe but... one day took an extra hour... the other took an extra 90 minutes.

I run the routes regular so I have my logs to defer back to. which I did...

cuz... creeping along it really felt like it should taken me twice as long to get where i was going and back home...

something else... some of the hills I hit at 70 normally and am running 45-50 at the top, sometimes having to downshift...

runing slower... the thing just chugged up over the top at 55.

this kinda thru me for a loop... hit the hill slower and finish faster made no sense at all...

second day I pretty much narrowed it down... (I think)

running slow... the truck was cool and the fan didn't kick on...

runing fast... the fan was running all the time just about and definately before I topped the hills.

read somewhere the fan uses about 15hp ???

so by running slower I had more power available when I needed it most.

I could be wrong???? any thoughts on that line of thinking?

also... If/when I get my own truck I will definately be running slower and banking that $300.00 a week in fuel savings....

__________________
Though your argument is very clever, I don't think it will lead to the results you desire. gandhi

Depends on how long the hill is as to whether the fan kicks in,one reason the higher horsepower engines often do better is that they don't have time to get hot so they don't run the cooling fan. 15 horsepower might be an average, but it uses more at high rpm and less at low rpm, the fan is also controlled by a refrigerant pressure switch, if the AC has too much pressure, the fan will be running even though the engine isn't too warm.

If your engine isn't running too slow to run efficiently, slower speeds will help economy, and not take that much more time to run the run.

I drove a setup for a long time that was terrible, 2.64 rears with a straight 10 speed with a 12.7 Detroit. you had to run 64 in high to be turning 1400 rpm, the truck actually did better at pulling a chicken trailer a gear down at 55 mph turning 1500rpm. High gear was lugging it unless you were going at least 68 mph, too fast for those hard pulling chicken trailers. The same tractor pulling a reefer up a long mountain grade would overheat unless it was run near the top of the horsepower curve at about half throttle. A new radiator fixed that

I had to make 2 runs with a Western Star, geared tall with a 13 speed OD. That truck did better in 12th at almost any reasonable speed than it did in 13th at any speed. Half a mpg better fuel economy.

I have about so much time to run out and get loaded, going slow isn't much of an option. the cargo has to be loaded on time and back at the plant almost as soon as possible.

15hp wont get you 5mph up a hill lol. Definetly something weird, maybe you were empty? lol

__________________


LOAD EM LIKE RAIL CARS, DRIVE EM LIKE RACE CARS!!!!

not empty, I had 48k of aqua ammonia on...

my other thought was it might be all in my head...

wind, ambient air temp, hemroids itching??? lol

maybe because I was in "test" mode I just didn't down shift wanting to see what would happen...

vs.

being full throttle I dropped a gear when it got close???

I know there is a huge difference between 7th and 8th. 7th is 1600 rpm at 55mph which might be out of the torque curve for the motor so it naturally slowed down to 50mph.

I dunno??? just reporting the observations...

__________________
Though your argument is very clever, I don't think it will lead to the results you desire. gandhi

Thanks for this tidbit...I took my truck in to Penske because the air-conditioning was working poorly (would kick off on hills and then take 30 minutes before it would start blowing cold air again)...the air conditioner works fine now, but anytime I run the ac the fan kicks on and off at low temps. Would anyone happen to know the high/low pressures that the system should have?

__________________
Neal


Support the Semper Fi Fund!

Wind it up! Accelerate it!
'nother hundred miles in a dead man's shoes,
Diesel fuel, I smell burnin'
White line fever and the interstate blues.

I don't know the pressure but with the ac on the fan should continually cycle on and off. every minute or so

The Star has good AC but it also cycles the cooling fan unless you are driving down the Interstate, the Columbia has a bad fan switch but the AC switch works good , it doesn't cycle on and off much unless I'm idling, but I shut that one off at every opportunity. It might be that the switch is off a bit on the pressure side. If the AC works when it is hot that is the main thing right now, not working means you are sitting in a heavyweight sweat lodge. If you get near that Penske shop, you might get them to look into it further.

I drove a 96 FLD small sleeper for a couple of months one winter a few years back, the cooling fan never kicked out and the AC never kicked in, even though I was in Texas on an anti-freeze peddle with it, and the temp reached into the high 70s. That shop wouldn't check it out while I was in it,but they fixed it later, that's when I found out the high limit switch was bad

I miss that darned Columbia. I can back that thing into a shoe box without pulling up