I didnt read all of the posts about tire height but here is what I can tell you.. (disclaimer: I am by no means an expert on this)

Taller tires with the same gear ratio does NOT increase gas milage. you need lower gears to match the offset of the larger tires in order for your truck to run at the correct RPM/transmission gear range range. Think of it this way.. The tire is taller so it takes more to get one full spin out of it. It's either going to take more engine to rotate it or lower gears to rotate it. Wider actually doesnt matter as much. However obviously weight of the tire wheel combo does play into account..

Here are some fun sites for you guys

Chart Gear Ratio

Ashcroft Transmissions - Ratio Calculator

And diffs/ratios explained

Introduction to Differential Gearing

 

I agree with you except in the situation of already having a gear ratio that is too low. Then it would help. That is not usually our issue with Discos though. My Ranger has 3.08s and 26" tires, gets 24 MPG. If I geared it to 4.56 for some reason, it would be tail happy and nearly impossible to drive sanely, and gas mileage would plummet especially at higher speeds. Throw on a 31-32" tire and I'd gain back some of my original gas mileage, so it can help. But if I put a 31-32" tire on it now with 3.08 gears, it would be a dog and get 10-15 MPG.

The Discovery is so underpowered that this scenario would rarely be true for us. But just be aware that by choosing a gear and tire combo, you are choosing the range of speeds at which the vehicle will be relatively efficient. Gear too high or too low, and you are selecting an unrealistic speed range so economy will suffer at all speeds. The gear and tire combo must be a good match to the engine, transmission, and vehicle weight.

But you're definitely right, and I'm just picking at the threads or your statement.

 

This is never true. I cant think of any vehicles that come from the factory under geared. Vehicles come from the manufacturer with "the best" gear ratio set ups for the tire size and RPM range the engine should run in.

Sorry you edited so I have to edit now..

The only way to compensate for the larger tires and small engine in the discos is gears. It stands true even in low range on the trails with 32's. The only way to get your get up and go back is gears. With the said Gears and big tires isnt gonna change the fact that discos get **** gas milage out the gate and if you care about that you should get a Toyota hybrid.

 

I hadn't even seen your post when I edited... Sorry man! Yeah the only reason you'd have a gear ratio too low is if someone changed it before you. Also I think you can request towing or highway gears when buying a pickup, so if somehow you had towing gears but wanted highway gears, a slightly larger tire might compensate. But yeah, picking at threads. I definitely plan to gear mine down to 4.10 as soon as I have money and time, those two things don't often coincide though.

 

I did 4.11's with the 35's and I am sure I am going to want to go lower when I start playing off road. Especially with 37's I'll probably eventually end up with..

They are coming out with 5.16's for Rovers now.. Not surprising since the 4.75's are the new 4.10's

Think I'll just go after this next

Ashcroft Transmissions - UNDERDRIVE

in my dreams

 

Now that would be sweet. I've often wanted to be between high range and low range.

I think 4.10/4.11s will be adequate for me, as I can hardly imagine going past the 32s I'm on. Perhaps a 33 someday if I go really crazy, but I'm more interested in expedition/overland driving, and getting to remote areas for backpacking. Mostly 2-tracks with the occasional technical rock trail or stream crossing in between. Dune driving too. I find 32s wholly adequate for all that. I'm sure I'd have a blast in Moab too even on 32s.

 

The underdrive is 8.93 crawl ratio.. I cant even imagine what they would be like. My low with 4.11's is low already

Makrinos Defender 4.6 Ashcroft Transmission - YouTube - This is the only reason I want to move away from the US so I can get into challenge racing. And not be the winch bitch

33's and 4.11's is a good set - honestly 4.10's 4.11's on 32's your revs may be high on the highway if you drive the truck as a DD. However your revs in low range off road - the thing will be a damn rocket.

 

Yeah, it never has been and probably never will be a DD. I drive it once a week to keep everything lubricated. And if I had to slow down to 60 or 65 on the highway it wouldn't kill me.

Plan is when I move I'll sell the Ranger (my current DD), keep my Nissan and my Rover (tow the Rover behind the U-haul), I'll drive the U-haul cross-country and let my wife drive the Nissan. When I settle down, I'm looking for a bare-bones 2005+ F150 to daily drive, preferably a V6, stick shift, and regular cab. Pretty sure I can find one for $6 to $8 grand and it will last me a couple decades. I'm going into the construction industry so I'll need a truck.

I'll leave the Nissan for my wife to use because she loves that car, and once life has stabilized I'll rebuild the engine and differentials on the Disco.

I should list "fleet management" on my resume, LOL.

 

So this reminds me of something I've been meaning to ask; what the hell is normal RPM? Idle for me has always been around 750, which seems about right. However, highway cruising around 65-70, RPMs are around 2,500-2,750, always have been, at least since I got it two years ago. That seems damn high to me. I'm riding on stock tire size. Are these RPMs normal?

 

That sounds about right. I run about 2500RPM at 67-68MPH on stock gears running 235/70-16 tires @50% tread left.

The gear drive transfer case along with the fact it's all wheel drive makes it seem like it's screaming but it really isn't. Switching to 85W140 dino in the transfer case and diffs cut alot of the noise out of my '97.