Here is an example
The other day my folks came to town for a visit.My dad has a 03 GMC z71 xcab 5.3 3.73,all stock except a catback.Near where I live is a straight stretch that I have marked from a stop sign to a power pole,This is somewhere near 1/4,I dont know ,never measured it and dont care.Well I was driving him in his truck and came to the stop sign and nailed it,his truck laid down a solid 83MPH.Ever since I did my 6.0 3 years ago it has yet to fail, laying down 94MPH every time.

 

we have the same type thing only are is to a mail box. when my truck was stock i could gat 98. now with mods i can only get 95. havent tested it with the new parts but i would think that it would be 95-96. i know a 6.0 would be more benifical to me and what i want, but it is defently the cost. i know going faster isn't free. another thing is i dont want to put the money into the 6.0 and have the trans blow. i planed on putting some tm back in and taking it easy. but then i have to start saving to replace it after the 6.0

 

Why do you think it slowed down in your marked spot?Did you do from 245 to 285s? that would do it.

 

Is your dads hd 03 a 1500 or a 2500? If its all stock then I would think it should get worse milage. A 5.3 with bolt on mods can match the power output of stock 6.0. This is achieved by raising overall efficiency. If the same mods were done to the 6.0 its mpg should come up and the 5.3 would fall behind again. Once you switch cams the whole overall characteristics of the engine will change. At a certain level n/a the only way to improve power is by moving the powerband skyward (think vtec with hp aproaching 100 hp per liter up near its rev limit) and the fuel efficiency should drop in proportion. This works in an f-body but with heavy trucks it seems kinda pointless (think of a vtec under 5000 rpm with its mighty 80 pounds of tq). As for the 4l60 it consumes like 25hp to turn it and the 4l80 uses at least 45hp to turn. This is all from memory but I remember once in a magazine they dynod a turbo 350 and a turbo 400 behind the same engine and vehicle and those number seem to have be sticking in my head. If the 5.3 is stout enough the 4l60 wouldn't last in stock form. A built one should last and I dont think the 5.3 would ever put out enough tq to kill it. Unless you beat the crap out of it and then your mileage would be way bad anyway.

 

HD's dont have 5.3 and Z71 = 1/2 ton.My dads fuel mileage is fine.On my truck with the 5.3 UPD,S&P custom tune hamms headers and catback my truck ran a 86MPH in the same spot.Even if I did a head/cam on the 5.3 it still wouldnt be close to the 6.0 due to the weight.In a RCSB 2x4 it maybe a different story,they might be closer.

 

Oh sorry I was talking about The Ladies Mans dads truck, he said it gets like 10 to 11 mpg.

 

its a 2500hd. all stock. i know the 80e sucks hp up! im just hoping my 60e will hold for a little bit.


i think the reason my mph were slower was because i was getting there faster. that would be my guess. i get there faster and there isn't enough time to get my mph higher. i dont know

 

you would not get any worst gas mileage with the 6.0 than the 5.3 because you will take the bind off the motor.one of the guys @ work gets about 17 to 19 with a 6.0 in an x-cab with 3.73.