Speed going into top gear?

[footdoc]

There has been alot of recent discussions about the best ways to save fuel....alot of folks claim they dont get top gear until 62 or so...justifying that faster is better..I have been watching my tach, etc and find top gear drops in right at 50!...Any one else comment?

[footdoc]

There has been alot of recent discussions about the best ways to save fuel....alot of folks claim they dont get top gear until 62 or so...justifying that faster is better..I have been watching my tach, etc and find top gear drops in right at 50!...Any one else comment?

[BigSkyBob]

In Econo mode, I can attain 6th at 61 MPH on the level. It will stay in 6th down to about 52 on a slight grade then catch 5th. I try to run about 62 MPH cause that's right on the peak of the torque curve which should be a sweet spot.

[Lug_Nut]

Mine will not up shift until 57/58 mph. It will however, hold sixth, once in, down to near 50, so the key is to set the cruise, say at 55, then override the speed manually to attain the shift. After that the cruise will take care of it. Oh, and you will need the Econ mode to hold the gear if you are in hilly terrain.

Peter

[footdoc]

How bout anyone else with my drivetrain...ISL 400 and the 06 Mountain Aire??? Am I normal or not

[69RoadRunner]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by footdoc:
How bout anyone else with my drivetrain...ISL 400 and the 06 Mountain Aire??? Am I normal or not </div></BLOCKQUOTE>

We're RVers, not psychiatrists!

[RVDude]

RR - beep beep!

There is probably more lore about fuel savings and shifting and rpm and cruise control and, and, and, and... than the entire Harry Potter series. Do your research and it probably should be very heavy with the engine manufacturer's web site - they have tons of info on both the Cat and Cummins sites as it will likely have less "opinion' and be based in tested facts.

With that said, be extremely careful with trying to achieve "early" shifts and trying to "hold" top gear slower that you do not wind up lugging the engine. That can be exceedingly damaging to these big boys.

That's my opinion!

[Norm Payne]

If I accelerate slowly my motorhome goes into 6th gear at 55 mph and when in Economy mode it won't drop back to 5th until 48 mph.

I have a 2005 Dutch Star on a Spartan Mountain Master chassis with a 4.88 rear end. It came with 275/70 tires and when I upgraded to larger 275/80 tires it changed the rpm and shift points 5%.

There are a lot of variables in shift points including engine, ECM settings, the way you drive, the rear end ratio and the size of tires.

I would not worry about lugging the engine because the engine / transmission is programmed to not let that happen. My engine (and most other engines) reach maximum torque around 1300 rpm and I am well above that speed when cruising at 55 mph.

[Jay W]

Doc, not exactly the answer you requested
According to Cummins, my rig should be geared to cruise between 1600-1900 rpm's. It also recomends not cruising under 1600 rpm's.

Here is a website to Cummins Power Spec

Good Info on MPG

2005 DSDP 3810
370 ISL Spartan Mountain Master

[Aridon]

We hit top gear around 50-53 and we cruise at 59-61. Averaging 8.3 towing a Vue on very flat terrain right now. We are heading to Canada in August so we'll get a better idea.


One thing to consider is the wind resistance on these things is massive. That what most of your fuel is going towards is overcoming that.

Simple physics tells us that if you slow down it is less of a factor and will improve gas mileage regardless of your gear. Naturally you don't want to run these engines with no load so it is a balancing game.

IMO its a cool game to play to see what you can get but it really shouldn't make much of a difference getting 7.9 vs 8.5 or whatever. Most of us are driving in the 55-65 MPH range anyways, which is about as low to be economical and practical as we can really go.

[Harm]

I'm with you footdoc, it drops into 6th gear at exactly 50 mph. I have 20k miles on now and in the last 2500 miles of this trip I have averaged only 7.0 mpg driving 55-60 mph.

[footdoc]

Wayne....On a flat 100 mile leg with no wind doing 55 pulling a toad, I was able to get 9.1 (by Triptek) just before a 7 mile long accident jam up..I have 11K miles....I guess I got the high test diesel

[JerryKelly]

6.9 mpg running between 60 and 75 and pulling my toad for 420 miles. I prefer to spend 20% less time driving at the expense of fuel (about $60).

[GaryKD]

My signature coach goes into 6th gear at 48 MPH. This is in economy mode, normal acceleration, flat road and towing 5100+ lbs.