horrable fuel milage above 90 km/h (2002 sierra 5.3 automatic)

[brucekramer613]

hello, i bought a truck because i run a construction company and i need one i use the box everyday. after i bought it , i figured out it had stock, upgraded 4:10 gears, and it has 265/70/16 tires ( stock tires and gears but there lower )... so im pritty sure the lask of mileage comes from the fact that i rev at 2200 - 2400 at hwy speed and im on the hwy all the time. i know for all of your yuppie cars putting bigger tires ( not mud tires just upgraded hight) on causes it to burn more fuel, but what about on a truck, where i would be lowering the cruseing rpms ?

[brucekramer613]

2002 sierra 5.3 automatic any other questions about her?

[mcrews]

Welcome!
There are many threads about slightly larger tires, have you tried a search?
Larger diameter tires can decrease rpm and thus increase mpg at highway speeds
What mph is 2400 rpm?
Maybe you just need to slow down ?
My 2002 Infiniti Q45 saw at 1750 rpm at 60 mph ( I had larger diameter tires) that 5.3 v8 is probably reving a little high.

[MetroMPG]

Need more info: what mileage are you seeing at what cruising speed?

What is the truck's highway rating (EPA, not NRCAN)?

[darcane]

Are you running LT tires or just standard radial tires? LT tires are heavier but have higher load ratings which you may or may not need.

Taller tires are also heavier, so you'll have to watch for that. If you switch from steel wheels to light aluminum wheels at the same time as increasing tire diameter, it may be enough to cancel out. But, higher tire pressures will likely help more than increasing tire size.

I had a similar truck. Check out the link in my signature for more details.

[brucekramer613]

even if i dont use the load rateing all the time, i would still like it , in the tires, and i dont drive to fast, thats the speed limit, or very mildly above it i see 17 mpg ish at 80 and it drops the faster i go, by alot. vs my dads f150 revs at the same rpms at 130 km's. i would like to get rims and tire upgrade, but not in the budget right now

[aardvarcus]

I also have a similar GM truck with 4:10 gears.
Is your tow/haul button pushed in? I don't think your transmission is shifting into overdrive, or the gearing is a lot lower than 4.10. Start from a dead stop and count the number of shifts it makes.
You stated 2200 - 2400 at highway speed, I am assuming that is your mentioned 90kph, or about 56 mph. If the speedometer in a GM truck with factory diameter tires reads 56, your are probably actually doing about 51 mph. (Your 265/70R16 is roughly the same diameter as factory 245/75R16.)
51 mph is 4488 feet/minute, and with 31” tires you will go about 8.111 feet per revolution, thus your tires spin 553 times in a minute. With 4:10s in the back, that means your driveshaft is spinning 2268 times a minute. With your transmission in direct drive (3rd), that would mean your engine is also spinning about 2268, so right in the middle of your 2200-2400 range. Your truck should not be in direct drive, it should shift into overdrive (4th) and have a 0.70:1 ratio, dripping your rpms to 1588, which would be about right.
Either that, or someone put rock crawler type gears in it, around a 5:86 ratio to get those rpms at that speed.

[oil pan 4]

Originally Posted by aardvarcus

I also have a similar GM truck with 4:10 gears.
Is your tow/haul button pushed in? I don't think your transmission is shifting into overdrive, or the gearing is a lot lower than 4.10. Start from a dead stop and count the number of shifts it makes.
You stated 2200 - 2400 at highway speed, I am assuming that is your mentioned 90kph, or about 56 mph. If the speedometer in a GM truck with factory diameter tires reads 56, your are probably actually doing about 51 mph. (Your 265/70R16 is roughly the same diameter as factory 245/75R16.)
51 mph is 4488 feet/minute, and with 31” tires you will go about 8.111 feet per revolution, thus your tires spin 553 times in a minute. With 4:10s in the back, that means your driveshaft is spinning 2268 times a minute. With your transmission in direct drive (3rd), that would mean your engine is also spinning about 2268, so right in the middle of your 2200-2400 range. Your truck should not be in direct drive, it should shift into overdrive (4th) and have a 0.70:1 ratio, dripping your rpms to 1588, which would be about right.
Either that, or someone put rock crawler type gears in it, around a 5:86 ratio to get those rpms at that speed.

Sounds a lot like you have one of 2 problems.
Some one swaped out the gears or the transmission isn't going into over drive for some reason.
I would supect that the person who owned the truck before you realized the truck was having some kind of transmission issue and off loaded their problem onto you.

Another possibility is the torque converter lock up isn't working. But I think your RPMs would be closer to around 2000 or less.

[darcane]

Originally Posted by brucekramer613

even if i dont use the load rateing all the time, i would still like it , in the tires, and i dont drive to fast, thats the speed limit, or very mildly above it i see 17 mpg ish at 80 and it drops the faster i go, by alot. vs my dads f150 revs at the same rpms at 130 km's. i would like to get rims and tire upgrade, but not in the budget right now

Is yours 2wd or 4wd? I bought a used set of aluminum wheels off a 4wd Silverado. They have taller tires stock than the 2wd. The used set with good tires were half the price of just buying new rubber...

Definitely sort out your tranny issue, that should be your first step.

After than, look to aero mods. What is on your bed (if anything)?

[LeanBurn]

Pretty sure factory tire size is 245/75R16. My truck revs at 1800 rpm at 90kph or 55mph, but I have 3.73 gears.

I plan to change mine to 235/85R16...It will correct my speedo error to match GPS, lower my highway cruising revs ~150 RPM and they are 10mm narrower than stock so less aero drag.