Higher speed equals lower mileage? Not always...

[glenhead]

<div class='quotetop'>QUOTE(jayman @ Jul 12 2006, 08:40 AM) [snapback]284906[/snapback]</div>

<snip>That is, does your vehicle have 3.73 gearing or 4.10 gearing?<snip>

<snip>It's my opinion the 5.3 Vortec has a sweet spot around 2200-2500 RPM. Any slower than that, you're not really "lugging" the vehicle but you're using more gas. I also carefully tracked the fuel consumption as I'm given an allowance for business mileage.

Another odd thing: Driving around Winnipeg, at speeds up to 80 km/h, I got around 1.5 MPG better by leaving the shift in 3, instead of OD.
[/b]

My Suburban has a 3.73 rear end. It's a real pig on startup when towing a fully-loaded trailer, but it sure is quiet on the highway!

It's nice to see someone else with similar results - makes me think I may not be hallucinating (at least about this...) The sweet spot at a bit over 2200rpm is exactly what I'm seeing. I hadn't thought about dropping PRNDL by a notch to keep it revved a bit higher in local traffic - I may have to play with that.

All that aside, I'm **really** looking forward to delivery of our Prius next week (Silver Pine #4) so I can quit using the gas hog except when I need it! My Loving Bride and I have already negotiated how we can share the Prius for nearly all our driving, and we expect our monthly fuel bills to drop by better than half. Right now, with all local driving on this tank, the Surbubban is showing average fuel economy of 12.1mpg - makes me want to scream. Fortunately, I walk to work, so I only put 40-50 miles on it a week locally.

[hobbit]

You may actually be running the engine under a higher load, and thus
maybe more efficiently, against air resistance at the higher speed.
If your transmission had an additional gear, I bet you'd top 20 mpg
more often.
.
_H*

[jayman]

<div class='quotetop'>QUOTE(glenhead @ Jul 12 2006, 08:55 AM) [snapback]284911[/snapback]</div>

My Suburban has a 3.73 rear end. It's a real pig on startup when towing a fully-loaded trailer, but it sure is quiet on the highway![/b]

I'll also assume you have the Z82 "locking" rearend made by Eaton. With towing of about 600 initial miles, the shop manual recommends taking the rear cover off the differential to wipe out accumulated sludge and to clean the magnet off. My 2000 had a drain plug, if I recall the +2003 did away with the drain plug. Your axle is probably made by AAM (American Axle and Manufacturing).

Hint: it's far easier to wrestle the cover off if you first lower and remove the spare tire. That's always a good time to lube up the spare tire crank too.

After I did the initial service, the inside of the case was sludgy, there was a dark gray film on the sheet metal cover, and the magnet was loaded with particles. I became a bit concerned so did another service about 5,000km later, and the inside was spotless. It really does pay to do an early initial axle service, especially if you do it yourself.

On the issue of gearing, from what I have seen of the newer pickup trucks and SUV's, they are grossly too high for gearing. For example, the 2005 Ford Expedition a co-worker has will cruise on the Trans Canada at 110 km/h and around 1,500 rpm. When he pulls his camper, the thing will barely get out of its way.

I'd have to guess the domestics for some reason geared all their pickups and SUV's for empty 75 MPH cruising. Load them up and they barely get out of their way. The 4.10 gearing, or even lower, would actually IMPROVE fuel economy at city and interurban speeds, especialy when the vehicle is used for its intended purpose of towing or hauling.

<div class='quotetop'>QUOTE(hobbit @ Jul 12 2006, 09:43 AM) [snapback]284929[/snapback]</div>

You may actually be running the engine under a higher load, and thus
maybe more efficiently, against air resistance at the higher speed.
If your transmission had an additional gear, I bet you'd top 20 mpg
more often.
.
_H*
[/b]

One co-worker has a '05 Expedition, another has an early production '06 Lincoln Navigator. The Navigator is obviously more luxurious, but it also has a ZF 6 speed auto transmission instead of the Ford 4 speed Slush-o-matic.

They've compared notes and the Navigator routinely gets 1-2 MPG better, and is rated to tow more too. The co-worker with the '06 Range Rover also has a ZF 6 speed auto, and that beast always seems to know exactly what gear to be in. Very impressive for such a huge thing.