Poor Highway mileage

[morebiodiesel]

I've been running Innovative Aggressive Street tune for a few months now and average 17.5mpg city driving (21 stoplights in a 24 mile commute). The best tank I've seen in the city with a VERY LIGHT FOOT was 19.8mpg. 17.5mpg average is up 1.5mpg from my complete stock city driving average.

I averaged 11mpg with the aggressive Tow tune taking the camper back and forth to the beach (mostly flat). That is 2mpg less than my stock average when towing the camper.

Yesterday I needed to drive back and forth to Richmond VA from Raleigh NC for business. It was about 360 mile round trip. It was all highway at approx 70mph. I had 3 passengers and our average weight was probably 200 so I had about 800lb load in the cab

I expected to get >20mpg on the highway mile round trip but only got 17mpg. I filled just before and refilled immediately after the trip and had filled up to the neck both times.

I'm disappointed with the mileage. I used to get 18.5mpg on the stock tune on unloaded highway trips.

So my average city driving is 1.5mpg better than stock but 1.5mpg worse than stock on the highway.

Any ideas?

[bmoore884]

Do you ever run a fuel treatment or cetane boost?

[morebiodiesel]

Yes I run the stuff in the silver bottle... Power Service I think it's called

[bmoore884]

I been using amsoil fuel treatment and cetane boost and I can tell a difference in milleage when I dont use it thought that might be a factor in your situation.

[tech10002]

You could have gotten some bad diesel. I seem to lose 2 MPG when I get mine from Walmart. How long has it been since you've changed fuel filters and oil? My mileage starts tanking once I hit the high 5k range on oil changes and in the 15k range or so with fuel filters. On my last trip to Florida, I thought I could make it back home before changing, but I ended up doing it on the side of the road in my mom's old folks trailer park since the economy got so bad. LOL I was down to 16 something on the road. Once I changed the oil/filter and fuel filters, I was back up to high 19s.

[HunterJ]

What speed are you runngin exactly? when driving 65-68 i got around 19.3 when going 75-76 i got 17.2

[morebiodiesel]

Originally Posted by tech10002

You could have gotten some bad diesel. I seem to lose 2 MPG when I get mine from Walmart. How long has it been since you've changed fuel filters and oil? My mileage starts tanking once I hit the high 5k range on oil changes and in the 15k range or so with fuel filters. On my last trip to Florida, I thought I could make it back home before changing, but I ended up doing it on the side of the road in my mom's old folks trailer park since the economy got so bad. LOL I was down to 16 something on the road. Once I changed the oil/filter and fuel filters, I was back up to high 19s.

The fuel filters were changed less than a 1k ago. I'd gone 45k on the frame mounted filter before the change

The Diesel was from an Exxon station that sells a lot to truckers so I think it was good.

I've never noticed a mileage change during an oil interval but I'm at 6600 miles on this change (I change at 7500mi).

The highway mileage definitely seems to have dropped with the new tune even though the city mileage went up.

[morebiodiesel]

Originally Posted by Harohunter

What speed are you runngin exactly? when driving 65-68 i got around 19.3 when going 75-76 i got 17.2

Speed was about 73-75mph for half the trip and 68-70mph the other half (VA has a 65mph limit).

[morebiodiesel]

One other note is that I'm running a slightly larger Ham radio antenna (a Tarheel antenna) now that I used to but I wouldn't think the increase in drag would account for 1.5mpg.

[tech10002]

Wow. I can't believe the truck still ran with 45k on the FICM filter. LOL

You might try cleaning your EGR valve. That can cause poor fuel economy if it's getting sticky. I'd also go ahead and change the oil. I definitely start losing economy at around 5K.