"""I wouldn't bet changing gears to 4.56 will lower gas mileage further if you already had 35" tires. Mainly because when you have tires like that, your RPMs will be much higher because your engine is working "out of its powerband". Basically- its having to try harder. Even a few hundered RPMs all the time can make a big difference. Although usually 4.56 gears would lower mileage on a stock truck- on one with 35 tires who is already going several hundered RPMs over normal, those gears will lower it back down (esp. in overdrive, where mine right now is about 2100 RPM overdrive!!!), and the truck will see more normal RPM numbers, which should bring back the gas mileage some, not take it farther away. """


Acutally its the other way around. With larger diameter tires the engine RPM's decrease, resulting in a loss in power, bogged down engine, loss of efficiency, lower gas milage. Regearing would increase the engine RPMs back up into the "power band" so the engine dosen't have to work so hard, increased efficiency, better milage (maybe). Performance upgrades will help only slightly, the honest, real, solution is a higher gear ratio. Ya can't rewrite the laws of Physics.

 

Well, i've always been told this way, and it's held true to my truck so far. The bigger diameter tire you have, the fewer rotations you get on the tire per rotation. Lets just for simplicity's sake use this example.
stock tires:
1 rpm = 1 rotation of tire
with bigger tires, you have more to rotate so therefore:
1 rpm = 3/4 rotation of tire
so naturally you're gonna have to give it more gas to get the rpms to get the tire rotating. (not b/c the RPMs are lower, just because it takes more power).
I understand what you are saying, but the 'loss of power' is not from lower RPMs....its only noticeable because you realize you have to give it more gas to rotate those meats on there. big tires cause the same effect as heavy-foot syndrome. Youre always holding down the pedal longer. What you are thinking is, 'its lower rpms so you have to give it extra gas to get up to regular.' Which makes sense. The rpm's are really just a sign of how hard your truck is working. So for instance, sitting at a stoplight with an unlifted ford, he and i could both take of the line at the same time and our engine's RPMs (if we had the same powertrain setup) would most likely be the same, only he's gonna pull ahead, because his tires take less to rotate- it's not that the rpms are less- the engine's still trying the same. NOW, in overdrive, i can attest that the truck has higher RPMs, cause i'm in that most of the time, and my truck will not relax at anywhere UNDER 2000 rpms in overdrive, pretty much no matter what speed. This is because the truck is tryin to maintain speed and it has to keep giving it more RPMs because the tires are slowing the truck down faster than usual, not because the RPMs are low. Anywayz, ok, this is a long post. I hope someone will clear up the subject for sure.

 

No genius.

Say you have 4.11 gears in your diff. this means that for every 4.11 turns of the driveshaft (input), your axle turns 1 time.

If you put on bigger tires, your axle is still turning 1 time which means your wheel and tire are turning 1 time. (see a trend)

However, with bigger tires, your truck ends up further down the road for each rotation.

Therefore, your engine is having to work harder because it is now moving you further along with the same 4.11 turns of the diff.

You should up your gear ratio to match the stock ratio in relation to your new tire size.

It's not rocket science.

And yes, your speedometer and odometer will be off because you are now going further down the road with each rotation.

dms
99' Expedition

 

Ugh, what i am trying to say is that the engine's each have the same power. The engine's are trying the same until you press down on the accelerater. If I raced next to unlifted ford of the same powertrain setup, then the engines would be trying the same if we pressed on the gas the same (he would still pull ahead though), but like you said, the truck needs to have higher rpms ('work harder') to keep up, therefore you have to accelerate- which increases RPMs. Which is really the debate i thought...uh...this thinking makes my head hurt.

But your truck measures miles traveled and speed by the axle turns i believe, which is why, like you said dms, the truck is travelling farther per rotation that stock, which means its actually going more miles than the odo/speedo says.



Brandon

 

now that i look back at this post in entirely, sag. was actually agreeing with me, i think he must have misread my post, as i misread his. hm...not fun.

brandon

 

BigBadRedLiftedFordMan,...Yes, ....partially.

"the truck needs to have higher rpms ('work harder') to keep up, therefore you have to accelerate- which increases RPMs."

No.

The engine is operating at lower RPM's "to run the same speed, with larger tires". thus does not have power in reserve it normally would with stock tires running at a higher RPMs. You may be pushing the pedal down, but not much happens until the engine RPM's slowly increase.

You can observe this by watching a vaccume gage in the cab while driving. With large tires and stock gears, you will have very low vaccume indicating the engine is bogged down. Regearing will increase engine rpm, the engine will have to work less to move the same truck, the vaccume will increase, and fuel economy will increase.

correct, it is not rocket science. If you could drive a truck before and after a regearing, you would swear the engine had been beefed up.

 

So when you upgrade to say 4.11's and you have an auto, AND you increase tire size, how does your computer know when to shift and what not? Aren't the shift points off majorly? Can ole' Mike Troyer help or the dealer?

Bafffled,
Travis Clark

 

Paco,

Ideally, if you upgraded to 4.11's and increased tire diameter the right ammount, THE TWO WOULD CANCEL EACH OTHER OUT! The transmission would not notice the difference. Thats why you regear with larger diameter tires, so the truck is not under powered.

 

Ok I'm still a slight bit confused still. I have one more quick question. Here is what i have. I have 30.1 inch tires (255/70/16) along with te 3.55 gear ratio. I would like to switch to 4.10s and 305/70/16 (On the black rock crawlers, sweet.....) Bfg M/t Kms. The calculator says I will need a 3.9 gear ratio to egual what i have right now. This is a slight difference from the stock setup. Would this have to be reprogrammed with a chip or the dealer? I have a 99 so i can't change out tooth gears.


thanks
travis

 

Paco, yes a chip can be programmed so the computer knows what everthing is.As for the particular set up you are talking about doing , yes a 3.9 would equal everything out, but (as far as I know) you can't buy a 3.9 ring and pinion set(gears) for our trucks.You could either get the 3.73s and be a little underpowered(Running an actual ratio of 3.30ish with the bigger tires) from stock(not my suggestion) or put in 4.10s and be running better than stock(an actual ratio of 3.60ish)(My suggestion).
*don't quote me on those #s for ratios I gave you.I researched the same thing and I have the same ratio has you and I think thats what it all came out to be.

I decided that I want to put 4.30s (this would give me an actual ratio of 3.73) in so I will have noticaeably more power than I did with the stock tires and gears.Look at my. "gears to get me back to stock ratio" thread in the other drivetrain forum.It has all the details about what the actual ratio numbers would be.Hope this helps.

 

Thanks for your help lightningcrash I really appreciate it. Would the dealer be able to retune the shift points or would I have to buy a chip?

Thanks
Travis

 

I think the dealer can reprogram your computer for the changes but it it will cost you(I have heard quotes as high as 200 bucks).For that kind of money you might as well go ahead and get a chip its not that much more and it will give you more power to boot.