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

[darcane]

Originally Posted by 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.

Again, it depends on what truck he has...

There are four different tire sizes used on 2002 Silverado 1500s:
235/75-16 (2wd)
255/70-16 (2wd)
245/75-16 (4wd)
265/75-16 (4wd)

So, if he has a 2wd, switching to a set of tires off a 4wd will result in an overall higher ratio over stock (either a small bump, or a significant one).

The set of wheels I got were from an '05 Tahoe 4wd with 265/70-17 tires which were about 2" taller than what I had stock. Silverados, Tahoes, Suburbans, and their GMC twins can all swap wheels ('99+, '88-'98 too, but occasionally may run into clearance issues with the smaller wheels) so there are a lot out there to choose from.

[ijames]

Do you have a handheld GPS, or a smartphone with GPS? I downloaded a free app from Samsung for my phone that shows a speedometer and odometer, so I could use it to verify that my speedometer was dead on (within .3 mph from 45 to 70 mph). When I first got the truck (05 gmc 2500hd duramax diesel 2wd) I checked the odometer on the highway several times using 10 to 40 mile distances and it's about 0.02% high so I figure that is also "dead on" :-). Anyway, check your speedo and odometer to make sure your ecm knows what rear end gear and tires you have, so you can be sure you are starting with accurate speed, distance, and mileage data. With my bone stock truck I get about 21-22 mpg at 60 mph on the highway, but I lose about 2 mpg per 5 mph as I speed up from there, so drive as slowly as you have time for.

[Hersbird]

I vote you are not shifting into overdrive, 4.10s or not for 55mph your rpm is way to high to be in overdrive, either that or your kph it really mph but it think you would notice with all the speeding tickets you would get.

[brucekramer613]

hello, to be honest, my trucks light is out behind the odometer so i cant read the 1/2/3/d/r/n/p whatever order, but i can feel it shift from 1-2-3-4 th gear, does overdrive feel like its shifting into fifth, and the truck only has 145 thousand kms on it , i hope the tranny's good

[aardvarcus]

No if you have the 5.3 engine your truck should have the 4L60-E, so four forward gears and you should feel three shifts. Your gear ratios are 1: 3.059 2: 1.625 3: 1.000 4: 0.696 R: 2.29. Do you observe an RPM drop after each shift? Is your truck 2wd or 4wd?

You mentioned the gear selection part of the instrument cluster is out, does your truck have any electronic issues? Can you tell by listening to the engine if the truck is actually spinning that many rpm, or is there a chance your tachometer is reading incorrectly?

[Hersbird]

You should feel the shifts and then at the end the torque converter locking up which seems like another shift. When at speed try pushing the tow/haul button on the end of the shifter (I think that's where it is), and it should rev up even higher as I think on a non diesel the tow locks out overdrive. You may have to shift down a notch in the shifter, or seeing you can't see it maybe you are already not using the in drive not overdrive.

[99metro]

2k rpm at 65 is about right, IMHO. 4.10s were great when the speed limit was 55, the rpms would be down to a nice 1750 or so. Today 4.10s are for towing/hauling and not great for highway MPGs unless you are using oversize tires and the gears are for compensation. So... 4.10s with stock tires, your MPGs will fall off over 55 mph. Over 2k rpm, all bets are off - expect low fuel mileage.

IMO, I wouldn't drive faster than 65 mph with 4.10s. I've had two trucks with 4.10s and some with taller gears. If MPGs are a concern to you, then your best and cheapest bet is to drive a little UNDER the speed limit. But if getting from point A to point B in the least amount of time, time is money, running late, etc is the name of the game, then MPGs gets thrown out the window. Up to you.

[Hersbird]

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.

See the math has been done, you are in 3rd gear with those RPMs, not 4th. No way somebody swapped in 5.86:1 gears.

[spacemanspif]

The shifter points should be (top to bottom) P - R - N - OD - 3 - 2 - 1. When on the highway bump the shifter up a click and make sure you are in OD and not 3. If you are already in OD all that will happen is you'll go into neutral and the truck will coast for a little; if you are actually in 3, you'll engauge OD and you'll see a noticeable RPM drop.

Still need to know if it's 2 or 4 wheel drive. Transfer case and locked hubs could be contributing to poor mpg, shouldn't affect cruise RPM but might still kill MPG.

should look into scangauge too. SG is better than an ultra gauge in your case because it can read the computer's trans temp sensor - I don't think the UG can do that.