Why is the mpg so bad on my 6.5 diesel??

[ecro77]

I just bought a 1997 GMC k2500 with the 6.5. It has a K&N and working boost guage (around 7-11 psi), and the mpg's are awful! I filled it to the brim. Got 115 miles to 3/4 tank. And I was at empty at 244!! Any ideas about what is wrong, or what to do?? Thanks

[nickhavlat]

What do you have for a transmission and gear ratio? Any smoke??

[ecro77]

I need to check the tran....minimal smoke, when cold. I do have oversized tires, but that is an acceptable loss. I do not see them costing me an approximate 7mpg.

[sonjaab]

Plain and simple those motors are pigs!

They work hard with the puny 130 hp pushin that 5500 lb. flyin'

brick so mileage ain't that great.

 

Not to mention with larger tires the odo/speedo will be off (read low)

so calculating mpgs accurately is pretty tough.

 

As long as it starts daily, runs good, doesn't smoke out the neighborhood

your in luck and will have to live with lousy mileage.

I am sure there are some aftermarket tweeks avail. that may help.

[Heineken022]

I had a similar problem with a 1991 6.5 non-turbo version. Took it to 3 dealerships and none of them could figure out what was wrong with it. Walked by the back of the truck one morning and noticed the passenger side wheel was very hot. Ended up being one of the drum brakes was locked up.

 

May not be your problem but it doesn't hurt to check the brakes out.

[djmercer1]

Plain and simple those motors are pigs!

They work hard with the puny 130 hp pushin that 5500 lb. flyin'

brick so mileage ain't that great.

 

Not to mention with larger tires the odo/speedo will be off (read low)

so calculating mpgs accurately is pretty tough.

 

As long as it starts daily, runs good, doesn't smoke out the neighborhood

your in luck and will have to live with lousy mileage.

I am sure there are some aftermarket tweeks avail. that may help.

 

 

these trucks are far from being fuel pigs. assuming the op has a truck that is stock, mileage in the 17-22 mpg range can be expected.... assuming he las a 25gal tank for 244 miles that about 10mpg which is low, even with oversized tires....

[jro909]

its an underpowered motor...their was a reaosn GM made the duramax....

[joshw1303]

Something has to be wrong. My friends dad has a 97 suburban loaded red on red with the 6.5L and he gets great MPG. Hes told me he can go 800km or more on a tank.

[yz2009]

My dads 6.5l tahoe will get about 340 miles per tank

 

As a comparison my 5.3l sierra has a 24 gallon tank. With my stock 245 tires i would get about 340 miles per tank (about 14 mpg). I just got off road 265 tires and my milage dropped to 11mpg. So now i only get about 260 miles per tank.

 

I think your tires could play a big part in your decreased mpg

[Carder09]

My dads 6.5l tahoe will get about 340 miles per tank

 

As a comparison my 5.3l sierra has a 24 gallon tank. With my stock 245 tires i would get about 340 miles per tank (about 14 mpg). I just got off road 265 tires and my milage dropped to 11mpg. So now i only get about 260 miles per tank.

 

I think your tires could play a big part in your decreased mpg

 

 

Wow, I have aftermarket 17s with the biggest optional tire on my truck and I'm gettin well over 400 miles on a tank. Other factors include but not limited too, its a 2wd, crew cab ssb with cover, and a bb tune. But even before that I was gettin 398 miles a tank. And I believe my tank is a few gallons smaller than yours. Also forgot to add that it has the 3:23 gears. And I can't believe that motor only churns out 155hp, what a dog of a motor!

[jro909]

My dads 6.5l tahoe will get about 340 miles per tank As a comparison my 5.3l sierra has a 24 gallon tank. With my stock 245 tires i would get about 340 miles per tank (about 14 mpg). I just got off road 265 tires and my milage dropped to 11mpg. So now i only get about 260 miles per tank. I think your tires could play a big part in your decreased mpg

Wow, I have aftermarket 17s with the biggest optional tire on my truck and I'm gettin well over 400 miles on a tank. Other factors include but not limited too, its a 2wd, crew cab ssb with cover, and a bb tune. But even before that I was gettin 398 miles a tank. And I believe my tank is a few gallons smaller than yours. Also forgot to add that it has the 3:23 gears. And I can't believe that motor only churns out 155hp, what a dog of a motor!

 

hp dosen't mean much..torque is the number you want...but yes the main focus of the 6.5 deterd diesel was MPG where the cummins and powerstroke were more work horse motors...don't ask me gm did this, regardless they blew it out of the water and the duramax took over

[plowking]

My 6.5 gets decent mileage,if I stay out of the throttle. I had the intake and 4 inch exhaust done. I also have the marine injectors and #6 resistor. I have a dump body insert in mine and it does just fine. Another thing is the motors sweet spot is 180 degrees ,I had a cooler thermostat and hood louvers installed to keep motor from running warm or overheating. The motor is suppose to be 190 -210 hp. I have the H.O. edition in mine,it is a 3500 series. plowking