I'm another guy that would like taller gears, was thinking 3.73 but after reading this I think I'd go all the way to 3.42. My question is how hard it is to change them out. I wouldn't do it myself, have 2 different mechanics that I might use. One of them told me the rear was easy but the front looked like a bear, had to do a lot of disassembly to get it done. Is he right? How long should it take a decent mechanic to get it done?
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Diff gear ratio suggestions
- Thread starter Dave Barbieri
- Start date
Dave Barbieri
Member
Wow, that's impressive! A 10.5" full floater on a half-ton vehicle. I didn't realize they were that heavy duty!
Dave Barbieri
Member
Dave -
Looks like you have the same drivetrain setup I do - 4L80, 4.10 gears and 235/85's. What do you get for mpg? I'm considering a swap to 3.73's. That would be approx 10% reduction in rpm. If my math's correct, I'd be looking at 2250rpm at 65mph. I kinda figure I'd see around 1.5mpg increase, which still would put me shy of 15mpg. I'm about to head out the door for OKC to check on mom & dad. That's 325 miles each way. Outa curiousity, I'll try to hold my speed to 65 (instead of 70 - 75) and see if there's any improvement. Most of the other folks responding to this thread seem to be doing a lot better consumtion-wist than me. I realize there's a difference in vehicle weights, but frontal area stays fairly similar. We'll see.
Interesting day to ask me about mpg! I had been getting around 17 mpg since I've owned the Suburban, bought it in May. I recently had a new injection pump installed and mpg dropped to 13-14 mpg. We checked timing and it is WAY off, I'm bringing it in this morning to be timed. I was surprised it ran so well with the timing off but the mechanic said they'll run great, just love the extra fuel but it does you no good. You might want to look into pump timing, even given your gearing you should be seeing 15 plus mpgs.
I'll let you know what I have after its timed but will take a week or more to run through a tank, I'll be away for the holidays.
I'll let you know what I have after its timed but will take a week or more to run through a tank, I'll be away for the holidays.
pgguru
Recruit
Mine is a 342 semi floater with 8 lugs. That is what is stopping my from a rear disk conversion.
On a other note. If you are running Bio you will be giving up 5 to 10% fuel milage to it. I run it all summer and then Dino in the winter and I get the same mileage as the Bio. On long Road trips when I have bought Summer diesel I can see 200KM (120 Mile) more on the tank.
OK, the pump was timed this morning to 2.5-3 degrees, it was over 10 degrees before this. TDC offset was at .61. On the scan tool the fuel rate at idle was 8 mm3, not sure what the normal figure is for this.
Dave, I think there is something wrong with the rpms you are seeing, either you have shorter than 4.10 gears or something else is off. I see (corrected for my oversize tires) 2000 rpms at 60, 2150 at 65, 2300 at 70, and 2500 at 75. I can't see why you'd be turning 2300 at 60 mph.
Dave, I think there is something wrong with the rpms you are seeing, either you have shorter than 4.10 gears or something else is off. I see (corrected for my oversize tires) 2000 rpms at 60, 2150 at 65, 2300 at 70, and 2500 at 75. I can't see why you'd be turning 2300 at 60 mph.
Dave Barbieri
Member
Invested in the GMTDScan program from Stefan Enghe. Definitely worthwhile! Got my pump timing set and the TDCO is spot on at -1.94. A little clanky in the morning, but I sure enjoy the way it runs.
I need to pop the diff cover and count gear teeth. One of the things I've wondered about is if I still have the stock gears or if one of the PO's changed them out. First PO used it to pull some super duper horse trailer/traveling palace; Second PO built metal buildings and used the truck to pull a twin axle trailer full of sheet metal and tools. I just drive around and have a good time.
Anyway, I'll pop the cover this weekend and see what I see.
Sunshine
Northern Lights
Invested in the GMTDScan program from Stefan Enghe. Definitely worthwhile! Got my pump timing set and the TDCO is spot on at -1.94. A little clanky in the morning, but I sure enjoy the way it runs.
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I felt i had to copy this bit of Fred to "An up Date" i hope it`s okay..i know sometimes i`m a pain.. ):h
midniteplowboyy
Recruit
Just got back from a trip up to McAlester OK, ran an average somewhere around 75mph highway(70-80mph most of the time), turnpike speed limit is 75mph, kept most of my upshifts below 2700rpm. 413 miles took 21.54 gallons, that comes to 19.17mpg's(hell yea), well I think its good for a 4door dually.
I wanted to keep it @70mph, but I was traveling with my sister and her family, anyway it did pretty good.
How was your trip up to the big OKC?
Dave Barbieri
Member
I am soooo jealous! You're seeing about 30% better mpg than me, and you're in a heavier truck!!
Went well. It's great to see family, and I like being able to do things for mom & dad (they're in their 80's). Got all the Christmas decorations up. Thank heavens they just do 'lawn art' now. I HATE being up on a ladder and hanging lights from the roof. And that star that went on the highest point above the garage.... Man, it was hours before I could talk in anything but a soprano!
On the trip up, I got a late start (7am Wednesday instead of 3pm Tuesday). Traffic was heavy from Dallas to Denton, but moving pretty steadily. I had planned on holding my cruising speed to 65-ish, but that didn't work out. Traffic was moving much faster, and most of those folks didn't seem too happy. Mostly 70-75. My mileage for the trip up was 13.8. The trip down was a little calmer; I kept it around 68. Mileage was 14.1.
McAlester, huh? That's a little further than OKC. Hope your Thanksgiving was a happy one!
Clipper
New Member
Been running 3.21 for about two years now. Love it.
RPM dropped from 2500 to about 2100 @ 75 mph.
1800 used to be 55 mph, now it's about 65. 70 is about 1950.
I am mostly solo, occasionally have 1-3 other people with me, rarely haul a load.
When I do pull with it, I just downshift to keep the engine near 2500.
I installed a manual TCC lock to keep the trans from heating up.
With the 3.21's, it *does* get a little funky sometimes when climbing... "hunts" a lot.
When it does this, I just downshift and lock the TCC.
1800 is the "sweet spot" on our engines for mileage...the peak of the torque curve.
2500 is the "sweet spot" for pulling...the peak of the HP curve.
Our trucks ARE sensitive to rpm and speed...general rule of thumb, you lose about 1 mpg for every 5mph over 55. That's in the stock config...chip it, and everything changes.
I consistently got 16.5 driving at 70 before the 3.21's.
WITH the 3.21's Now I get 17.5-18 at 70.
BTW, our engines are not supposed to be any good for engine braking without an exhaust gate, but I've found that with the TCC lock, I *do* get some braking effect. Maybe not as much as with a gate, but enough for me when running light.