how well do these trucks pull overdrive?

[chris142]

My 87 with a stock 6.9,t19 and 3.55's runs 3000 rpm @70 and really struggles on the smallest hills even empty. It really sucks fuel running like this and I know an overdrive would improve the mpg but can it pull od? I've never pulled a trailer but I'm guessing that id be down into 2nd gear on any hill so I don't think it would pull od on flat ground.

 

[Shadetreemechanic]

You sound like you have a timing or fuel delivery problem. These trucks run fine with od, I regularly do 7 mile grades with my stock 6.9 in od.
Check your filters and then your timing.

 

[chris142]

Timing is at 6.5. Any more and it runs worse and rattles bad. Fuel filters are new within the last 1000 miles or so. Air filter 2 weeks ago. Running out i40 to az really sucks on the small rolling hills and it gets 12.5mpg in town or the hwy. Slowing down to 60 gets me 13.9 mpg but is unsafe with trucks running 70 and cars running 100+.

 

[jaluhn83]

That seems pretty low to me, not sure what's going on there. I get 10-11 mpg running 60-65 at 14k.

If you want to go ZF5 I've got one for sale.....

 

[chris142]

I want to keep my t19. If I went with an od id rather bolt one on behind the trans if they make one. a Turbo scares me. I don't know much about installing one and id be afraid of blowing my heads off my 26 yr old engine lol

 

[jrad235]

How many miles on the pump and injectors?

 

[Shadetreemechanic]

I would focus on your power issue first. Do you know the history on your pump? You should be knocking down mid teens for mileage with no power issues unloaded. If you can't get your pump to time that makes it suspect as well.

 

[freebird01]

with my truck (cclb drw 2wd zf5 4.10's) it wasnt happy unless i was running 65-70. now rolling hills was more like 60-65... steep hills i was down in 2nd gear... that was with a way overloaded truck and heavy trailer...if i were to guess i was in the 14-16k in weight...my truck was on the overloads before putting the loaded trailer on the back.

i averaged 10-11mpg with it... thats with a NA 7.3 that hammered like hell and had a set of stans headers on it. unloaded 70mph was fine...a bit high in the RPM scale but still 12-13mpg. i think between the internal issue and an original ip with appx 130k on it thats what was killing my mileage.

 

[chris142]

Pump was new when I bought the truck last year. Rebuilt by a pump shop in anchorage alaska. No clue on injectors. Runs smooth though

 

[jaluhn83]

Chris, what part of socal are you in?

 

[chris142]

Hesperia

 

[funnyman06]

Something seems off. My 85 with C6 when NA ran better than that; and the worse I have EVER gotten was 16mpg, and that 10K going up the rockies in AZ/NM.