BlackWorksInc
New member
I meant heavy duty options like cryo-treated gears and such that hold up to abuse more?
Would regearing help with MPG for larger tires?
- Thread starter dyizhere
- Start date
WXman
New member
Um... no.
The driveshafts are connected to the differentials. The ring/pinion gear determines how fast the driveshafts spin. If you gear down, the driveshafts are spinning faster. That's how you get the mechanical advantage of using lower gear ratios...because the engine RPMs increase and the driveshaft RPMs increase relative to the axle shaft speeds.
In my case it had absolutely nothing to do with pinion angle. And it wouldn't for a 4Runner either. Nobody lifts these things enough to make pinion angle an issue.
KidVermicious
New member
Bolding mine, and that's nonsense. The transmission spins the driveshaft, and the differentials transmit that energy to the axle shafts. By changing your differential gears you change the number of driveshaft rotations it takes to create an axle rotation. The only way to change the speed the driveshaft rotates at is to change the gearing in the transmission or transfer case.
Your driveline vibration issue in your Jeep had exactly jack and squat to do with what gear ratio you had installed. Nevada is exactly right.
nevada
Senior Member
you can have the treatments done to the new ones. i dont think its even that expensive really.
but, ive been running a v8 on toy running gear for years in my wheeling truck. spooled and 5.29's, and dual cases. the only think ive broke are front axles.
you're still WRONG.
if you regear back to STOCK, the driveline is going to spin the SAME speed it did when you had stock tires, and gears. so will the engine rpms.
I have some expertise in this area... The bigger tires and change in gear ratio aren't really causing all the much trouble by themselves. The system does act differently though as a result.
1. Biggest factor is probably the tire type/brand, not so much the size. I have a 2006 sport that came with the stock size dunlops. I got 20 mpg with the dunlops, then switched to Cooper ATR's in the bigger 265/70/17 size. My mileage immediately dropped to 18.5 and stayed there for the life of the tires. More recently, I got new Michelin MS2's in the same 265/70/17 size, and the mileage jumped right back up to 20 mpg. This is suggesting the size of the tire didn't have a big impact, it was just the type/brand.
2. A little bigger tire changes the gear ratio enough that during highway driving, the truck may have to downshift more often (depends on terrain in your area). If it does downshift more often, that means the torque converter is unlocked more often, and whenever the torque converter is unlocked, you're wasting lots of fuel. Increasing or decreasing the engine speed is a tiny factor compared to a locked vs unlocked torque converter.
3. The mass or inertia of the tires can be a factor too, but tends to show up mostly in city driving. A heavier tire takes more energy to get up to speed, but doesn't really take any extra energy to maintain that speed. So, for city driving, you have to keep accelerating the tire over and over again, and it impacts the fuel economy. On the highway, it's a pretty small factor again.
SO... If I were you, I'd just keep the stock gear ratio. In your case, you won't pick up noticeable fuel economy by changing it, even if you went up a couple more inches in tire size. The only reason to change gear ratios would be to get back some of the performance and "snappiness" it had with the smaller tires.
Good luck!
Corbs
1. Biggest factor is probably the tire type/brand, not so much the size. I have a 2006 sport that came with the stock size dunlops. I got 20 mpg with the dunlops, then switched to Cooper ATR's in the bigger 265/70/17 size. My mileage immediately dropped to 18.5 and stayed there for the life of the tires. More recently, I got new Michelin MS2's in the same 265/70/17 size, and the mileage jumped right back up to 20 mpg. This is suggesting the size of the tire didn't have a big impact, it was just the type/brand.
2. A little bigger tire changes the gear ratio enough that during highway driving, the truck may have to downshift more often (depends on terrain in your area). If it does downshift more often, that means the torque converter is unlocked more often, and whenever the torque converter is unlocked, you're wasting lots of fuel. Increasing or decreasing the engine speed is a tiny factor compared to a locked vs unlocked torque converter.
3. The mass or inertia of the tires can be a factor too, but tends to show up mostly in city driving. A heavier tire takes more energy to get up to speed, but doesn't really take any extra energy to maintain that speed. So, for city driving, you have to keep accelerating the tire over and over again, and it impacts the fuel economy. On the highway, it's a pretty small factor again.
SO... If I were you, I'd just keep the stock gear ratio. In your case, you won't pick up noticeable fuel economy by changing it, even if you went up a couple more inches in tire size. The only reason to change gear ratios would be to get back some of the performance and "snappiness" it had with the smaller tires.
Good luck!
Corbs
WXman
New member
Actually, you're both exactly wrong. The transmission has NOTHING to do with how fast the driveshaft spins. Read what you wrote, sulk, and then try again.
At the diffs, it's a solid connection. At the transmission it's not. You've got a torque converter. If you put in deeper differential gears, the driveshafts are spinning faster, period.
WXman
New member
True, IF you could actually regear back to stock. But if you go up 3% in tire size, you're not going to find a 3% deeper gear ratio. You've got to take what's available and 99% of the time that's going to be much much more than a 3% or 5% or whatever increase. Heck, just going from 3.73 to 4.10 is a 10% change in itself and it would be retarded to spend that kind of money for 150 RPMs difference. So most are going to do 4.56 or 4.88 and that makes the shafts spin a LOT faster. Which is what I did. Which makes my example 100% correct. You guys suck at math/mechanics. LOL.
KidVermicious
New member
For a guy who thinks he knows what he's talking about, you've said a lot of silly things in several threads now, but this takes the cake. Manual trucks don't even have a torque converter, so how now brown cow?
The driveshaft is driven by the transmission, not by the differential. It has EVERYTHING to do with how fast the driveshaft spins, full stop.
aha2988
New member
nitro gears is pretty much whats out there. you can go directly to toyota they have a 4.11 option but it's pricey and i believe nitro may offer a 4.11 now but im not sure
aside from all the theoretical talk of what might happend, could happend, should happen...
I've regeared to 4.56's to aid in turning my 33's and I didn't get any mpg's back from what was lost due to tire size. I only regained my low end torque numbers, and acceleration. 4.11's would put 33's back at factory rpm from my calculations and 4.56 is perfect for 35's (speaking from the 4.7 perspective) the V6 is going to be different. I see a lot of 4.88's in those doing good work.
I run about 2500 rpm at 70mph so not to high of rpms on the highway. I did a top speed run shortly after I regeared and it wouldn't budge over 94mph. well 98 on a down hill stretch. so my top speed was reduced.
The main thing regearing overcomes is the shear torque it takes to roll over a larger tire gets you from stop to rolling more quickly as well as helps trailering larger weight after larger tires are installed.
My purpose for installing gears was strickly to gain climbing torque offroad with larger tires as well as help with pulling trailers. MPG gains or MPH gains are completely negligible. well another benefactor was my speedo has become a little more accurate being off by 1 or 2 mph instead of 3-5.
aha2988
New member
i believe your comparing apples and oranges, different transmission gearing in different vehicles largely effects the rpms of the engine compared to the gear ratio of the axle. As well different torque/hp output in engines changes a lot as well. Where a 350hp 400ft/lb engine will roll out on some 4.56 gears where as a lower hp lower torque vehicle isn't going to gain much acceleration and not have as much ability to turn the gears.
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