Alright final attempt on fuel mileage before I trade my truck in.

I have a 99 silverado 1500 4x4 200k miles, runs and drives amazing, but when I bought it Bone stock, was getting 10mpg, so I did a full tune up, new plugs, wires, fuel air and oil filter, fresh synthetic fluids, o2 sensors, couple cans of seafoam and injector cleaner, cleaned maf and throttle body, no dragging brakes, electric fan conversion, proper fuel pressure and absolutely perfect compression, and to top it all off I have a custom tune, none of that changed my fuel mileage one bit surprisingly, now.im running 34 inch tires and only lost 1mpg which is expected but still 9mpg with everything I've done, and a custom tune is ridiculous, I bought the truck and me and my dad worked on it before he passed, so it's hard to get rid of, moving into a new place and gas will be the downfall of paying rent, if anyone has any other ideas please help, thank you

 

What do you mean a custom tune is ridiculous? How old are the cats?

 

What gears?

 

And is this all city driving? What size converter if not stock? Trans slipping any?

What was done in the tune?

 

I don't know your financial situation, but there's no shame in having a $1500 Honda or Toyota to get around in, saving the truck for fun or truck duty.

 

I dont think the reported MPG is far off what should be expected for city driving

 

Yep. My daily is a 15 Mazda 3. It was tough to swallow at first but my fuel cost on daily driving my truck pays for the car payment , insurance and gas. Plus if it gets door dings I don't give a crap!!

 

Cats are about 3 months old, custom tune he made sure timing was perfect, and air fuel ratio, shift points ect,

Trans doesnt slip, and everything works as it should, I have a streetbike for commutes but in the colder months I enjoy driving my truck, would just like to make it a bit cheaper to drive

I'm running 3.73s now with my 34s, I had 3.73s with stock tires and only got 10mpg that's why I'm worried gears wont help much niether, I feel like it should get more mileage as it is before I spend thousands of dollars on upgrades

 

If it wasnt done on a dyno, fueling and timing arent perfect. Do you know the specifics of the tune? Did he change commanded AFR at low load?

My Tahoe is 17mpg when she is babied on a road trip and ~15mpg on my mixed commute. Its been a long time since she was only city driven, but I recall ~220-250mi to a tank in the city, which is right around 10-11mpg. I dont think anything is wrong with the truck, this is the nature of the beast.

I have a 5.3, cammed and bolted with 33s and 4.10s, for the record. ~6k lbs with full load of fuel

Improvements you can make are to increase flow in and out of the engine (intake, long tubes, no cats), lean it out at cruise, slow down, coast hills and pulse and glide driving methods

 

Leaning out the cruise would be nice if the OS would allow you to do it, but it won't. Unless you ran open loop, and I don't think anyone wants to do that.