RPM cruising speed seems high??

[cowboy4123]

Hi,

New member and 6.5 owner. Have owned the truck for about a month now. My fuel mileage is terrible 9.5-14mpg

its a 1994 2wd crewcab dually c3500 automatic.

It cruises @ 65-70mph @ 2600-2900 rpms. I have no idea what gear ratio this truck is...

its got 117000 miles on the odo and supposedly the long block was replaced at 83000.. any ideas? Is this cruising rpm normal? I'm certianly not happy about the fuel mileage, I know somebody that has the same truck 1999 but with 4wd and he gets 16mpg towing and 18-20mpg highway.

[a5150nut]

Well I have a 94 4x4 and it has 4:10 gears and about the same rpm your talking about. If I slow down to 60 / 62 mph the fuel mileage improves. That and the 285 tires. With the taller rubber I'm right at 3:73 final gearing. I had to replace the DRAC in the speedo and it was reconfigured for 3:73s and it is dead on. My guess is you have either 4:10 or 4:53 gears and stock rubber. Look on your dash door and see if there is a whole page of codes there. We can figure out which one is your gearing if it hasn't been changed. Otherwise set the back end up on stands and have someone rotate the tire one full revolution and count how many times the drive shaft goes round.

Oh, and WELCOME to TDP!!

[More Power]

Searching for the keywords "alternator pulley" turned up a slew of threads. The following threads get to the heart of it regarding tachometer accuracy.

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=29535
http://www.thedieselpageforums.com/tdpforum/showthread.php?t=562

Jim

[rhsub]

Here is a list of GM production codes for axle ratio's as per the sticker in the glove box. this is a 98 listing but I think they are all the same for different years
GK9 4:63
GT4 3:73
GT5 4:10
GU4 3:08
GU6 3:42
G80 Positraction
HC4 4:56
HC7 5:13
If the sticker in the glove box is missing or unreadable contact a dealer and give them your vin and they can print out your production code list
These engines seem to be most efficient/ get the best mileage around 1800 rpm:cool:
Hope this helps
Ron

[Robyn]

My Dually has 4.10 gears and slightly oversized tires (4X4)
19-20 Hwy is normal and 16 around town.

The Alternator is often changed out and the pulley size is usually wrong on the rebuilts.

Mine is one of those and the tach reads about 500 RPM or so Higher than normal.

The proper pulley can be scrounged up and swapped easily.
As far as the poor mileage, you may want to check the Cat and be sure the thing is not plugged up.

Be sure the vaccum pump is working and that the turbo waste gate is operating.

Also a check of the fender well on the RH side to be sure a little creature has not moved into this space and set up houskeeping.

If the airflow to the filter is stopped up it will drastically effect the mileage.

Injectors that are worn out can also cause poor performance as well as sour mileage.

Gearing can certainly effect the numbers but you should see far better than what you are posting.

Take the truck out on a "marked" highway and check your odometer to be sure it reads the same as the mile posts.
Many times gearing and tires have been changed rendering the odo goofy

Mine reads 1/10 shy per actual mile so to get real mileage I have to multiply my total by .1 and add this to get real time miles.

Best

Robyn

[cowboy4123]

Wow thanks for all the quick responses....

Naturally, the stickers on both the door and glove box are worn out. The tires I'm running that came with truck are 225/75/16's....

There is no cat, there was stupid obnoxious 4inch exhaust I just changed out yesterday to the stock size exhaust because I wanted a quiet ride!

I'm using an air intake from SSDiesel...

As far as the turbo wastegate thing, the truck doesn't smoke and i can hear the turbo spool up and down (it even spools up and down on a steady run, doesn't stay boosted like other diesels, thats another subject though i guess)

Finally, interestingly enough, the alternator looks newer like its not the orginal. I'm goin to read about the different pulleys' after i post this. I'll call a dealership tomorrow to check my vin for the gear ratio.

I'll also add that I tow a 10000 pound 26 foot tag along. I've put about 3 miles on the truck so far in the month month i've owned it and so far its treated me well.

I installed the pmd cooler (with a new pmd), upgraded to the dual thermostat cooling and duramax fan. New OPS because it hesitated when i bought it. And in reference to this mpg problem the fuel filter is new. I also run with Lucas as a fuel additive in all my diesels.

[Matt Bachand]

Hi,

New member and 6.5 owner. Have owned the truck for about a month now. My fuel mileage is terrible 9.5-14mpg

its a 1994 2wd crewcab dually c3500 automatic.

It cruises @ 65-70mph @ 2600-2900 rpms. I have no idea what gear ratio this truck is...

its got 117000 miles on the odo and supposedly the long block was replaced at 83000.. any ideas? Is this cruising rpm normal? I'm certianly not happy about the fuel mileage, I know somebody that has the same truck 1999 but with 4wd and he gets 16mpg towing and 18-20mpg highway.

You need to keep the RPm's around 2k for maximum fuel efficiency... on 4.11's its around 57mph.... get use to it if you want good MPG

[AKMark]

Are you getting that mileage while pulling, or all the time. While pulling, that isn't too bad. Especially for a 10K trailer.

My poor 91 Suburban with a 6.2, 3.73 gears, and 31" tires got 5 mpg towing a 10K trailer from Alaska to here. The trailer was almost four feet taller than the Burb and had the wind resistance of a large barn. Yeah that was a long trip.

[cowboy4123]

9mpg towing. 14mpg is rounded up for driving unloaded

[cowboy4123]

Ok its midnight here now, just read the alternator pulley articles, being that I said earlier that it looks like I have a newer alternator, I went outside and measured that I do HAVE the smaller 2 1/4 (or) 55mm pulley...So tomorrow I'll look into to getting that new pulley, and according to the article that would throw my rpm's back down to where everybody else is.

This could also answer the unquestioned fact that I've wondered why this truck would rev out to 4100rpms if I really romped on it. I'll repost after I install/get a pulley.. Hopefully this would give me better mpg for some reason.

[cowboy4123]

Just wanted thank you guys and let know I followed through on the 62mm alternator pulley. That turned out to be my rpm problem. Now my truck kicks down when it hits the redline like it should. And I can clearly see running 60mph @ 2200-2300 rpms. My next thread/step will be to figure out why my fuel mileage is 12-14mpg unloaded.

Thank you all once again!!!!:)