I'm looking at buying an 87 6.9. I really like it, but its a 3 speed and the mileage sucks, it screams down the road at 60 mph. I was wondering what some ways to improve it would be, and how much they would improve it by.

 

I have the same problem, but I'm not going to do anything about it..

I have 85 6.9 non-turbo with c6 and 4.10 rear. I average 11-13 loaded or not.

Things that can be done:

1) Turbo, better performance and better mileage. 2-3 mpg?
2) 3.55 Rear, if 4x4 must also change front differential. 1-3 mpg?
3) Under Overdrive transmission add-on (Pricey, but they say it helps) 1-3 mpg?


I only put about 1-3k miles on my truck a year. ROI does not make any sense for me to make the change.

 

I feel your pain guys.

My truck was originally a 2wd with C6 auto and 4.10 gears. Its ideal for low speed urban conditions or towing up to 55 MPH.....not much else.

I spent just under $300 for some 3.08 gears (only possible on 2wd trucks, or 4wds that have the D44 front end). It is by far the cheapest way to bring down the RPMs for highway speeds, especially if you spend a lot of time in 70 MPH speed zones. Even 3.55s are better, but for freeway speeds, you will still be reving pretty high to hold 70.

Next most expensive option is to find a used gear splitter overdrive, or swap a 5 speed manual in there.

The most ex*****ve option is to swap an E4OD transmission, which has the tallest overdrive of all the transmission options of the era.

3.08s will limit your ability to tow really heavy trailers, but you can still do it if you are careful about it. The advantage of a gear swap is you don't have to worry about the mysteries luking in any used transmission you might find. I rebuilt and upgraded my E4OD before installing it, and spent a lot in the process, but since I put up to 30k miles on my truck per year with most of those miles on the freeway, it was an investment that is paying off.

 

belive it or not biger tires also help i have 37 on my truck i get 15-17 i swap in 3.55 and the tire size lowerd them down to 3.08 active ratio i have a 4 speed t19 with a crew cab

 

have you tried adding sea foam to the fuel filter ? I have 4:10 4x4 and a t19 manual and i get 20 mpg if i baby it and round 18 if i run 55 -60 mph only thing i did was use sea foam, changed the air, filter straight piped it and almost always fill up at shell

 

ur truck has biger them stock tires i think thats y u get the numbers u get

 

there 33s idk what the stock tire size for these trucks are arent they like 235s? lol

 

ya like 30 or 31 inchs lol

 

I'm running 285/75-16s on mine, using online interactive calculators tells me I have an actual ratio of 3.43:1 if my real ratio is 3.55 (I think it is, not positive, if I recall I was turning between 1900 and 2100 rpm at 60mph). Just returned from a 1000+mile round trip, and I haven't fully calculated my entire trip mileage, but the first and second stops of the trip on the way down I averaged 18.8 mpg. I know at some points on the return trip, we got a bit over 20, but I haven't calculated it. I will calculate the entire trip at some point today, but one thing I did do was to make sure I ran a higher tire pressure. My rims won't take over 65 PSI, so I put all four at 55 PSI, and I ran between 55mph and 65mph (for the most part), with a few 35-45 areas, and some 70+, but not a whole lot.

 

joe 31s are 265 i think so if stock is like 235 thats somewhere around 28s which are some small tires!

 

good for off the line bad for top end speed

 

That depends on the aspect ratio though. 235/85 is actually a fair sized tire, while a 235/75 is significantly shorter. Inch for inch conversion, a 235/75-16 is 29.9 inches tall, while a 235/85-16 is 31.7 inches tall.

 

got a point there

 

well said jester

 

I have the same questions. I have a 1990 IDI that does not have a turbo. It has a 5 speed manual transmission, and it has 4.10 gears. I receive 16-17 just putting around and 19 on longer trips as long as I don't push it much over 65 mph. This pickup is a 4 door long box, and it is a 4x4. I also have a 1989 IDI, but it has an ATS turbo installed on it. It has a 3 speed automatic. It is a 2 wheel drive dually 4 door long box. It also has 4.10 gears. My problem is the dually. With it, I get around 14 mpg no matter whether I'm towing anything or not. The biggest problem is the transmission. It raps the engine up to around 2300-2400 at only 55 mph. I'm ok with the fuel mileage I get out of the 1990, but I'm not ok with the fuel mileage on the 1989.

Here's what I'm thinking of doing, and I'm definitely open to suggestions if you have any. The 89' with the turbo will pull tree stumps. I tow around a 44 foot dually flatbed, and this pickup handles it with ease. I'm thinking about putting 235 85R 16's on it replacing the stock 215 85% 16's. This will help a bit, but really not that much as there's not a huge difference in the height. The biggest reason for doing this is because the 235's are so much more common around here anyway. Then, I want to replace the 4.10 gears with 3.55's. I don't need to travel 70+ mph, but I'd at least like to travel the speed limit which is 60-65 around here on the highways. Here's my question for all: how much power am I going to sacrifice going to the 3.55's? The way this pickup is set up now, it will hold its own with the new diesels up to 55 mph before I get run over because I'm not comfortable rapping my diesel over 2300-2400 rpm. Am I going to lose so much torque that I'll regret going to the 3.55's when towing? I guess I should give a little background. This pickup rarely ever is just driven. It was bought specifically for towing, and that is about all I use it for. I just want to be able to bring the rpm's down around that 2000 range and hopefully be able to drive the speed limit. Thanks for any input.