Gear Ratios

twodogsdaddy · Feb 4, 2005

In looking at different trucks I'm in a quandary as to what would be a more benefitial gear ratio. ON the class 8 larger company trucks that seem to pull some prety heavy stuff their ratios generally tend to be the 3:70...with a cruising speed somewhere between 65 and 70 MPH. Now if one were going to convert and stretch a class 8 to make it a straight truck what would be a more optimal gear ratio for fuel economy? Figuring that your load would never be anywhere near what the truck was originally intended to pull. Would say a 3:42 ratio be better than the 3:70...or would it be better with a 3:90?...I know I have seen this somewhere and have had it explained to me but I guess I'm having another one of those Senior Moments and am totally lost. I've heard the old adage Gear fast drive slow but I can't remeber which way is the faster ratio. Thanks

Mortgage Banker 20+ years
OTR company driver 3 years
Car wash quick Lube mgr 3(too many) years
Expediter wannabee ~3 years

twodogsdaddy · Feb 4, 2005

OOps I see that twmaster ans msinger have the same idea..so I guess I'm not original I just saw their posts in truck talk

Glen Rice · Feb 4, 2005

Your dealer has a program to determine engine speed using different rear axle gears and different high gear ratio's. Check with them and don't forget tire size also effects speed/rpm ratio, tall rubber/short tire comparison. Hope this helps.

gojack · Feb 4, 2005

Cummins has a site that gives gearing ratios:

http://www.powerspec.cummins.com/site/home/

Dave

twodogsdaddy · Feb 4, 2005

Thanks for the info.. will keep all in mind

davekc · Feb 4, 2005

As mentioned, Cummins has a good site for this. The size and type of engine and transmission have to be figured in to ensure a proper match.
Davekc
owner
20 years

DocRushing · Feb 4, 2005

While dealing with fuel economy and engine wear, the differential ratio is one of the factors most important but least considered.
I prefer to choose a ratio which allows me to run (when necessary) at 55-58 m.p.h. at an r.p.m. at or near the upper-left end of the torque curve for the engine type which I intend to specify.
In the double-nickel states Marda and I most often ease along at a speed of about 58 -- unless we feel that the enforcers are willing to let us nudge it up to about 63.
Sooo -- while we cruise at 58 or so, I want the engine to turn as slowly as possible (or nearly so) without dropping off the upper-left edge of the torque curve.
That uses the least fuel, causes the engine wear most slowly, and gives a quieter ride.
Most heavy-duty diesel engines nowadays have torque curves with upper-left ends around 1050-1200 r.p.m.
(You can get a copy of the performance curves from a dealer or from the website of the engine manufacturer.)
For example, our next new truck will have full-size 24.5-inch tires, an overdrive ratio of 0.74, and a differential ratio of 3.42.
An engine speed of 1100 r.p.m. will give a road speed of 54.7 m.p.h., 1200 r.p.m. will give 59.7 m.p.h., 1300 will give 64.7, 1400 will give 69.6, and 1500 will give 74.6.
Not bad, eh?
I've designed several trucks that way, and I've thoroughly enjoyed driving them with those specs.
This is not empty theory; it really works.
Eaton Fuller offers a handy-dandy little feature on their website.
Go to www.roadranger.com, then click onto "road-speed calculator" near the lower-right corner.
You can play around with it by changing the tires, the transmission top-gear ratio, and the r.p.m.
It will then tell you the road speeds corresponding to the various differential ratios.
Enjoy!
Doc.

long-time trucker
(with strong background in physics and engineering)

ATeam · Feb 4, 2005

In the double-nickel states Marda and I most often ease
>along at a speed of about 58 -- unless we feel that the
>enforcers are willing to let us nudge it up to about 63.
>Sooo -- while we cruise at 58 or so, I want the engine to
>turn as slowly as possible (or nearly so) without dropping
>off the upper-left edge of the torque curve.
>That uses the least fuel, causes the engine wear most
>slowly, and gives a quieter ride.

DocRushing makes a KEY point. He talks about the states he most often drives in and his and Marda's own driving habits. With those in mind, he THEN noodles his gear ratio specs of choice. This is one of the advantages buying new provides. You can custom-tailor your truck specs to the loads you haul, the places you drive, and the way you do.

Why run the engine at low RPM if you can? There are a few reasons.

First in importance to us, it's quieter.

Second, an engine running at say 1,500 RPM and 60 MPH rotates 1,500 times to get the one mile down the road. An engine that moves the truck one mile at 1,400 RPM rotates 100 times less in that minute. Thus, the wear and tear is less and the hard engine components (berings, rings, gears, etc.) engine can be expected to last longer.

Operating cost savings can also be derived through the use of new truck spec'ing. One example is tires. If you can get good gearing AND use "tall rubber" too, your tires may last a bit longer. Larger-diameter tires rotate fewer times per mile than low-profile tires.

DocRushing · Feb 4, 2005

Larger wheels give a softer ride than do smaller wheels,
and full-size tires give a softer ride than do low-profile tires.
So full-size 24.5-inch tires really give a better ride than do low-profile 22.5-inch tires.
The differences are not dramatic, but they are noticeable.
Every little bit helps!
Doc.

ATeam · Feb 4, 2005

>Doc.
>
>long-time trucker
>(with strong background in physics and engineering)

Kicking myself here! Dang, Doc! Had I learned of your background in physics and engineering sooner, I would have invited you to serve on the truck-building team Diane and I formed for our new truck. Too late now, delivery is just weeks away.

I think we'll do OK. As is, the eleven-man brain trust brings 232 years of combined industry experience to the project, with an average of 21.1 years each. But still, I'd have loved to have you on board. Truck driving experience is easy to find. Physics and engineering smarts seated behind the wheel is a rare commodity.

Weave · Feb 4, 2005

I did not design my class 8 truck, but bought it used with the following setup on a hunch it would work fairly well:

S60 Detroit engine, 470 HP
10 speed manual OD transmission
3.58:1 single axle rear
22.5" rear tires

Setup spins low RPM's on the highway, so low I don't even think about them, around 1300 at 60 MPH. With the power the engine has, I think the rear axle ratio could go even lower yet, to the 3.42:1 Doc Rushing mentioned. You don't want to go so low that the engine starts to lug on upgrades and forces a lot of downshifting. The setup of mine with truck loaded to its 33k gross can climb the steepest of highway grades like its a sack of feathers with no downshifting. With MPG in the 10 range, I have no complaints with this setup.

-Weave-

twodogsdaddy · Feb 5, 2005

Thanks guys I'm going to print this and put away for good use later. You're all really a big help.

Weave · Feb 5, 2005

No Bazooka, but again funny how A-Team comes in this post and mentions with exact figures the years of experience the people who helped design and build his truck have, even using the term, "Brain trusts."

With 21.1 years average each in that group, I guess Mickey Mouse wasn't present in the group when it came to his truck self described only as, "expensive." Was there a shiny new engineer there pointing out to all the others with more time under their belts in the group all the errors in their ways who was agreed with the most?

-Weave-

Tom Robertson · Feb 5, 2005

ATeam...

not wanting to become a napalm bomb tossing monkey myself...

however...

Did you find the experience of this panel important in your decisions on the specifics of your truck?

Thats the point we have been trying to make with you for weeks.
Both you and your wife have been defending a position which differs from your actions with your own truck.

Perhaps an explantion would clear things up for me.

Please explain

davekc · Feb 5, 2005

Certainly a strange post. It seems we have went for weeks discussing how experience doesn't matter, and yet that appears to be the cornerstone of your truck building process. Now I am confused?
Davekc
owner
20 years

Doggie Daddy · Feb 5, 2005

i don't know why you guys are confused,after all you're just truck drivers...what do you know. DD.

RichM · Feb 5, 2005

A Team ,can you advise me the names and qualifcations of some of the 11 man team.I am thinking of getting a new truck soon,the 1953 FL 70 should be retired soon and on it's way to Mexico.