Engine/Transmission (1998.5 - 2002) heavy differential oil and poor mileage

[Wyomingcowboy]

I replaced my differential fluid with some heavy Schaeffer's brand in the rear awhile back. I haven't done alot of driving since but have noticed a real loss of mpg's. My thought at the time was that the Schaeffer's was free, and most of my driving would be pulling a heavy horse trailer in the summer time so it wouldn't make too much of a difference. My mileage has dropped from about 18-19 empty and hwy speeds, to about 15 empty and hwy speeds. Pulling a trailer is even worse - about 10.

Is this type of mileage loss normal for running heavier oil or should I start to look elsewhere?

TIA

I was running Amsoil 80W-90 Gear Lube and switched to Shaeffer's 75W-140

 

[RonaldLarson]

My guess probably matches yours. That non-synthetic heavy stuff is the culprit. No need for it even in summer. I'd check your manual and see what weight is recommended. Definitely go with synthetic.

 

[SAndreasen]

Shaffers is synthetic I've ran 75 140 synthetic and never noticed a mpg drop I think you got plugged air and fuel filters maybe I don't know .

 

[SAndreasen]

Some info on the oil also my manual says to use 75 140 when pulling heavy

 

[AH64ID]

The Dana axles like a heavier fluid for heavier towing, but is highly doubt that going from a 90 to a 140 is giving you a noticeable reduction in fuel economy. The fluid would have to be like molasses.

What else was done around the same time?

 

[Wyomingcowboy]

Nothing else was really done. I changed a leaking real pinion seal, heater core, and clutch all within 6 months..... I just replaced the clutch last night actually. Maybe it's been slipping? I had an incident 2 nights ago where I fried the clutch but prior to that there was nothing obvious. No RPM/speed ranges being out of whack and no smell.

 

[Wyomingcowboy]

I checked and replaced the fuel filters. It's been 2 years and 25k miles.....

Would fuel filters cause poor mileage? If that was the case I would be starving the IP too, correct? I have gauges but my fuel sending unit is dead and has been for awhile. I was consistantly 10+ PSI no matter what before the sender died though

 

[AH64ID]

It could. How new is the rebuild?

 

[Wyomingcowboy]

I've got 80k on the rebuild. i probably should fix my signature.....

 

[AH64ID]

Yup, probably nothing with that

 

[SAndreasen]

When have you done your air filter and yes fule filters can effect millage m I had two trucks where the filter minder never moved no matter how plugged the filters were

 

[JR]

15mpg might not be all that far out of the norm this time of year. You say you haven't been driving the truck lately, if you are comparing your mileage from the warmer summer months to now there will certainly be a change. Colder temps, winterized fuel, longer idle times, they all add up. My mileage varies because I do alot of mixed driving (loaded and unleaded) but typically 2-3 mpg loss is not uncommon compared to the summer months.

 

[Wyomingcowboy]

The mileage drops have been with no idle time. I did a few long trips over Christmas and that included pulling in to fuel up warm, and then get right on the hwy, drive for 200-300 miles, and fill up again. I have changed fuel filters now and we'll see if anything improves. I'm not planning on driving it much for the next few months though so who knows what will happen.