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Discussion Starter · #1 ·
I drive a 1993 k2500 extended cab 8' box truck with the mechanical version of the 6.5TD with a TH4L80e, 4:10's and 235/85r16 tires. The truck came stock with 245/75r16's. She's only got 127,000 on the ticker, and the long block was replaced at 85,000. My question is what kind of mileage should I expect. I drive 100 miles daily at 60-65 mph country road driving (about 2500 rpm). No major hills. One day a week I do 120 miles on the highway at 75 mph ( at I believe about 2900 rpm). On my 34 gallon tank I am pushing it hard to get 400 miles out of it. My mileage figures are 11-12 mpg. I am really disappointed with this. I'm thinking on putting some 285/75r16's on it to lower the rpm's. I just drive this truck for the most part. I rarely tow anything heavy with it.
I believe one issue is my injectors. I have been told that they are the reason my truck bucks and skips and smokes on cold start ups. There is considerable injector knock on cold mornings. Also over 3000 rpm the thing falls flat on it's face, BAD. It contiues to rev to 4000 before it shifts, but I generally lift and let it shift before that.
I also have an exhaust manifold leak. It's on the passenger side rearmost cylinder(#8?). Would this cause a problem with the turbo being on this side and all ?
Any ideas on how I can improve my mileage? My last vehicle was a 88 3/4 ton 4x4 suburban with the 6.2L and I got about 13-14 mpg with it. Both trucks weighed about the same (within a couple hundred pounds) I was hoping for better mileage with this truck with the 6.5TD.
 

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70-75 mph @2900 rpm and you wonder why your mileage is disappointing....hmmm, it must be the 4:10 gears.

Best mpg I have gotten recently was 21.6 and running an average of 60 on the highway. Want better mileage, ease off the throttle a bit.
 

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I remember sking another membir why my range was much better....doh!, it's the 4.10s...
Your starting issue migt be GPs...
 

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235's would look good on a jetta. combined with 4.10 gears, you've got some good torque at the rear wheels. Very bad combo for fuel economy. 285's would help. if you rarely tow, swapping gear ratio's to 3.55 will be even better.

Since you've got smaller than stock tires, your odometer says the truck is traveling more miles than it actually is. Is your mileage calculation corrected for tire size?

Check the usual suspects, air filter, engine oil, cts, propper operating temps while driving.
 

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Are you sure the tranny is shifting to 4th, and the converter is locking? That is REALLY high rpm's for 75mph. I know mine is a dodge with 3:55's, but I only turn about 1800rpm at 75mph. That has to be the reason you are getting horrible mileage.

Eric
 

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235/85's are the same height as the 265/65's, also his tach is probably off. The tach on the 6.5's is driven off of the alternator, mine reads around 2,750RPM's at 75 with 4.10's. The easiest way to help the milage is to SLOW DOWN. It worked for me and I'm sure it will work for you. Get the timing checked out and make sure your getting good boost. And a good exhaust system and air filter will help the milage some as well.
ON EDIT: after re-reading your post I would definately say your tach is off. And the exhaust will help tremendously with the engines power up in the RPM's.
 

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Your problem in order:
1) 8' box truck
2) 4:10's
3) 235/85r16 tires
4) 60-65 mph country road driving (about 2500 rpm).
5) 75 mph ( at I believe about 2900 rpm).
7) injector knock
8) exhaust manifold leak

Fix the above for better fuel consumption figures.

>6.2L and I got about 13-14 mpg < Not turbocharged=better fuel economy
 

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Your problem in order:
1) 8' box truck
2) 4:10's
3) 235/85r16 tires
4) 60-65 mph country road driving (about 2500 rpm).
5) 75 mph ( at I believe about 2900 rpm).
7) injector knock
8) exhaust manifold leak

Fix the above for better fuel consumption figures.

>6.2L and I got about 13-14 mpg < Not turbocharged=better fuel economy
Why do you think 235/85 tires are a problem DP?
 

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missed the /85 part....
 

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Discussion Starter · #10 ·
I double ckecked my rpm's today. I'm at 2500 at 60 mph and 3000 at 75. What's the big deal with putting larger than stock tires on? They should drop my rpm's. 245/75r16 (stock) are 30.5" tall and spin 682 revs per mile. My 235/85r16's are 31.7" tall and spin 656 revs per mile.
 

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Your RPMS's will not change in relation to your speedo,....but your vehicle speed will change. that means with the larger tire you are going further, so ideally you can drive slower (as per speedo) but be traveling faster (you likely knew that)It sounds lke your running in drive not OD I dropped mine in drive today and I hit almost the same rpms as you are posting, I'll confirm what mine turns in OD, later today.
 

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Also over 3000 rpm the thing falls flat on it's face, BAD. It contiues to rev to 4000 before it shifts
I think you need to find the problem here, the governor in your truck should be set at 3,300-3,400 RPM's. The 4,000 RPM's is what makes me think your tach is off. Since this is a mechanical engine, are you sure it is going through all of the gears and then lockup? The mechanical engines had problems with not getting into overdrive or lockup due to the engine running to cold(bad t-stat or aftermarket), and bad TPS's and CTS's.
 

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If the alternator has been changed and the pulley is not right your RPM's will be off. If you change tire size from stock you need to adjust the VSSB to get proper speedo reading.
 

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70-75 mph @2900 rpm and you wonder why your mileage is disappointing....hmmm, it must be the 4:10 gears.

Best mpg I have gotten recently was 21.6 and running an average of 60 on the highway. Want better mileage, ease off the throttle a bit.
You should never be doing 2900 rpm's @ 70 - 75 mph unless you are gunning it or going up a very steep hill gunning it.
I cruise on the highway at 75mph w/ 2550rpm's or so with 4.10's running 245-75-16's right now. I get between 15-16 mpg overall between highway and city driving and I assure you I do not drive conservitively(although I probably should), just like the sound of my exhaust):h .
You may have another issue going on here. Check the tire pressure and alignment as well as the other issues brought up.

Mark
 

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Are you sure the tranny is shifting to 4th, and the converter is locking? That is REALLY high rpm's for 75mph. I know mine is a dodge with 3:55's, but I only turn about 1800rpm at 75mph. That has to be the reason you are getting horrible mileage.

Eric
Eric,
Your gearing is the big difference in that respect. Running 1800 @ 75 would be golden for me since I rarely travel slower than that on the highway. OD may be an issue as well like you and others have mentioned.

Mark
 

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Discussion Starter · #16 ·
thanks for the input. I checked out every thing today. The tranny is working perfectly. All rpm/mph readings I have given have been in 4th gear after the converter has locked up. I know it's locked up because at a steady speed I can press my brake pedal slightly and unlock my converter and then release the pedal and it locks back up. I even checked it in 3rd gear, 2600 rpm at 50 mph.
The alternator is a Pep boys special and was replaced a few years ago, before I bought the truck. Does anyone know the diameter the pulley should be? If it has the wrong pulley on it, this would probably cause my rpm problem, right? I understand that the alternator is what runs my tach, is this right? How else could I correct it or is the pulley the answer?
 

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I checked mine today I'm approx 2100-2200 @ 70 MPH with 3.73 gears
 

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I have the exact same truck except 3.73 rear end gears. It revs 1700 at 60mph and I think 1950 at 75. Are you sure its in 4th gear and the lock up is working. If so I would look for some 3.73 gears. I make about 18 to 19 mpg on the US gal at 60mph and 18 mpg at 75mph. MY engine will hardly rev past 2600rpm and have only done so maybe 3 times in its life.
 

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I would get your rpm checked at a shop easy to do with hand held rpm gauge, I find it hard to believe your reving that high and are you sure they are just 4.10 gears.
 
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