Weird MPG issue

[Prairie Dog]

Last winter I made two changes to my truck. I installed 285 75/17 Toyo AT's and replaced my problematic TST Powermax (3/3) with the Smarty Jr. which I keep at 100HP (rarely use the power though). I have corrected the speedo for the new tires.
Towing last summer I found that I had about a . 5 to 1 MPG gain in MPG.
Fast forward to yesterday when we finally took a long solo drive on a route I had taken many times in the past. Before I had gotten 22~23 MPG but yesterday I got 19. The only other change was that I dropped tire pressure from 75 to 50 all around.
So how is it my towing MPG is better but my solo is so much worse?

Thanks in advance,

Scotty

 

[Rockranch]

Scotty,

I have no real answer to your problem, except to say that such a big drop in air pressure could account for the decrease in milage.



What was the problem with your "problematic" TST tuner? I have one as well.



Thanks,

Larry

 

[Prairie Dog]

Scotty,
I have no real answer to your problem, except to say that such a big drop in air pressure could account for the decrease in milage.

What was the problem with your "problematic" TST tuner? I have one as well.

Thanks,
Larry

Larry, It kept throwing codes the whole 5 years I had it. In the end the display died and I was looking at buying a used one from TST. While the SmJr. can't fuel as hard, I'm really happy with it and don't really want anymore power. I like the idea of no hardware being added to the truck too.

I figure that calibrating the speedo cost me some MPG too since it was reading fast by nearly 5 mph. Now it's less than 1 mph fast so that would account for maybe 1 mpg.

 

[Lordsman]

Why such a low tire pressure on the front end? Real heavy there with the Cummins weight. . I'd run at least 60 to 65 psi there and about 40 on the rear if empty and increase the pressure based on load. You want an even tread on the ground across the width of the tire to maximize mpg and proper tread life.

 

[Prairie Dog]

I run the AT's that low because it seemed to be the general consensus here that the AT's (which have a higher weight rating) had a good wear pattern at that pressure. Am I misinformed?

 

[LRutigliano]

Last winter I made two changes to my truck. I installed 285 75/17 Toyo AT's and replaced my problematic TST Powermax (3/3) with the Smarty Jr. which I keep at 100HP (rarely use the power though). I have corrected the speedo for the new tires.

Towing last summer I found that I had about a . 5 to 1 MPG gain in MPG.

Fast forward to yesterday when we finally took a long solo drive on a route I had taken many times in the past. Before I had gotten 22~23 MPG but yesterday I got 19. The only other change was that I dropped tire pressure from 75 to 50 all around.

So how is it my towing MPG is better but my solo is so much worse?



Thanks in advance,



Scotty

If you didn't do this already...



You need to measure the height of your rear tires using a level and a tape measure. On the Smarty, stock default is 30. 65" for 265/70-17, but if you go by the tire manufacturer's specs, its approx 31. 7"



When I measure my stock tires, its 30. 65"



Can't explain the difference in towing vs non-towing... winter fuel??



My towing mileage is always 10 to 11 mpg



What kind of RPM drop on the highway did you see ?? I am going with 285/70-17 or 285/75-17 with my next tires so I can get closer to the highway RPMs of a 2nd gen with 3. 55 gears...

 

[p-Bar]

Are you using the "lie-o-meter"" or topping off and hand calculating?

 

[Motorhead]

Try the Smarty at the 70hp setting. That is where I get my best MPG. One thing I have found but only in my old 60hp 240D Mercedes is it seems like the diesel fuel varies between producers. I noticed that with certain brands of fuel, I had to pedal the old 240D harder to get it to go. Running GOOD it ain't no power house! Our trucks have well ample power that we might not notice a slight change in fuel..... Only in the fuel mileage.

 

[Prairie Dog]

Thanks everyone.
p-Bar ~ Everything was hand calculated and the recalibrated speedo was verified with GPS . The speedo is much more accurate than it was when everything was stock.

LRutigliano ~ I did calibrate my speed. It's set to 32" tall and thats about 1/4" short (I measured the Toyo's at 32. 3"). The RPM's dropped about 250 RPM's which frankly if I was towing any heavier, would not have worked out. It's just barley into the torque and if I didn't have a little extra power I would never get into 6th. As it is I think it has worked well. I find 5th gear is much more usable and allows (or requires) me to stay in it more. If I drop to 60 MPH or hit anything more than a moderate hill then I have to drop out of sixth to prevent lugging. If my trailer was as heavy as yours it wouldn't have worked without a lot more power like you have but I'm a little afraid of pushing that much torque through OD.

Motorhead ~ I'll try the 70 HP setting. I tried to research what setting was best but never got a good answer from somebody with experience. Thx

 

[CToney]

I have a similar truck. My milage can differ a fair bit. What were the boost and pyro reading? Wind really affects my milage. A little higher boost or egt and milage goes down.

I have a Con O going in next week. I will either run on tow or detune with pod.

 

[Prairie Dog]

Boost and pyro were normal but the route I drive takes me up and down hills so they're really all over the place. For those that are familiar with the area, it's between Bothell and Leavenworth Wa. which is a 210 mile round trip. Much of the time my speeds are between 50 and 60 mph. There was no traffic or wind and only a bit of rain.

 

[Regcabguy]

I run the AT's that low because it seemed to be the general consensus here that the AT's (which have a higher weight rating) had a good wear pattern at that pressure. Am I misinformed?

My Toyo AT's wore prematurely down the center at 60psi with stock rims and my camper load(2000#). I'm running the MT's now at 55 psi.

I miss those AT's.

 

[Prairie Dog]

My Toyo AT's wore prematurely down the center at 60psi with stock rims and my camper load(2000#). I'm running the MT's now at 55 psi.

I miss those AT's.

Thx for the info. I better start measuring my treadwear!

 

[CToney]

I'm a flatlander. Few hills so watching gauges tells a lot about milage. If I drive 65mph or less milage is usually better than 70. I buy the same fuel in bulk all the time. Some people say fuel from different places makes a difference too.