what is the mileage difference between a 4 speed man. and a auto. transmission

[mike davis]

my tdm got 9-11 mpg  with a 671/4 speed What is the mileage with an 8V71/Auto.

oh the TDM converted weighed 29,000

                     thanks

                                      mike

[Dallas]

on a good day.... 8 on a normal day about 5.5 to 6.5 I won't tell you what a bad day of climbing into a headwind pulling a 18' flatbed trailer is.

[mike davis]

on a good day.... 8 on a normal day about 5.5 to 6.5 I won't tell you what a bad day of climbing into a headwind pulling a 18' flatbed trailer is.

Dallas,

Lets say one of my best friends uncle owns one of the largest diesel salvage yards in the North East.  Lets say on the weekend said friends uncle likes to line his pockets with a little bar money his wife doesn't know about and it not on the books.  And, the currency conversion rate is very favorable. 

Would it be worth switching one of these more modern coaches Like the one I'm looking at to a four speed?  They have every machine known to mankind to fabricate linkages, etc. etc.  There might even be a coach that I could yank them off of.  They have the big smasher.  It smashes buses.  But they can't sell buses 

What I'm looking at is half the fuel millage from an auto.  Is it the 8v71 vs the 671 or is it the tranny. 

Thanks,
Mike

[Utahclaimjumper]

The size of your foot has to be factored in..>>>Dan

[HighTechRedneck]

What I'm looking at is half the fuel millage from an auto.  Is it the 8v71 vs the 671 or is it the tranny. 

An automatic transmission is a little less fuel efficient, but I've never seen, or even heard of, it cutting fuel economy in half.  I've always understood that, all other things being equal, a manual transmission will get 1-1.5mpg better mileage.   If one is getting half the economy of the other, I would beleive it to be a combination of several factors.

• Engine
• Transmission
• Engine and transmission health
• Injector size and tuning
• Total operating weight of the buses
• Driver habits (even if it's the same driver, the extra power can go to ones head and from there the foot)

[mike davis]

so It's the 8V71 vs the 671 at 30,000 lbs also a steel bus


         mike

[Jerry Liebler]

Mike,
    The fact that there is more horspower available with the 871 means it can burn more fuel, However if the 871 is driven so no more power than the 671 could give is used it is possible to get the same mileage if gear ratios, weights, etc. are the same.  But most of us use all the power we've got accelerating and climbing hills.  In the case of 'V' drives the popular V730 has a lower gear ratio than the typical 4 speed in addition to being less efficient.  More modern fuel controls, turbochargers and 4 stroke engines all are more fuel efficient than the WWII designed 2 cycle Detroits.
Regards
Jerry 4107 1120

[Beatenbo]

I have owned 4 MCIs

MC8 8V71 Auto 7mpg
MC9 8V71 4sp 7mpg
96A3 6v92T 5spd 7-8mpg
102C3 6V92T Auto 7mpg

I run highway miles @ 70-80mph if limit and traffic permits 30-40 K a year. 65 mph will drag every coach I ever owned down on the grades. I keep mine wound up when safely possible

[mike davis]

Mike,
    The fact that there is more horspower available with the 871 means it can burn more fuel, However if the 871 is driven so no more power than the 671 could give is used it is possible to get the same mileage if gear ratios, weights, etc. are the same.  But most of us use all the power we've got accelerating and climbing hills.  In the case of 'V' drives the popular V730 has a lower gear ratio than the typical 4 speed in addition to being less efficient.  More modern fuel controls, turbochargers and 4 stroke engines all are more fuel efficient than the WWII designed 2 cycle Detroits.
Regards
Jerry 4107 1120

Jerry,

So I guess besides the TDM aluminum exoskeleton construction and the newer coaches steel rib construction, the addition of ? 35% ? more power, then adding an automatic transmission are what reduces the gas mileage by 35-40%?   

I think the 8V71 was introduced in '56 or '57? but it wasn't the total factor in the less miles per gallon.  I thought it would have been a shocker to greyhound as an example to absorb 35-40% loss of economy at one shot.  Even though diesel fuel was 10-12 cents a gallon then. 

Just trying to wrap my head around the loss of 40% of the fuel mileage.  Also, thinking that there might be a single cause, but apparently I was mistaken. 

My TDM was governored out at 73MPH.  I usually drove it between 65 and 70 because that was what the laws were at the time.  I did notice that when I kept in wound up it got better fuel mileage.

Thanks everyone for all of your help and insight

Mike

[HighTechRedneck]

Just trying to wrap my head around the loss of 40% of the fuel mileage.  Also, thinking that there might be a single cause, but apparently I was mistaken. 

A 40% drop (assuming that weight and driving terrain/habits are similar) still sounds like there might be something wrong.

Going back to the driving habits though.  Especially if it is driven in an areas with a lot of grades or even just rolling hills.  Keep in mind that 8V71 will climb a grade faster than a 671. Likewise, a 6V92TA will climb it still faster and a 8V92TA will climb even faster.  But each will burn progressively more fuel to accomplish it.  All other things remaining equal, faster climbing takes more horsepower and that takes more fuel being burned.

Likewise, if both are driven in a city or other environment where frequent stops/starts or at least accelerating/decelerating is common, the fuel economy difference of larger engines to smaller ones will be more pronounced.  The larger engine will get rolling/accelerate faster, but use progressively more fuel to do it.  Once it is up to speed and on average terrain, driven at similar speed and with all other variables equal, fuel consumption should be comparable.

MC8 8V71 Auto 7mpg
MC9 8V71 4sp 7mpg
96A3 6v92T 5spd 7-8mpg
102C3 6V92T Auto 7mpg

40' RTS II 6V92T V730 Auto 7mpg (30,000 pounds plus towing my pickup)

Maybe some folks here with 671 experience could post their numbers to give a good comparison across the full range.

[TomC]

With my AMGeneral transit pulling my MB 300 turbo Diesel (34,750lb total) cruising at 58mph and 1850rpm I averaged 5.15mpg on the round trip to Las Vegas.  But with the 8V-71TATAAC engine I have, it is such a pleasure to drive it, not having to down shift but once going up the big hill at Baker and Cajon, and that it doesn't smoke at all anymore, I don't mind 5.15mpg.  Good Luck, TomC

[Jerry32]

88 102A3 8V92T 6.5 to 7.5 at 65 to 75 MPH 740 auto Jerry

[steve5B]

   What you also might keep in mind, the quality of fuel (cetane rating) that the engine is running will make all the differance. The national advarage is 42.5.numbers.  Higher the number, the better the preformance.  2-EYTEL-HEXEL NITRAIT is the componet they put in diesel fuel for combustion.  The more BTUs , and less pre- engine knock with this product will produce unbelieveable results.  I say this because I'm in that type of business!


MY 2cents worth.

Steve 5B.....

[JohnEd]

Steve 5B,

A while back there was a post about adding "acetone" to your fuel.  Gas or diesel both were claimed to benefit significantly.  The theory, as I vaguely understood it, was that the acetone aided in the atomization.  I said vaguely!  The amounts added were very small and I thought almost any increase in power/efficiency would have justified the expense.  This was claimed to be based on some real science.  I think cetane rating was something it boosted.

You said you were in the business and I thought you might be able to comment with authority.  Anything that adds power or decreases cost in of interest to me.

Thank you,

John

[Charles Seaton]

Hi Mike,

The 8V71 was first put into buses in 1961 (GM PD4106)  The fuel hit was probably worth the extra power.  As good an engine as the 6-71 was and even the 6V71, they are both underpowered in some circumstances.  Are you still considering the 4905 or have you moved on?


-- Seaton