MPG and manual vs automatic transmissions

[LDB]

I've read and heard arguments for both getting better mpg than the other. Does anyone know of a reputable source for information on which transmission will give the best mpg with all other factors being equal? I want to put together a truck that will get the 10.5 - 11 mpg that a number of drivers have told me they are getting. Thanks for any and all input on this.

Leo
truck 4958

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[davekc]

Information will be difficult to find on just transmissions.
Type of engine, rear end ratio, type of truck, will all effect MPG.

Depending on all factors, a 10 speed manual in one truck may net 7mpg, and in another net 12 mpg.

Forgot to mention computer settings

Davekc

 

[LDB]

I know it won't be too easy but that's why I said all else being equal. One says the torque converter eats mpg and another says the auto shifts more smoothly and evenly and gets to the optimum gear sooner so it's better mpg. I hope to find out how two identically speced trucks, except for transmission, would perform on mpg with the transmission choices being either auto/man but all else the same.

Leo
truck 4958

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[davekc]

I would say a manual or auto shift would be very simular.
A full blown automatic will be less.
On a full auto, the torque configurations are different in addition to a great difference in weight.
Davekc

 

[mcbride]

We have the Allison heavy duty 6 speed automatic transmission and a Detroit 50 series engine in our truck. We were told up front to expect 1 to 2 miles ~less~ per gallon compared to the same truck with the Detroit 50 series and the auto shift.

I forgot to add that we weight 26,000# completely empty and get approx 8.5 to 9.0 miles per gallon. This MPG does not ever get higher or lower even if you consider load weight, hills, climate, etc...I.E, We get the same fuel milage running unloaded on flat roads at 55 MPH as we do loaded with a 15,000# load, on 9% grade hills at 70 MPH!


mcbride-
O/O-D unit-Team-6yrs
--What goes around comes around--
LEGAL NOTICE: The views and opinions expressed in this post are those held by the posting author and do not represent the views or opinions of Expediters Online.com, On Time Media, LLC, it's agents, subsidiaries, affiliates, advertisers or sponsors.

 

[LDB]

I spent some time talking to the Volvo salesman today. We discussed mpg and the effect of engine and transmission choices. He said the Volvo engine is 2-3% better mpg than the Cummins. He said the Freedomline 12 speed automatic is 2-3% better mpg than a manual. That's if rear end ratio, gvw etc. are all otherwise identical. He also said setting the engine for the lowest power setting would probably be most economical provided it wasn't being run at max gvw most of the time or otherwise run toward the max of rpm/power. That goes along with what I figured.. get a big block and set it to the lowest setting so it's just loafing along and not working anywhere near it's max potential.

Leo
truck 4958

Support the entire Constitution, not just the parts you like.

 

[Broompilot]

Won't the Engine be (LUGGING) since it is not performing but running at such low RPMs?

Cannot speak for my truck, but if I put my mtcy in to high of gear, forget it it starts jerking and won't run right. I know I am comparing two different engine types here but this is what the Engineers feared would happen in my Cummins when they switched the Transmission from a 5 to a 6spd.

Finially got someone at Allison with some brains who said do it the rpms are way to high where I was at. But if you added which cannot be done a 7th gear the truck would lug, its perfect now 1400 rpms at 60.

 

[Loose_Cannon47]

Engine RPMs in are a function of axle ratio and tire size..regardless of engine size or horsepower, it needs to be operating in the optimal RPM range according to the engine manufacturers specs...while producing enough power to perform the required work...big engine loafing is way better than small enging working it's heart out just to pull your truck down the road...

Loose

 

[davekc]

I spent some time talking to the Volvo salesman today. We discussed mpg and the effect of engine and transmission choices. He said the Volvo engine is 2-3% better mpg than the Cummins. He said the Freedomline 12 speed automatic is 2-3% better mpg than a manual. That's if rear end ratio, gvw etc. are all otherwise identical. He also said setting the engine for the lowest power setting would probably be most economical provided it wasn't being run at max gvw most of the time or otherwise run toward the max of rpm/power. That goes along with what I figured.. get a big block and set it to the lowest setting so it's just loafing along and not working anywhere near it's max potential.
Leo

In a tractor configuration I think the salesman info is accurate.
Using a Volvo as a straight truck changes a few things. With that bigger engine, a majority of the time you will be running a tag axle down, or running a tandem. That will drop your fuel economy. Volvos are very heavy on the front end.
With regards to the 12 speed, they are new and pretty expensive.
Because of them being new, finding qualified service and or parts could be an issue.
Just an observation
Davekc

 

[Loose_Cannon47]

Good points, Dave...and all that needs to be considered...my personal preference is with the manual...while you may have clutch replacement issues to deal with, I think probably the overall service costs outweigh the initial and long term expenditure of the automatic...I'm certainly not an expert here, just my personal observations...I'm not sure about the tag/tandem issue...with the tandems, you are pushing them all the time (no different than a T/T unless it's a single) and with the tag, it should only be on the ground when you need the added load capacity...with the larger (loafing) engine, I think I would be inclined to shoot for a higher axle ratio, as long as you can keep the engine turning in the optimal rpm range...

Loose

 

[davekc]

You could still do a 10 speed autoshift. Reasonably priced, and have past the test of time. Service and parts are readily available. They do cost more than a manual, but some of that can be recovered through resale, mpg's, and some of the maintenance.

Adding a tag changes all kinds of costs.
Everything from taxes,insurance,licensing,maintenance,tires,loss in fuel economy and initial investment.
You would certainly want to be with a carrier to recover all of the costs. This cost is even more magnified when running a tandem.
My personal experience is that they have never paid for themselves unless it came on a used truck.
But, that may have to do with the carriers we have been affiliated with.
Davekc

 

[Loose_Cannon47]

Good points, for sure...I'd like to see some real life numbers just to satisfy my curosity...my response regarding the tag/tandem issue was strictly from a fuel economy standpoint...the other costs related to tags, purchase and installation costs, toll savings, etc. are another whole forum issue...I guess I'm just not understanding how the fuel consumption issue would be different on a T/T vs. a straight truck when it comes to tag/tandem axle installation...

Loose

 

[mcbride]

>Adding a tag changes all kinds of costs.
>Everything from
>taxes,insurance,licensing,maintenance,tires,loss in fuel
>economy and initial investment.
>You would certainly want to be with a carrier to recover all
>of the costs. This cost is even more magnified when running
>a tandem.
>My personal experience is that they have never paid for
>themselves unless it came on a used truck.
>But, that may have to do with the carriers we have been
>affiliated with.

Yes--Dave, it does depend on what carrier you are affiliated with. Our truck having a tag/pusher was a necessary add-on as we spec'd the truck originally as a FedexCC D unit. At that time, their D units required a 13,000# pay load capacity so we had to have a pusher installed to meet their requirements. Therefore, with regard to the investment it has paid off for us as we are not able to carry any load weights above 6,500# with out it. In addition, we now can carry tractor freight, although this alone would not be a good enough reason to have one installed IMO, as we have not been used enough as a tractor to justify the cost of the axle. With regard to tires, we have to replace our original tires now as they are weather checked..they still have very deep tread.


mcbride-
O/O-D unit-Team-6yrs
--What goes around comes around--
LEGAL NOTICE: The views and opinions expressed in this post are those held by the posting author and do not represent the views or opinions of Expediters Online.com, On Time Media, LLC, it's agents, subsidiaries, affiliates, advertisers or sponsors.

 

[LDB]

I spoke to a driver with a D unit based on an '01 Volvo 770. It had a Detroit 430 and 10 spd but I am not sure if he said autoshift or automatic. I'm pretty sure it was automatic as I believe he said something about nice not having a clutch pedal. In any event, he said he is getting 10.2-10.3 mpg with it running at 62-64 mph. That is what I would expect as far as mpg from a good class 8 D unit.

I may be somewhat short of that because if I do it I'll put an ARI or ICT or similar sleeper on it which will up the weight a bit. I don't think it will make too much difference though. I'm also thinking about super singles on the rear because of less weight and better mpg than duals. I'm still debating a lot of options while waiting to see if this is going to happen or not.
Leo
truck 4958

Support the entire Constitution, not just the parts you like.

 

[Gary and Linda]

Hey Leo
I've found, over the years, that small power works best for town trucks and big power works best for highway trucks. The reasoning behind this (my opinion)is:
1) town trucks are always using the torque/horsepower curve, so the lower setting saves fuel
2) hwy trucks get best fuel economy when idling along at the lowest rpm (as per mfgr rating). That way they are still in the optimum torque curve, but still running at the lowest rpm possible.