Please explain rear axle ratio
10-08-2014, 10:29 PM
#1
Please explain rear axle ratio
I'm not very truck savvy when it comes to rear axle ratio. Why is it important? I think it has something to do with towing, but is that all it's used for? Why is it important for towing? What exactly is this the ratio of?
I was building a Lariat on the build site and these were my options:
3.15 electronic locking axle ratio
3.15 non-limited slip axle ratio
3.55 electronic locking axle ratio
Why would I choose one ratio over the other? What difference does it make?
I chose max tow package and saw that it had the 3.55 locking axle ratio. If I sometimes pull a boat, do I need the max tow package? If not, which ratio is best for occasionally pulling a boat?
What ratio is best just for everyday driving going down the highway?
What are the advantages and disadvantages of each ratio?
And what is limited slip vs. locking? Why and when would one be better than the other?
Thank you for the explanations.
1969TxCowboy
I was building a Lariat on the build site and these were my options:
3.15 electronic locking axle ratio
3.15 non-limited slip axle ratio
3.55 electronic locking axle ratio
Why would I choose one ratio over the other? What difference does it make?
I chose max tow package and saw that it had the 3.55 locking axle ratio. If I sometimes pull a boat, do I need the max tow package? If not, which ratio is best for occasionally pulling a boat?
What ratio is best just for everyday driving going down the highway?
What are the advantages and disadvantages of each ratio?
And what is limited slip vs. locking? Why and when would one be better than the other?
Thank you for the explanations.
1969TxCowboy
Last edited by 1969TxCowboy; 10-08-2014 at 10:32 PM.
10-08-2014, 10:42 PM
#2
Senior Member
Locking differential is what I have in my truck. I have the 3.55 locking differential, it's good for pulling boats out of the water for example. Or when you need traction (posi traction) up to 20ish mph like in ice/snow. A limited slip will transfer the power from a spinning rear wheel to the stationary wheel so that you gain momentum/traction to move the vehicle.
I'm assuming that you have the Ecoboost engine selcted from the rear end options that you gave. The lower the rear gear ratio, the less your engine will turn at a given speed.
For example, all other things being the same:
3.15- 2050RPM @ 70 mph
3.55- 2150RPM @ 70 mph
While everybody has their personal choice, I prefer the 3:55 gear. It's in the middle of the highway/mpg gear (3.15:1) and the towing gear (3.73:1). I am able to comfortably tow the trailers I have and get decent mileage when unloaded. Hope this helps.
EDIT** The 3.15 Non-Limited Slip rear end is an open differential. More commonly known as a "One Wheel Wonder" when you stomp on it. Lol
2nd EDIT** I did have my RPM's backwards. Thanks. Fixed now
I'm assuming that you have the Ecoboost engine selcted from the rear end options that you gave. The lower the rear gear ratio, the less your engine will turn at a given speed.
For example, all other things being the same:
3.15- 2050RPM @ 70 mph
3.55- 2150RPM @ 70 mph
While everybody has their personal choice, I prefer the 3:55 gear. It's in the middle of the highway/mpg gear (3.15:1) and the towing gear (3.73:1). I am able to comfortably tow the trailers I have and get decent mileage when unloaded. Hope this helps.
EDIT** The 3.15 Non-Limited Slip rear end is an open differential. More commonly known as a "One Wheel Wonder" when you stomp on it. Lol
2nd EDIT** I did have my RPM's backwards. Thanks. Fixed now
Last edited by Manuellabour247; 10-09-2014 at 08:50 PM.
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Misterwlm (10-09-2014)
10-08-2014, 10:47 PM
#3
Cycle For Fun and Health
The axle ratio determines the final gear ratio for your truck.
The 3;15 = the highest gear ratio you mentioned. This one best for non towing highway driving as the engine RPM's will be lower and the gas mileage will (should) be higher.
The 3:55 is the best all purpose ratio as the lower ratio will provide more torque for towing .There are also 3.73 and lower ratios available. As the number gets higher, the engine RPM will increase to maintain the same speed; With the 3.15 to 1 ratio, you may be turning 1600 RPM to do 70 MPH, with the 3.55 to 1, the RPM may be 2000 to do 70 MPH.
If you plan to run oversize tires such as 35", you should go with the 3.55 or the 3.73 to 1 ratios.
Hope this brings some daylight to the issue for you.
Manuellabour - take a look at your RPM for the different ratios - The 3.55 should have the higher RPM, the 3.15 should have the lower RPM.
The 3;15 = the highest gear ratio you mentioned. This one best for non towing highway driving as the engine RPM's will be lower and the gas mileage will (should) be higher.
The 3:55 is the best all purpose ratio as the lower ratio will provide more torque for towing .There are also 3.73 and lower ratios available. As the number gets higher, the engine RPM will increase to maintain the same speed; With the 3.15 to 1 ratio, you may be turning 1600 RPM to do 70 MPH, with the 3.55 to 1, the RPM may be 2000 to do 70 MPH.
If you plan to run oversize tires such as 35", you should go with the 3.55 or the 3.73 to 1 ratios.
Hope this brings some daylight to the issue for you.
Manuellabour - take a look at your RPM for the different ratios - The 3.55 should have the higher RPM, the 3.15 should have the lower RPM.
Last edited by joe mcmillan; 10-08-2014 at 10:52 PM.
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Misterwlm (10-09-2014)
10-08-2014, 10:48 PM
#4
Senior Member
The rear axle ratio is the last gear set in the chain. If you've driven a manual (stick shift) then you know 1st is the gives you the most mechanical advantage so you can take off. Once you're rolling, 2nd gear can help you go faster, and so on until you hit your top gear. Here your engine speed is the lowest (rpm) for the actual speed you travel (mph).
A high rear axle ratio has the affect of making all your 6 gears have even more mechanical advantage (more rpm for a given speed). So, if you tow a lot then you want the additional mechanical advantage over your truck and trailer. Your engine will spin faster for a given road speed and a faster spinning engine makes more power. The downside of a high rear axle ratio is you will burn more fuel (faster spinning engine).
If you do a lot of city driving, the downside isn't really all that bad. If you do a lot of interstate cruising with no load, then it will cost you some miles per gallon. Hard to say how much.
A high rear axle ratio has the affect of making all your 6 gears have even more mechanical advantage (more rpm for a given speed). So, if you tow a lot then you want the additional mechanical advantage over your truck and trailer. Your engine will spin faster for a given road speed and a faster spinning engine makes more power. The downside of a high rear axle ratio is you will burn more fuel (faster spinning engine).
If you do a lot of city driving, the downside isn't really all that bad. If you do a lot of interstate cruising with no load, then it will cost you some miles per gallon. Hard to say how much.
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Misterwlm (10-09-2014)
10-09-2014, 07:58 PM
#5
Senior Member
For me, I will put a lift and oversized tires on. Because of this, I prefer a higher gear ratio (would choose 3.73 if an option, but with 3.5EB will have to settle with 3.55). With the oversized tires, the higher ratio maintains closer to stock RPMs at the same actual speed. If I kept it stock, and since the limited towing I do is a 2-place snowmobile trailer and a 17' fish/ski boat, I'd probably stick with the stock 3.15. Might a little underpowered when towing but not bad with the Ecoboost. But 97% of the time I'm not towing, so worth the extra mileage benefits. Getting around hunting, the e-locker is very nice/handy. In the end, it really depends on your use/desires. If you want to race people off the line at a stop light, go highest possible if that's your thing and suffer a bit on the mileage for the other 99.5% of your driving time. (my 5 cents)
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Fungooli (09-22-2018)
10-09-2014, 08:53 PM
#6
Senior Member
People seem to be finding that not having 3.73 gears is fine for the Eco as the turbo boost is actually better. Not sure though as I have the 5.0 and haven't driven an Eco for very long.
10-09-2014, 10:18 PM
#7
Better OUT then IN
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3.15 - best mpg, least towing
3.55 - good mpg, good towing
3.73. - least mpg, better towing. Understand that the mpg differences are minimal but need to be noted.
With that said (and others have also said), if you lift and upsize the tires, with the 5.0, go with 3.73s. The 5.0 torque band is much higher and you'll need the higher rpms to maintain the peppiness...especially if you tow any serious weight.
If you do the same with the 3.5 EB, 3.55 is plenty because of the lower torque band...and...I believe Ford only offers the 3.55 as the max gear for this reason. No worries about towing...this engine is stupid torquey.
Can't speak to the 2.7 in these regards.
Oh yea... WTF happened with the 6.2? Pissed me off!
My next truck would have been the 6.2 hands down.
3.55 - good mpg, good towing
3.73. - least mpg, better towing. Understand that the mpg differences are minimal but need to be noted.
With that said (and others have also said), if you lift and upsize the tires, with the 5.0, go with 3.73s. The 5.0 torque band is much higher and you'll need the higher rpms to maintain the peppiness...especially if you tow any serious weight.
If you do the same with the 3.5 EB, 3.55 is plenty because of the lower torque band...and...I believe Ford only offers the 3.55 as the max gear for this reason. No worries about towing...this engine is stupid torquey.
Can't speak to the 2.7 in these regards.
Oh yea... WTF happened with the 6.2? Pissed me off!
My next truck would have been the 6.2 hands down.
Last edited by justjimmy; 10-09-2014 at 10:26 PM.
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10-10-2014, 12:20 AM
#8
If u normally drive hills u will like 3.73 electronic locker. An naturally bigger wheels n tires.
10-10-2014, 02:02 AM
#9
The axle ratio refers to the ratio of driveshaft revolutions to the tire revolutions. If you get the 3.55 that means for every ONE revolution of the tire your driveshaft will turn 3.55 times. If you have the 3.15 axle, for every ONE revolution of the tire your driveshaft will spin 3.15 times. The slower your driveshaft is spinning the slower your engine is spinning. ( less RPM's) .
10-10-2014, 02:25 AM
#10
UR driving the EB. Make it with *****. 3.73 locker an big wheels n tires. If u buy 3.55 an put big wheels n tires on it you will wish 4 3.73. So why not start right. Do it right off the block. Ur talking $3 for gas every 500 miles more until u do the bigger wheels an gear ratio comes back to better mileage. U buy the EB because u got ***** dont whimp your ride when u want torque to the tires.
Last edited by papa tiger; 10-10-2014 at 02:29 AM.