After driving my 01 Ram for the better part of a month now, I notice how much of a gas hog it is! Granted, I expected it - but not as bad as it’s been at 8.5 MPG.

I recently did a tuneup, and had the ACCEL TST24 kit installed. This includes: Cap, Rotor, Plugs, and wires. Also installed was a new MSD 8228 Coil and a generic Distributor Pickup Coil. The truck runs very smooth, and is otherwise bone stock.

Rear gears according to the tag are 3.21’s. Originally the truck had 225/75r 16 tires. I purchased it with upgraded 20” rims from a 5th gen, new tire size being 275/60r 20. Could this really affect my fuel mileage that bad?

I know the 5.9 gasser doesn’t sip fuel, but I also know it should get better mileage than that. Especially in a little single cab! I appreciate any advice and help you may have to potentially increase fuel mileage. It’s getting the same mileage that my 1968 Chrysler 300 with a 440/727 gets and that’s wild to me. After driving 128 miles, it took 15 gallons to get her back to a full tank.


Plenum gasket isn’t blown, and she doesn’t burn any oil whatsoever. Transmission is very healthy.

 

You actually traveled 144miles now up to 9.6, still sucks. FYI. When driving at 67mph you're actually doing 60mph.
https://www.blocklayer.com/tire-size-calculator

 

Is it possible the inputs you entered were backwards? Originally the truck had 16’s but now it has 20’s. When the speedo shows 60 MPH I am really going 67 and so on and so forth.


Doing some more research, I know the truck has all original sensors (ie: MAP, TPS, IAC.) I wonder if a failing or dirty MAP could significantly affect fuel economy.

 

I stand corrected. Speedometer at 60=67 actually. If you replace 225/60/16 with 275/60/20 for Odometer reading 128
the actual distance traveled = 159 ~ (23.9%) (If your Odometer is accurate for original tires) I would use a GPS for miles traveled.

 

I suspect your rear end gearing is causing a fair bit of your fuel mileage issues, especially if you're doing a fair bit of around town driving. You're running a 33.5" tall wheel/tire combo with 3.21 gearing. I'm running 33" tall tires with 4.10 gearing and get about 12 MPG in a Ram 2500 4x4 with 5.9 magnum engine. I changed from the stock 3.54 gearing to 4.10 gearing to compensate for the taller tires than stock and to restore some of my truck's get-up-and-go from a stoplight. Your motor is working mighty hard to turn those tall tires with 3.21 gearing...

In my humble opinion, to ensure that you're getting the best fuel mileage that you can, the cheapest things to do 1st is to change the O2 sensors and thoroughly clean the throttle body.

 

Put everything back to stock and drive like a grandpa and you'll see 13+ in town.

 

Outside any sort of driving style.... I reckon the 3.21 gear and bigger tires are hurting you. When do you hit overdrive? 80mph? I would go to at least a 3.90 gear.

 

Yep. Gearing sure ain't helping. So, smaller tires, or better gears. It's a toss up which is cheaper.

May want to change your O2 sensors as well. If they are original, they are gettin' might long in the tooth... NTK or Denso sensors only please.

 

Where is your temperature gauge reading? I've worked on numerous computer controlled engines over the ears where a cooler thermostat was used to try and get more power. That works on carburetors, it doesn't on computers if you don't flash them. I worked on a Chevy C3500 once that got 6 mpg. It had a Failsafe thermostat that had failed. Replacing the thermostat with a 195 unit and mileage went from 6 to 11 mpg. Not great, but for a dually with 4.11 gearing, it was acceptable. If your engine never warms up, it will stay in open loop mode and inject extra fuel trying to warm the engine up.

 

Good suggestion, I’m going to “guesstimate” and say that my temp sits at around 180 when the truck is warmed up and stays pretty consistent. I’ve considered a 195 but wasn’t sure if it was worth it or not? ALSO, I’m not even sure if it’s overkill for my truck - but that Hughes airgap intake looks to be pretty promising. I’ve heard there is some MPG’s gained with it as well. I am wondering if it’s worth it to install that intake as well.

Smaller tires it is! I’d like to throw on some 17’s. A 245/65r 17 should be almost equal to the 225/75r 16 so I don’t think it would be an issue. Also good suggestion on those O2 sensors! Will put that on my list of things to do…


I hit OD pretty early it seems. Gradual acceleration I seem to hit OD at like 55 or thereabouts. But if I am accelerating to get on the freeway it kicks in closer to 65/70. I don’t like the big tires. Gonna go down to a size closest to the factory.