Could someone tell me what I should expect in the way of fuel economy with the 3/4 ton Cummings diesel, with auto transmission with overdrive, in my 1993 Dodge D250.

Also would it be possible to put in a five speed stick shift? The reason I ask is the overdrive is not working properly and it might be better in the long run on fuel economy to go with the stick shift. Thanks

 

In good tune about 22 on the freeway and 18 city. Sounds like your TPS is not operating correctly. Do a search on here to find solutions to overdrive problems related to the throttle position sensor.

brad

 

I've been averaging 16-18 MPG in town, but haven't taken the truck on a long freeway trip (empty) to check the mileage in a while - last time it got 22 MPG.

The OD problem can be one of several things. I'd start with cleaning the electrical connections at the TPS, speed sensor, and trans temp sensor. Use some contact cleaner, and dielectric grease in each one before reconnecting them. The TPS may also need to be adjusted - you can search this forum for the instructions. I have mine set at .70v, which seems to work well for my type of driving.

Your truck can be converted to a 5-speed, but it would be a lot of work and wouldn't be cheap. Besides the transmission, you would need the pedals, linkage (or hydraulic system - not sure what the 1st Gen uses), flywheel, clutch assy, pilot bearing, etc. I thought about the same swap, but couldn't find a wrecked truck to get the parts from. A 5-speed may give a little better MPG, but personally, I don't think it would justify the cost or work involved.

 

92Diesel's truck ('92 D250 auto) got 22 mpg @ 75 mph before I messed with the pump. He only gets 21 mpg now.

The automatic actually has a taller overdrive (.69 vs .77), so RPM is a little lower on the highway. Fuel economy should be a little better with the auto.

I would not bother with swapping in a 5 speed, it would take a long time to pay off even at current fuel prices. The OD problem is electrical and can be fixed for free or pretty cheap. No reason to take drastic measures to fix a minor problem.

 

i have a 92 d250 auto... on my last trip, average speed 83mph i got 20.1 (1 turn of the fuel screw, everything else stock) before the turn, not that i think it effected anything highway. i took a long trip and killed myself to stay at 70 the whole way, i got just over 23mpg.

 

The three trucks that are on the road in my sig get around 17-20 mpg in mixed driving, 50/50 mix of highway and some stop and go.

 

It all depends on the gearing and how ou drive it. The auto does better for fuel economy. If you have the 3.54 gears mids 20s is the norm for highway around 70-75 mph. Ive turned a bunch of these trucks up and got feedback of 1-2 mpg increase, I got that on my truck also.

 

Then why is my truck back averaging between 13-15 mpg. Smoke screw is now only one turn, everything else is stock?

 

Well for starters, you have a 1 ton (extra wheels) you have 4 wheel drive and I dont know what gearing you have. If your talk about the "smoke" screw, does that mean the full fuel screw? If you've turned up the idle screw and softened the spring under the cap and have 4.10 gearing, you've done everything you can to decrease mileage.

I've backed off on the AFC spring on the and idle screw all the way up. The full fuel screw controls all the above and I've cranked my about 1 1/2 turns from stock and my timing is 1.7mm.

This SHOULD give about the best mileage one can expect from a Cummins engine. I also have 3.54 rear end but have yet to put in the tighter converter which I'm told could help by 1-2 mpg.

brad

 

I average 16-17 city and 22 + on the highway. Most of my driving has been city lately and I usually achieve 20 mpg average when I fill her up. My truck is also unmolested...otherwise, bone stock. The best mpg I've had so far was coming back Highway 99 N from Tehachapi CA to Modesto CA where I was traveling 75 mph w/cruise on and managed a killer 26 mpg! Way, way too cool!
Of course, the harder(faster) you drive it the less mpg you're gonna get. Thats a given.

 

3.54 gears, stock 2300 spring. Smoke screw= Torx 25 screw w/ 13mm lockscrew under the cap. I think it it the no/low boost fuel screw. Newbie so you'll have to excuse me.

 

torque screw should be all the way up. AFC spring should be tighter to limit fueling to higher boost levels. How about the timing? betweet 1.5mm and 1.8 is where most of us have our timing on our fuel pumps.

When you start it up does it blast a lot of black smoke? when you accelerate normally down the street does you exhaust blow black smoke?

Not until I'm getting close to half throttle does my pipe show any smoke.

brad

 

I would like to just clearify what wannadiesel said. My truck @ 85 mph gets 21! 75 mph I get about 22. and lately I've been doing the speed limit and no WOT launches and last fill up got 23.8....

Thank you Dave

Michael