mc5 fuel economy - yic

[viento1]

Howdie guys,

I just sold my MC5 that I used for many years with never a problem other than no fuel gauge (I am quite efficient at priming now) Anyway, I recently purchased that 1968 MC5 off ebay last year. The coach is georgeous and was built as a motor home from day 1 and has very few miles.

The new coach only gets 5.8 MPG top speed is only 70 I run out of torque at even the slightest hill...

The only other thing I have to compare it to was my 9.9 MPG MC5 that needed constant feathering of the throttle to keep it under 80 !

Both are 8v71 both have functioning mechanical throttle linkage.
The old coach is a 4 speed spicer
The new coach is a 4 speed allison automatic
No idea on injector size for either
Neither of them smoke (once warmed up)

I hope it is not because it is an automatic - if so, why on earth did I ever change

[Blacksheep]

Automatics have always been known to produce less in MPG! 

Ace

[JohnEd]

Maybe you switched so that you could justify installing that 7 speed all synchro and getting 10.5.  Ya think?  Lots of things you can do for MPG.  Tire selection, tire pressure, alignment, synthetic in differential, syn in trans., insure trans temp is about 180, insure oil temp is about 180.  Check injectors and report size.  318 upgrade will kill MPG and increase operating RPM and the 740 would keep the r's too low to get any advantage out of the cam advance.

Were I u I wood be unhapy tu.

John

[TomC]

It has been proven in the trucking industry that with proper training, an Allison automatic should get very close and sometimes better than a manual (especially if you have a driver that doesn't know how to shift and when to shift).  The biggest fuel gulping mistake that most do with Allison equipped buses is to just floor it from a stop.  If you remember back to your manual transmission bus and feather the pedal so you accelerate as fast as a manual transmission bus, you're fuel mileage will go up.  If you have a tachometer (if not, install one-it only takes about a day to do if you have to run the wires), don't do any continuous cruising above 1800rpm.  You'll get much worse mileage between 1800-2100, and horrible above 2100.
Try having your engine tuned, and see where you're cruising the engine at when on the highway.  I think you should be close to 8mpg with a 5.  Good Luck, TomC

[RJ]

318 upgrade will kill MPG and increase operating RPM and the 740 would keep the r's too low to get any advantage out of the cam advance.

John -

The Allison in this MC-5a, with the 8V71 engine, will not be an HT-740 - it's too long to fit.  Most likely he's got an MT-644 bolted to the 8V.


Viento -

A 3-mpg drop between the auto and stick is on the high end of average difference.  What concerns me more is the lower top speed and lack of power on slight grades.

As TomC suggested, I'd have the rack run to make sure the engine's in top tune, and that the idle speed and governed speed are set properly.  I'd also check to see what the rear axle ratio is - you might find it's lower than your old coach (3.70 vs 3.33, for example), which would contribute to the different top speed.

Either way, a 2 mpg loss of fuel economy is about average between an automatic and manual box in a bus.

FWIW & HTH. . .

[Jerry32]

Maybe it wieghs more than your old coach?

[Jeremy]

I don't have any experience of automatic transmissions in buses but instinctively it seems very wrong to expect an average 2mpg difference between auto and manual. 2mpg in less than 10mpg is ENORMOUS. I know your fuel is very cheap but I'm sure if I were a bus operator I'd still be desperately upset if my vehicles were using so more fuel than those of my competitors. I cannot imagine bus manufacturers would ever sell buses with automatic transmissions if they represented such a big percentage increase in costs / reduction in profit for operators.

Jeremy

[Ed Hackenbruch]

All good points!  With my 68 5A i get from 5.9 to 8.1 mpg.  8V71 with an Allison Mt 644.  It loves to run at about 1750 rpm in 4th and at 70-75 mph.  If i run at higher rpm the mileage drops rapidly.  I usually get right around 7 mpg depending on terrain, some freeway driving, a lot of secondary state road driving.  My speedo goes to 80 and i have gone faster than that on flat ground and still had lots of throttle left. I was just telling Jim Shepard that i would like to see what the top speed is but not by a cop.  Maybe Bonneville?   There were 2 different rear ends ratios available in the 5s, you have either a 3.36:1 which was standard or a 3.7:1 which was optional.  My engine is supposed to have been bumped up to 318 hp but i am not sure if this is true......we get mighty slow on some of the long grades but we just enjoy the view and know we will get there sooner or later.

[Ed Hackenbruch]

Ps.  My bus has gained about 1000 lbs. since we started fulltiming and we now weigh 29,000 lbs. Also if i drop down to 3rd gear and run at 21-2200 rpm instead of 4th and 1750 rpm i am now running at 25% higher rpms.......and using 25% more fuel for the same period of time.

[Damn Yankee]

I consistently get 7 mpg in my 5A (8v71 & allison 644) I keep the rpm's at 1800-1900 at 65-70 mph. Jack rabbit starts will murder your mileage. I accelerate very gently and slowly.

[Ed Hackenbruch]

Also letting off of the throttle and coasting up to a stop light will save fuel and make your brakes last a lot longer too!!!

[viento1]

Hello,

I have to admit, I have it to the floor almost the entire time it is on the road. Only because I am used to racing around with the old bus.

Maybe i should dissable the Jakes and coast more (although they are a hoot)

I am not sure what the ratio is in the rear end but the shift points are exactly the same it shifts at 35km 60km 90km and the top speed is  115km. The old bus top speed was the only difference.

I believe you are correct, this tranny is a 644? The previous owner did the swap and according to Al at "buss's R us" the installer did a great job, even re enforced some cross member that most guys do not bother with?? Is the 644 a good trany?

THe bottom line is the family trip to the ski hill cost $320 instead of $150. It used to be easy to rationalize using the extra fuel and offset hotels and add to the whole event... but now, she just stays parked

Al wants $200 per new injector and 12 hours labor to "run the rack?" I have never had a tune up so this is a bit pricey. Do I have to pull the motor? remove the Jakes? my manual does not go into such detail. I have every tool imaginable. all the warm shop floor I could ask for and I am always up for an addventure...  I am a little light on the know how.

[TomC]

If you're top speed is 115kph, and figure that is 2250 on the governor, then to get to the magical 1800rpm, then you should be cruising at 92kph.  1850 will be 95, and 1900 will be 97-and go no faster then that-just try it and see.  With these speeds, your rear end ratio is the 3.73 ratio.  Maybe consider changing to the 3.33 if you're going to keep the bus for a long time-it will be worth it since you sound like you want to drive fast.  Good Luck, TomC

[Lin]

It would seem that aside from having a different rear end, the top RPM could be set differently.  To go 80 mph with a 4-speed Spicer, you would have to be running at about 2250 rpm with a 3.36 differential.  I would not expect the fuel mileage would be so great there either.  By the way, do you know for sure that the speedometers are accurate? 

[viento1]

Ok - I think I am getting the picture here. It seems that going 55mph is perfectly acceptable in the bus community. I guess I could just live with it... if I must. well, actually that may not work. I remember trying to keep up with busses on the highway all my life - I dont want to be the guy in the "Bounder" holding up traffic sucking fuel at an astounding rate.

The speedo is consistent with the gps.

I consistently get 7 mpg in my 5A (8v71 & allison 644) I keep the rpm's at 1800-1900 at 65-70 mph. Jack rabbit starts will murder your mileage. I accelerate very gently and slowly.

So Yankee, what is your top speed? shift points? How on earth do you accelerate gently. my tranny shifts at full RPM and down shifts at about 80% of full RPM. I find it hard to not just get going and end all the screeming ASAP.

Ed Hackenbruch, Same question to you...

I have a Tach ready for install - first on the to do list

One more point that may be of note. The old bus I had to sneak up on the start button - just touch ever so gently and it starts every time. The new bus gives me a sore thumb pressing that button!!!