Engine/Transmission (1994 - 1998) Mileage woes!

[Got Smoke?]

Ok last year... ..... bought the truck with 122,XXX on it. It was as bone stock as it could be. The only thing that wasn't stock on that truck were the nasty 16. 5 chrome mods and 33" Wild Countrys.



Anyhow I bought this truck on a Monday then went to Gatlinburg on Friday. That was about an 500 mile roundtrip. With my math I figured I got about 21 MPG. That was do'n 70-75 on the way down and 75-80 on the way back.



Since then I've changed the air and fuel filters (although its been probably close to 15K miles ago)and straightpiped it.



Went to Gatlinburg this year do'n 80 both ways got 17. 5 MPG (my rack stop was out at the time).



Reinstalled rack stop.



Went to BMD yesterday morning, did 75 all the way up. Seemed like mileage came back STRONG! Did 80 come'n back and it seemed like it went away. Filled up and did my mileage this morning and it was 17. 5 again?!?!



Ok I know that with my foot in it (i. e. 80MPH!) that it's probably gonna hurt my mileage, but 21 MPG is a big difference from 17. 5 MPG.



Is there something I'm missing here? My tires are @ 40 PSI cold. Seems I heard a tale one time that you can get more mileage if you inflate your tires more?



I'm going to order a new fuel filter from Geno's tomorrow. Wonder if that will do anything for it. I'd almost love to swap tires with somebody who has factories for just one tankful, but I'd probably be so jealous I'd end up buying some 16" rims and tires.



Any thoughts/suggestions/comments on this? :{ :-{}

 

[Blakers]

I don't think your mileage is bad at all for running 80 mph. I have the same wheels and the same sized tires as you do and I get about 18 mpg running at a combination of freeway at 72 mph and rural highways at about 60.



I would question your 21 mpg from your previous calculations. Perhaps there was a difference in fill-up levels or perhaps you got some extra heavy fuel with more BTU. So easy to be off a little in your calculations - best to average over a long period.



One thing, I would seriously consider getting some 35" tires if you're gonna run 80 mph all the time. BTW, what is your speed limit down your way? I don't think I could get by running that fast around here for long.



Blake

 

[Got Smoke?]

In TN the interstate speed is 70mph, up Tom's way (KY) its 65mph :-laf



He's gonna read this and tell me to slow down! LOL I told him how fast I got home Tuesday night. But I was just ready to get home! I usually only do 5-10mph over the speed limit, but I was all over the CB on the way home. Made it from Tom's shop ('bout 250 miles) to my house in 3hrs and 45 mins. :--)



What pressure do you run your tires at Blake?

 

[Blakers]

On these tires I run 50 psi in the front and 45 in the back. Unless I have a load, then I run more in the back. These are Remington 33-12. 50-16. 5 Load Range D.



I really like them for summer weather and the wear beautifully. I run 100 miles a day round trip to work and they're hardly wearing at all. No cupping or anything.



I generally run 5-8 over the speed limit and run with the CB on too. But nowadays the truckers are so lax on their communications that sometimes they never mention a bear. They must be on their cell phones or have a CD on.



Blake

 

[Got Smoke?]

Originally posted by Blakers

On these tires I run 50 psi in the front and 45 in the back. Unless I have a load, then I run more in the back. These are Remington 33-12. 50-16. 5 Load Range D.



I really like them for summer weather and the wear beautifully. I run 100 miles a day round trip to work and they're hardly wearing at all. No cupping or anything.



I generally run 5-8 over the speed limit and run with the CB on too. But nowadays the truckers are so lax on their communications that sometimes they never mention a bear. They must be on their cell phones or have a CD on.



Blake

I had no problem getting help from a trucker finding out where the bears were at going to Gatlinburg and Tom's shop in KY.



Which Remington's are you running? Wide Brutes?

 

[Blakers]

Yes these Remington Wide Brutes .



A lot of the time the truckers are good with bear reports, but since I run everyday, I notice many could care less. 90% of my driving is in Ohio and, since they hate Ohio so much, and the cops are so nasty to them there, I think many truckers have just said heck with it and drive slow. It's not like it used to be on the CB.



Blake

 

[Got Smoke?]

Originally posted by Blakers

Yes these Remington Wide Brutes .



Blake

Yeah Remington's are good tires. I had a set of 33" Mud Brutes on a 93 Toyota for 2 years and when I sold it they still had around 1/2 tread. Took all the mud and off road I could throw at them without even blinking.



I thought about getting the Wide Brutes for the Dodge, as I'm going to be needing tires soon. I need a good MT/AT combo for farmwork/highway. I had originally planned on getting BFG MT/KM's but then found out I would be going back to school this fall. Which mean a lot more highway driving than usual so I think I'm just gonna settle on BFG AT/KO's. They cost the same as Remington Wide Brutes, and I think they will dig a little better. Can't be any worse than the Wild Country RVT's I have now. #@$%!

 

[CumminsPower98]

I run 33x12. 5x16. 5 BFG/AT's. I keep 60 PSI in the fronts and 50 PSI rear when empty, which is most of the time. I have not seen less then 20. 5 in town or running 75 since setting my timing to 16º.



Mark

 

[Got Smoke?]

Originally posted by CumminsPower98

I run 33x12. 5x16. 5 BFG/AT's. I keep 60 PSI in the fronts and 50 PSI rear when empty, which is most of the time. I have not seen less then 20. 5 in town or running 75 since setting my timing to 16º.



Mark

Thanks Mark. I will be heading back up to Black Mountain Diesel either next week or the week after to have Tom set my timing at 16 degrees. I was gonna let a shop do it down here, but the guy made me feel like he didn't know exactly how to do it.



Your tires aren't wearing HORRIBLY with 60PSI in the front?!!?! I'm surprised. When I got my RVT's the guy that put them on aired them to 50PSI which wore out a good amount of my center section as well as killed the factory siping the tires had. That thing must ride like a buckboard with 60psi in the front.

 

[CumminsPower98]

Well the tires call for 50 PSI so I figured with the extra weight of the Cummins up front I would add some pressure to help them hold up (I don't like to see the side walls poking out too much because that is a sign of more flex causing heat build up leading to a shorter tire life). They are wearing evenly all the way across that wide footprint (10. 5") so I'm happy. It is a little rough but these trucks are just that way when empty. The highest mileage I have seen with my present setup is 22. 4. I get 21. 5 much of the time just running around town. Good luck with the timing and I hope it helps the mileage.



Mark

 

[Got Smoke?]

Dumb question, but would a 3K GSK make any difference in MPG.



I highly doubt so, but I am just wondering.

 

[BWilson]

I am on my 8th set of 35x12. 50 BFG AT's and I get 60K everytime. I also drive like a bat out of H$%& all of the time. Just make sure that you are taking into consideration the tire size when you make your calculations. My speedo is 15% off. Of course, around Detroit, my speedo reads 75 and I'm still not the fast guy... ... I calculate my mileage every 130 gallons though, ususally about 2200 miles per fill up on the odometer.

 

[Got Smoke?]

oh DUH! I feel so stupid for not even remembering that my odometer is off with the larger tires.



Tell me guys how can I figure out my mileage with my larger tires? Math is my worst subject:{

 

[BWilson]

I would calculate the circumference of each tire and compare the difference. circumference=pi (3. 14) times the diameter. I would calculate it for you but I don't remember what the stock tires are???? However, this still doesn't explain the change you have been seeing in mileage with the same size tires??????

 

[BCFAST]

OK... for the um-ttenth time, try this: http://www.sgmotorsports.com/tiresize.htm#TireSize



Also, understand how your pump works... the fuel curve turns up VERY SHARPLY when the motor gets to about 2100rpm. This means that there is a big difference in economy between 72mpg and 76mph. I know it seems wierd, but watch your boost guage at these speeds; I think you'll be suprised at how much more is required for just a tad more speed! It all has to do with 2 things: pushing air, and the fuel curve of the pump.

Brian

PS. Notice my combo. At 70, I only turn 1760!

 

[Got Smoke?]

Originally posted by BCFAST



PS. Notice my combo. At 70, I only turn 1760!

Keep in mind I'm still "diesel dumb" Brian, but how are you only turning 1760RPM at 70MPH.



At 70MPH I'm right on 2K RPM, and I have 3. 54 gears. Is it the timing?

 

[BCFAST]

OK... that tells me that your truck has the 3:54 rear, as mine does. The Rickson 19. 5's with 265/70's have a circumfrance that's 12% more than stock... which means that for each mile, they rotate 12% fewer times. So, my RPM's are 12% less than stock at any given speed. Use the tire size calculator I gave you earlier to compare your tire size with stock. This will tell you how different you speedo and odo are from stock. Again, drive your truck with the boost guage and the pump's fuel curve in mind.

 

[Blakers]

Originally posted by got smoke?





Keep in mind I'm still "diesel dumb" Brian, but how are you only turning 1760RPM at 70MPH.



At 70MPH I'm right on 2K RPM, and I have 3. 54 gears. Is it the timing?

I too am only running about 1760 rpm at 70 mph. Or about 73 mph at 1800 rpm. But I use my GPS rather than my speedometer. My speedometer is off by about 5 mph low at that speed. For some reason my speedometer wants me to have bigger tires.



Blake

 

[CumminsPower98]

Blakers: How on earth do you get 70 mph out of 1760 rpms? I run the same tire size as you and I get 71 or 72 at 2k. I also have 3. 54 gears and a speedometer correction box.



Mark

 

[BWilson]

What is a speedomter correction box? I have been considering adjusting my speedo since I am running 35's. Can't we just change the speedo gear?