Changing rear end gears?

[teh Nub]

While driving down the road this morning, I had an idea. I'm not sure if it's a good or bad one and that is why I am here. I searched and didn't find what I was looking for.
My idea is to replace the gears in my stock 2003 explorer sport 2wd auto w/ o/d to lower the rpm as cruising speed on the interstate. I was thinking that dropping rpm by 500 to 750 rpm in high gear.

I know I couldn't get too aggressive and cause the engine to bog, but this change would cause rpm at 60mph to go from 2000rpm to 1250 - 1500rpm. Turning the O/D on/off changes rpm by about 750 rpm therefore, I could retain my current "gearing" with O/D if I were to make this change and turn O/D off.

The explorer could then be driven normally with O/D off, then switched on at highway speed for the extra low highway gear...

My questions are...
1. Is this a good solution at an attempt to achieve better fuel economy?
2. Where and Who can I find reasonably priced ring/pinion kits?
3. What other things should I look at/ need?
4. How labor intensive is this project?

I haven't read the rear end tag to find out what gears I have so I can't ask what ratio I should move to. Any other info would be great

Thanks

Jeremy

 

[jturquette]

I think there may other issues you need to consider. I'm not a mechanic, but I have spent the past year thinking about gearing, tire diameter, and engine rpm. The rear differential is the final link between the engine and the rear tires. Any gearing change there will affect the vehicle's overall gear ratio.

If you are looking for a 500 rpm reduction from 2000 rpm, that is a 25% reduction, or you are asking the engine to operate at 75% of its intended rpm. When your car shifts into a lower gear as you approach hwy speed, look at the rpm reading at each shift. I'm guessing your engine drops to about 1500 rpm. The gearing reduction means the vehicle will be operating at 0.75 * 1500 rpm = 1125 rpm. You may find that your truck will down shift again because the engine load needed cannot be met at that rpm. This will cause undue wear on the gear clutches. Now if you look into modifying your shift points, then this last issue may not be big. But modifying **** points up to compensate means the engine will rev higher (=more gas used). You might be taking a step backward here.

Additionally, your ability to carry a load or tow a trailer (even a small one) will put a strain on the transmission.

I'm certain a transmission guy or a motor head will be able to give a better idea of the affect on the engine.

FYI- I just re-geared my 2003 XLT (4-door) from a 3.55 to a 3.90 to compensate for the larger diameter tires (265/70 r16). My X now is now geared equivalently to a stock tire size and a 3.73 rear end. It cost me $800 for the re-gear, out the door. That would be an average price to pay.

Check out a member named aldive, he has a 1999 4.0 V6 SOHC explorer that gets over 30 mpg hwy. He is very knowledgable about the ways of mpg.

Good luck. I hope I have sparked other questions for you to get a better understanding of how these things work.

 

[410Fortune]

just run bigger tires, much cheaper.

3.55 to 3.9 is not a drastic enough change IMO

Go to at least 4.10

but to drop your RPM at highway speeds, just get the biggest tires you can.

I have 35" tires on my BII with a 5.0L V8, a wide ratio auto trans (low first gear, steep overdrive) and 4.10 ring and pinion gears... at 75 mph I sit at 2100 RPM approx in OD

I get great MPG on the highway with this setup.
my 96 explorer is similar, 33" tires, 3.73 gears in the axles... 2300 RPM at 75-80 mph

 

[FROADER]

That seems like the hard way and would be a lot of money wasted if it doesn't meet your expectations.

Find some cheap or used 31" tires and put them on the back for a couple of tanks, and see what happens. 31s aren't that much heavier and shouldn't affect the outcome.

EDIT: damn, stepped away for a minute while posting.

 

[wickedturns]

adjust gears to 70 % of all driving. towing , high way, ect..... take into account tire hight. and engine torque

 

[racprops]

You will need to figure out what your REAL miles are, changing wheels and or tires changes what miles and speed you car is reporting as driven, so with out correcting that you will get false incorect numbers.

Rich

 

[94Eddie]

What I have found with gearing is there is no free lunch. If you want to cruise at that low of an RPM at highway speeds it might end up costing you mpg at lower speeds and around town driving. The lower (numerically) differential gears make it more difficult to move a heavy vehicle from a dead stop and puts the engine under a higher load more often in general. The engine and transmission programming is optimized for the rear end gearing from the factory. Deviating from this a good bit from the stock setup might create inefficiencies that cost mpg under many driving conditions. If doing what you want to do would raise mpg overall then I think ford would have done this from the factory. Automakers go to great extents to eek out small gains in mpg and I think it is hard to improve upon all the R&D they put into balancing mpg with drivability, performance and long term reliability.

I noticed a decent jump in mpg and performance when I moved from my 2002 V8 Explorer to my current 2010 V8 Mountaineer. The six speed transmission was a big component of getting these improvements along with a more efficient engine. With six speeds the 6R80 transmission allowed higher (numerically) gears for 1st and other low gears while dropping RPM in 6th gear relative to the 5-speed in the 3rd gen Explorers/Mountaineers. One reason the higher sixth gear in my 2010 works well is because the 3V engine makes more torque at a low rpm to overcome the steeper gearing in 6th gear. Going from 2k RPM to 1.25k RPM is a heck of a drop and this will happen in all gears. This puts you in a much lower hp/torque level in the power curve that the OEM engine programming expects and I think this might cause some severe drivability issues and maybe a loss of mpg in many driving situations.

I agree with some others to try oversized tires first. This way it can be easily reversed if there are problems.

 

[racprops]

OK This is an area I am very well knowledgeable about. I have done a fair amount of work on this subject.

First tire and wheel changes: From a search: How does changing tire size affect speedometer?


Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That's because larger tires cover more ground in one complete revolution. When your speedometer reads 60 miles per hour, your car will actually be traveling 67.7 miles per hour!

Regarding this, how much does speedometer change with bigger tires?
When you go to bigger tires the circumference of the tires is larger, meaning the distance around the tire is longer. Thus, with each revolution you are traveling further than before. Therefore your actual speed is higher than your speedometer reads. Or lower if you have gone with a smaller tire than stock.
Similarly, how does wheel size affect speed? Newton's Second Law dictates that the force on an object is equal to its mass multiplied by its acceleration. So, increasing your wheel size will decrease the driving force from your wheels which will culminate in a decrease in acceleration of said wheels.
In this regard, how does changing tire size affect odometer?
This can also affect your odometer and speedometer as larger tires will have a larger circumference, causing you to travel a little bit further per each complete tire rotation. This also will cause your odometer to register fewer miles than you are actually traveling.
How much does tire size affect RPM?
So you'll be increasing tire size by 10%, so you'll decrease engine speed by 10% too. Assuming you're running about 3000 rpm at 70 mph with the small tires, you'll drop 10% with the big tires. 10% of 3000 is 300, so you'll go down to 2700 rpm.

How do I recalibrate my speedometer?​

Electronic Speedometer

1. Determine the distance of the test drive needed to recalibrate your speedometer.
2. Press and hold the calibration button on the speedometer, start the vehicle and release the button.
3. Press the button again and drive the distance prescribed by the speedometer manufacturer.

Do larger tires increase gas mileage?​

And the wider the tire the higher the rolling resistance is and the harder the engine has to work to move the tire, thus decreasing the car's fuel efficiency. Larger tires can in fact, increase a vehicle's gas mileage if the vehicle is regularly traveling with consistent speeds over 60 miles per hour.

How much is it to calibrate a speedometer?​

Any reputable mechanic who records results properly and can produce a valid report may run a speedometer calibration on your vehicle. Though it costs about $75 and lasts about an hour, obtaining a calibration report could help your defense.

Do bigger tires make your speedometer read faster or slower?​

Up-sizing or Down-sizing
Up-sizing, or installing a taller tire, will lead to a speedometer reading that is slower than your actual speed. This is because a taller tire will have a larger overall circumference, which will cause it to travel more distance per revolution than the original equipment tire.

Do smaller tires affect gas mileage?

How Do Wheels Affect Gas Mileage? For example, larger tires decrease your fuel economy because they are heavier, while smaller tires increase fuel efficiency. Bigger tires also have a higher rolling resistance than smaller tires which means they require more resistance and effort to get them rolling.

How much do 35 inch tires affect speedometer?​

33" OEM Tires

Speedometer	35" Tires	37" Tires
30 MPH	33.9 MPH	35.8 MPH
45 MPH	50.8 MPH	53.7 MPH
60 MPH	67.7 MPH	71.6 MPH

How far off is my odometer with bigger tires?

Divide your old tire diameter by your new tire diameter. Subtract from 1. This will be the % your odometer is off. If your vehicle odometer registers 250 miles and it takes 16 gallons to fill…

What happens if you put bigger tires on your car?​

Assuming you've gone 1 or 2″ bigger than the original size, you're tires are now going to be lower profile tires. If your wheel and tire package ends up being heavier, this will affect acceleration and fuel economy. Acceleration will be less and so will fuel economy. Steering may also become heavier.

What gear ratio do I need for 35 inch tires?​

Depending on the axle, the next lowest ratio from a 4.10:1 is most likely the 4.56:1, which should be selected for towing and performance. Because of the extra weight associated with off-road tires, we usually recommend going with a performance-minded gear ratio for any tire size above 35 inches.

Can you change tire size without affecting speedometer?​

No speedometer can be 100 percent accurate. But, if a car is modified, its speedometer may need to be recalibrated. Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That's because larger tires cover more ground in one complete revolution.
Can wrong size tires damage transmission?

Tires do matter, incorrect tire sizes can cause serious damage to your vehicle not to mention a safety hazard. Putting on the wrong size tires on some cars and trucks will destroy the transmission cause damage to the suspension.

Will changing tire size void warranty?​

Your warranty would not be voided by getting bigger tires. That's just nonsense. Changing your tire size shouldn't void your warranty unless they can somehow prove that the larger tires caused something to fail. Pretty unlikely with the tire size you are talking about.

Are Bigger tires bad for your truck?

A larger tire size can make your truck look and perform better in some situations. But changing tire size too much can affect speedometer and odometer accuracy, handling, steering response as well as safety issues such as tire load capacity.

Do smaller wheels accelerate faster?​

Yes, smaller wheels accelerate faster, but it doesn't matter. A 29” wheel has more mass at a larger radius from the centre of rotation (the hub), and thus has a higher moment of inertia than a smaller wheel and is harder to get moving. So yes, bigger wheels will accelerate more slowly.

How do I increase the size of my tires?​

How to Size Your Tires

1. Multiply the width by the aspect ratio expressed as a decimal (225 x 0.55 = 123.75).
2. Convert the aspect height to inches (123.75 / 25.4 = 4.87).
3. Multiply the aspect height by 2 (4.87 x 2 = 9.74).
4. Add the inside diameter of the tire (16 + 9.74 = 25.74).
5. Repeat for the size of the new tire.

How accurate is WAZE Speedo?​

It's pretty accurate, considering that your phone is triangulating satellites 22,500 miles up. I have not noticed a difference in my car's speedometer versus Waze. Since 60 mph = 1 mile per minute, you can test the accuracy of your speedometer with a good stopwatch along a road with known accurate mileposts.

Can you change tire size?​

It's possible to change the size of your tires without actually changing the diameter. Generally, you don't want to go with tires that have a diameter that is more than 3% different from the factory tires. It's also a good idea to consult with a trusted mechanic before switching to larger or smaller tires.

 

[racprops]

So without accurate Speed and distance you will get a false reading and not really see real changes in your MPG readings.

I have up sized my wheels to allow thinner tires to improve handling but I have kept my overall wheel and tire size as close as possible to the factory wheel sizes.

94Eddie Said:​

"I noticed a decent jump in mpg and performance when I moved from my 2002 V8 Explorer to my current 2010 V8 Mountaineer. The six speed transmission was a big component of getting these improvements along with a more efficient engine. With six speeds the 6R80 transmission allowed higher (numerically) gears for 1st and other low gears while dropping RPM in 6th gear relative to the 5-speed in the 3rd gen Explorers/Mountaineers. One reason the higher sixth gear in my 2010 works well is because the 3V engine makes more torque at a low rpm to overcome the steeper gearing in 6th gear. Going from 2k RPM to 1.25k RPM is a heck of a drop and this will happen in all gears. This puts you in a much lower hp/torque level in the power curve that the OEM engine programming expects and I think this might cause some severe drivability issues and maybe a loss of mpg in many driving situations."


From all my reading I agree mostly with this, BUT I would as: Do you know if the rear end gearing are the same?? With stronger lower gears (1st and 2bd etc.) a truck can run a lower rear end and thus get better highway MPG. I would ask what was the RPMs in top gear in both trucks?

Second I also look at the torque curves of Any engine for my planed adding on a second overdrive. The Ford 4.0 and 4.6 are reported as having lots of low end torque.

I also have tested the power and MPG at my projected RPM ranges. I currently use a Scan Gauge II for these readings.

In my 02 4.0 V6 Explorer I find running her at 1600RPMs shows a MPG of 27/32 at 50/55MPH.

In my 03 4.6 V8 Explorer I find running her at 1600RPMs shows a MPG of 25/32 at 50/55MPH.

Both trucks have 3.73 Rear ends and 5R55 transmissions.

In my 03 Crown Vic with a 4.6 V8 I find running her at 1600RPMs shows a MPG of 25/30 at 60/655MPH.

In my old 2000 Mercury Grand Marques I find running her at 1600RPMs shows a MPG of 25/32 at 60/65MPH.

I find all of these engines can pull these cars easily and the PCM has no problem dropping into OD and cursing at these speeds and can easily lightly accelerate without down shifting.

So in all of these cases all of these cars can get great MPG IF you can stand driving at those low speeds.

And I totally agree you want to keep all the current factory power and acceleration factors.

So as there are VERY small differences in all the OD ratios:

4L60 4L80 6l80 8L80 10 R80 Gears Ratios:

1St 2nd 3rd 4th 5th 6th 7th 8th

4L60 3.06 1.63 1.00 .70

4L80 2.48 1.48 1.00 .75

6L80 4.03 2.36 1.53 1.15 .85 .67

8L80 4.56 2.97 2.08 1.69 1.27 1.00 .85 .65

10R80 Transmission Gear Ratios

First – 4.70
Second – 2.99
Third – 2.15
Fourth – 1.80
Fifth – 1.52
Sixth – 1.28
Seventh – 1.00
Eighth – 0.85
Ninth – 0.69
Tenth – 0.64

So it is .70, .75, .67, .65, .64. in overdrives

So to get a engine into 1600RPMs at most highway speeds of 70/80MPH you need a major rear end gear change, around 2.40.

Which could net you the same performance of a older 4/5 speed transmission running a 3.73.

BUT what a nightmare in replacing all of that, transmission, rear end, and retaining the PCM.

My plan is to drop in a second OD. To run a double overdrive.

And before you get out the torches and pitch forks, there IS a company doing these: Gear Vendors. Home - Gearvendors

I am doing my much cheaper and I think perhaps stronger by using a 1938 to 1964 Borg Warner detatched OD unit.

Doing so will add a second .70 drop in gearing and give my trucks that 1670RPMs at 75MPH.

See the chart.

Rich

 

[racprops]

Last thing: Such a change will only improve your MPG on/at highway s and highway speeds.

It will not help with 90% or 100% of your city stop and go MPG.

SO if you do not do a lot of driving at 70MPH+ or higher and if your daily commute is normal stop and go “Rust Hour” driving it will not help there either.

So this will only help on long highway trips.

So if you do a LOT of freeway driving at speed, then this can help.

FYI Which I do intend to use my 03 Explorer for lots of Day Trips in and around Phoenix and AZ, so I will see the benefits of 25/30MPG at highway speeds of 75/85MPH.

Otherwise it will be a bust.

If you’re considering a Gear Vendor Over Drive you need to figure out how many miles it will take in the 10 to 20% improvement in your MPG over your original MPG, how many miles you will need to drive for those savings to pay for that $3 to $4,000.00 cost of getting one.

Sadly this can run into the 10,000+ miles and years to pay off.

The only way to get great MPG in a city is a small car.

My wife's 2000 Toyota Camry gets 25MPG city and can do 30/35 highway.

Rich

 

[racprops]

Did some number crunching:

A 3.73 runs at 2400 RPMs at 75 MPH

A 3.50 runs at 2260 RPMs at 75 MPH

A 3.27 runs at 2110 RPMs at 75 MPH

My double Overdrive can run:

A 3.73 runs at 1660 RPMs at 75 MPH

A 3.50 runs at 1560 RPMs at 75 MPH

A 3.27 runs at 1460 RPMs at 75 MPH

A change from a 3.73 to a 3.50 in a stock truck gains only 140RPMs and only 290 RPMs gain going all the way to a 3.27.


With my Double Overdrive the 3.73 is great as I can cruse at 1660RPMs at 75MPH and a gain of 100RPMs is not worth all the cost and pain by switching to a 3.50 and a 3.27 and only drops 200RPMs, which is too small and most likely will hurt MPG.

Rich