G80 replacement
- Thread starter Fifty
- Start date
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swathdiver
Full Access Member
What size tires are you running? That motor is going to hum with 4.56s on stock tires.
Marky Dissod
Full Access Member
If you've already a 3.42 rear axle, and the 8L90E, you really don't need to step up to more than 3.73.
Multiply your 3.42 rear axle, by 8L90E's 1st gear.
My DESIRED rear axle would be 4.10, times 4L60E's 3.06 = 12.54, for a frame of reference.
Do the same for 8L90E's final overdrive gear of 0.652.
For me, 4.10 times 0.7 = 2.87. Effectively I'd be improving city MpG to sacrifice highway MpG.
That's why I'd prefer 3.90 - which I don't think they make? - to 4.10, since I don't tow or offroad.
3.42 * 4.56 = 15.59 Your present 1st gear
3.73 * 4.56 = 17.01 Your 1st gear with 3.73 rear axle
4.56 * 4.56 = 20.79 This may break something, and make 1st gear too short, might as well be a seven speed
You REALLY don't NEED any more 1st gear torque multiplication than 3.73 will provide.
3.42 * 0.652 = 2.23
3.73 * 0.652 = 2.43
3.73 might actually save fuel on the highway, unless you enjoy speeding over 70MpH (as I do).
4.56 * 0.652 = 2.97
4.56 won't save any fuel on the highway (nor in the metro / urban).
If you want more axle than 3.73, 4.10 would be more than enough.
For best results, tune your ecm & tcm for the gear change, especially to improve the transmission's personality.
Multiply your 3.42 rear axle, by 8L90E's 1st gear.
My DESIRED rear axle would be 4.10, times 4L60E's 3.06 = 12.54, for a frame of reference.
Do the same for 8L90E's final overdrive gear of 0.652.
For me, 4.10 times 0.7 = 2.87. Effectively I'd be improving city MpG to sacrifice highway MpG.
That's why I'd prefer 3.90 - which I don't think they make? - to 4.10, since I don't tow or offroad.
3.42 * 4.56 = 15.59 Your present 1st gear
3.73 * 4.56 = 17.01 Your 1st gear with 3.73 rear axle
4.56 * 4.56 = 20.79 This may break something, and make 1st gear too short, might as well be a seven speed
You REALLY don't NEED any more 1st gear torque multiplication than 3.73 will provide.
3.42 * 0.652 = 2.23
3.73 * 0.652 = 2.43
3.73 might actually save fuel on the highway, unless you enjoy speeding over 70MpH (as I do).
4.56 * 0.652 = 2.97
4.56 won't save any fuel on the highway (nor in the metro / urban).
If you want more axle than 3.73, 4.10 would be more than enough.
For best results, tune your ecm & tcm for the gear change, especially to improve the transmission's personality.
Yea, I need to do the math like you just did. There is a really good website that breaks a gear comparison down with rpms, speeds etc etc etc. Tons of data.
Im running 275/65/18 cooper ATP tires right now. The goal is 35's eventually so my two trucks and the trailer can share spares. But even technically with my truck lifted (Border patrol suburban rear springs and ADS coilovers up front running about 1/2 inch lower than the rear) I am not sure the cutting will be worth making 35's fit.
Maybe if a decent set of small fenders that have a good liner or if Dirt kings ever makes their liners cheaper...
Ok back to the gears. I was shocked at the MPG increase I got on the colorado when going 3.42 to 4.56... and stock tires. However this 6.2l and the current 3.42 is really good once you hit 75 mph. That being said 50-70 is just eh ok. because it hunts gears any time its not flat.
I had not settled on 3.73 or 4.10 gears. I wanted something that keeps the pinion strong (4.56 on the d44hd/M220 on the zr2 the pinion cant safely get smaller than the 4.56 ratio. I wanted 4.88 for 37's). But I figured if I was going to change gears, I would look at two things and its pretty silly really:
1) which gearing kept the speedo as close to correct with a 33 inch or 34 inch tire. I live in commiefornia so correcting the speedo and not having to run a tune, or buy a boat load of credits on HPTuner and load/reload tunes every other year sucks.
2) Since anything higher numerically than stock will get me better mileage on the city streets and the freeway, find a number that didnt make 1st stupid on cement and actually makes 1-2 and 2-3 shifts in 4wd low usable because the shift pressure tuning etc etc is very complicated and I dont know any 4wd trans 8l90 tuners that are any good for the crawling stuff. High speed pre runner stuff yes... crawling... which is really "Im camping and towing an offroad trailer and going slow up a tricky section and need the truck to be in low crawl mode not spin the tires on each shift flare" mode
So yea, that and as much as I like the G80... its quite nice when they work and mine seems to have been built nice and tight, I would like to have a locker in the rear. I dont want an arb, been there and they fail open which sucks, and air lines leak and get caught on stuff. OEM Eaton E lockers are on two trucks, and they have been awesome other than the BCM from GM makes engagement delayed. So either an e locker or a TruTrac.
On the street and on fire roads etc, a trutrac or other (grizzley etc) are sufficiently open enough when you dont need a locker. But when you need it, man, they work. The suburban is heavy. I wouldnt want them on a lighter car although I had a 3100# Subaru with their version on my front diff and an OS Giken on the rear (which is similar to the G80...kind of...sort of)
But having a tight lsd (or locking lsd) in a pinch is better than a locker if its not a crawler in my mind. My TRX has that e locker and in Baja mode (75 rear 25 front power split) and sport (70/30) the rears just spin and it goes from both to one to the other spinning. The traction control makes it a pretend locker (same with front which is an open diff) but it just gets annoying.
Long winded... I need to do the math for stock gears and tires to new tires
Im running 275/65/18 cooper ATP tires right now. The goal is 35's eventually so my two trucks and the trailer can share spares. But even technically with my truck lifted (Border patrol suburban rear springs and ADS coilovers up front running about 1/2 inch lower than the rear) I am not sure the cutting will be worth making 35's fit.
Maybe if a decent set of small fenders that have a good liner or if Dirt kings ever makes their liners cheaper...
Ok back to the gears. I was shocked at the MPG increase I got on the colorado when going 3.42 to 4.56... and stock tires. However this 6.2l and the current 3.42 is really good once you hit 75 mph. That being said 50-70 is just eh ok. because it hunts gears any time its not flat.
I had not settled on 3.73 or 4.10 gears. I wanted something that keeps the pinion strong (4.56 on the d44hd/M220 on the zr2 the pinion cant safely get smaller than the 4.56 ratio. I wanted 4.88 for 37's). But I figured if I was going to change gears, I would look at two things and its pretty silly really:
1) which gearing kept the speedo as close to correct with a 33 inch or 34 inch tire. I live in commiefornia so correcting the speedo and not having to run a tune, or buy a boat load of credits on HPTuner and load/reload tunes every other year sucks.
2) Since anything higher numerically than stock will get me better mileage on the city streets and the freeway, find a number that didnt make 1st stupid on cement and actually makes 1-2 and 2-3 shifts in 4wd low usable because the shift pressure tuning etc etc is very complicated and I dont know any 4wd trans 8l90 tuners that are any good for the crawling stuff. High speed pre runner stuff yes... crawling... which is really "Im camping and towing an offroad trailer and going slow up a tricky section and need the truck to be in low crawl mode not spin the tires on each shift flare" mode
So yea, that and as much as I like the G80... its quite nice when they work and mine seems to have been built nice and tight, I would like to have a locker in the rear. I dont want an arb, been there and they fail open which sucks, and air lines leak and get caught on stuff. OEM Eaton E lockers are on two trucks, and they have been awesome other than the BCM from GM makes engagement delayed. So either an e locker or a TruTrac.
On the street and on fire roads etc, a trutrac or other (grizzley etc) are sufficiently open enough when you dont need a locker. But when you need it, man, they work. The suburban is heavy. I wouldnt want them on a lighter car although I had a 3100# Subaru with their version on my front diff and an OS Giken on the rear (which is similar to the G80...kind of...sort of)
But having a tight lsd (or locking lsd) in a pinch is better than a locker if its not a crawler in my mind. My TRX has that e locker and in Baja mode (75 rear 25 front power split) and sport (70/30) the rears just spin and it goes from both to one to the other spinning. The traction control makes it a pretend locker (same with front which is an open diff) but it just gets annoying.
Long winded... I need to do the math for stock gears and tires to new tires
