Filled tank to brim, got on interstate, set cruise @ 1700 rpm -- a paltry 56 mph. Drove over 30 miles. Filled to brim again, making sure truck was level both fillings. No big suprise, got 19.68 mpg. When get the cash, I think a gearvendors will finally be monitarily worthwhile, since cost of diesel has gone WAY UP --- I usually get maybe 14 driving about 2200 rpm. My speedo is dead on matching GPS after adjusting with Smarty. It is REALLY hard for me to drive 55 mph on I-40. Not to mention probably dangerous. Have not put pencil to paper to see how long it would take, but I think it would pay for itself sooner than 2 years since buying truck (obviosly). How much was diesel then???

 

If you really had some money to burn, install the updated G-56 like my 07 5.9 has in it. Tag code AE. THi is the new one that went into the 6.7. My truck was made Dec 28th 2006 so I got the good 5.9 and the NEW G-56. I can roll 70 and be at 1900-2000 Rpm's. My truck is only 4x2, 6 speed and In the same conditions as your test drive I get 22-23 MPG.

 

WOW!! I'm jealous. Unfortunately, or (fortunatly when I mash skinny pedal), you can see in my Signature where I really burned my money. Roper, how much you reckon a G-56 like yours would cost?

 

You think wopping 30 miles is really a good test?

 

Yes I do. I can for a fact tell you I have checked it several times with less than 50 miles, and always get similar results. However I didn't measure head or tailwind, nor grade of highway. Maybe you will do that for us and drive 55 mph for 500 miles. Please post your results. BTW what's a wop?

 

Tail wind, head wind calculations, are you serious?

I'm more interested in "real world" mileage than your "ideal highway situations at 56mph for 30 miles" scenarios, who has time to fill up that often anyhow?
I fill up when the tank's down around ¼ tank and enter the info into a spreadsheet that does the calculations, I'm getting nowhere near 20 miles per US gallon BTW.

 

Of course I wasn't serious, just being sarcastic. I guess you missed my point. Point being lowering rpm's by 500 rpm on interstate would PROBABLY net some serious gains in mpg or LPG. 2nd point --2 years ago, a $3000 gearvendors unit was maybe not worth expense. Now with fuel as high as it is, maybe the situation has changed. That's all. I wish someone with a gearvendor unit could tell us how much mpg increase they saw.

 

Yes, agreed, 6th gear is too low in these things.
I don't think the gear vendor unit will take the power you're making though...?

 

What probably got you that milege was your speed, not your RPM. I get near 20 MPG al the time at that speed also, unloaded.

Higher speed = more drag. If you lower your gearing, you may even make it worse because your engine will have less leverage over the wheels, and it will be forced to work harder to turn them.

I see this reflected in my EGT's all the time when pulling between 55 and 65 MPH. Since I have an auto, that speed range is kind of the no-man's land between 3rd and 4th - a little frenetic for 3rd and too low for 4th. If I keep it in 3rd, the revs stay up but the EGTs stay very low. If I let it shift to OD, EGT's go up by 200 degrees immediately. I generally keep it in 3rd then, and enjoy the mechanical advantage of the gears, and the fuel efficiency.

 

i thought so too, before the 35's.

 

What happened to your mileage with the 35's?

 

overall, it stayed about the same. i don't do much city driving, and with the drop in my highway rpm's, my highway mileage went up some. i would do it again in a heartbeat- makes highway driving so much better.

 

Dang it, you both are probably right. I was thinking a GV may help. HOV, are you saying if I drove 55 mph, but put it in a gear to make it 2200 rpm, mileage would be same?

 

^^ It's possible. There are a lot of factors, not just gearing. I don't think the GV would be worth the cost.

 

I agree, HOV, still I would like to drive a truck with one in it though, see how it felt.