Axle Ratio and tow rating

APT wrote:
RoyJ, the exception to your analysts is first gear and top gear. Otherwise, yes, they will be very similar if what you said is true.

Yes. Other than that the two trucks overlaps 7 out of 8 gears.

So if you tow all the time - get the 3.92, as the tall 8th would be no benefit. But those buyers are likely to go HD.

Otherwise, for someone like me that tows less than 10% (have an RV), the 3.21 can do 99% of the 3.92 trucks.

For that 1% I'll simply use 4Lo :P

IT all boils down to the effective ratio at the pavement..minus losses

The whole 'new' trannies with more gears is just the old 'close ratio' trannies
of racers, but with higher OD or double OD

The whole point of close ratio of old is that most were racers with
their peaky cams.

So higher number of gears and close ratio to keep the ICE in those
peaky torque/hp RPMs

RoyJ, the exception to your analysts is first gear and top gear. Otherwise, yes, they will be very similar if what you said is true.

Sport45 wrote:
The engineer in me just got hung up on the "power" term.

Power, force, torque, and work are terms with specific (and different) meanings in my line of work.

In your example with the torque multiplier above it takes twice the force with the 5-1 vs the 10-1 multiplier. The power used relates to how fast you turn the wrench. If you turn it at twice the speed using the 10-1 multiplier the power is the same. If you turn the wrench the same speed in both cases the 10-1 multiplier is using half the power. (Because you're turning the bolt half as fast.)

Love when fellow engineers are around so we can speak the same language!


For some reason the concept of torque vs hp confuses the most people; I feel is because people try to separate the two, rather than seeing them as coupled pairs like space-time. The coupler in this case is rpm, or angular velocity.

Here's one example I like show people. My 2014 Ram 1500, Hemi, 8 spd, is offered in 2 gear ratios: 3.21 and 3.92.

Either out of pure coincidence, or RAM's intentional design, it just so happens the gear ratio spacing of the ZF 8 speed is nearly identical to the split between 3.21 and 3.92.

Meaning, given a road speed of say, 60 mph, you get the exact same rpm (within 200 rpm) with the 3.21 truck in 7th, and 3.92 in 8th; or 3.21 in 6th, and 3.92 in 7th, and so on.

In this case, how does choosing the 3.92 gears have any advantage over a 3.21? The answer is it doesn't; except for the most extreme case of starting out at full GCWR, up a 25% grade... But considering the ZF's ultra deep 4.7:1 1st gear, you're almost never short on startability.

Otherwise, on every hill, ever speed, that you can ever imagine, the two trucks performs identical. The 3.21 truck is simply in 1 gear lower than the 3.92 truck. Neither trucks "puts down more power" than the other, because simple machinery like gear sets cannot create power.

I have owned the same model truck with both 4.10 and 3.73 ratios. The only difference towing is that I run in 3rd instead of 4th on the hills, 4th instead of 5th on the flats with very similar engine speed and road speeds with the 3.73 version. Same fuel usage. I am only talking towing at common road speeds, not racing or cruising empty. Gears are gears, whether they are in the rear axle or transmission.

TomG2 wrote:
You guys really think the mileage and wear and tear on the drivetrain is worse on a pickup with a 3.73 rear axle ratio running down the road at 60mph in 4th gear and 3,000 rpms versus one with a 4.10 rear axle rating running the same 60 mph and 3,000 rpms in 5th gear? I say that today's six or eight speed transmissions make it easy to find the right combination of road speed and rpm's that work best.

Empty or lightly loaded no but loaded or pulling heavy yes it makes a difference. What you choose for gears has to be balanced against what your using it for.

Denny

You guys really think the mileage and wear and tear on the drivetrain is worse on a pickup with a 3.73 rear axle ratio running down the road at 60mph in 4th gear and 3,000 rpms versus one with a 4.10 rear axle rating running the same 60 mph and 3,000 rpms in 5th gear? I say that today's six or eight speed transmissions make it easy to find the right combination of road speed and rpm's that work best.

Denny & Jami wrote:

Sport45 wrote:
I'm just trying to call it what it is. The power doesn't change. You just swap rpm for torque.

That's how the 400hp turbo-diesel climbs the hill at 50mph in 4th while the 150hp hot water six in can only make 20 mph in 1st. Gearing down won't give the diesel more power, it would just slow it down.

Simple example you need to torque a bolt to 1000 you use a 10 to 1 multiplier so you only have to exert 100 ft lbs to get that torque. If you go to a 5 to 1 multiplier you would need 200 ft lbs to gain the same same torque.

The lower gears affect every gear all the time. Diesels have to be geared different than a gas motor because of their rpm operating range, gears that work for diesels aren't the best for gas motors just like gears for gas aren't the best for diesels.

Denny

You're absolutely right.

The engineer in me just got hung up on the "power" term.

Power, force, torque, and work are terms with specific (and different) meanings in my line of work.

In your example with the torque multiplier above it takes twice the force with the 5-1 vs the 10-1 multiplier. The power used relates to how fast you turn the wrench. If you turn it at twice the speed using the 10-1 multiplier the power is the same. If you turn the wrench the same speed in both cases the 10-1 multiplier is using half the power. (Because you're turning the bolt half as fast.)

RoyB wrote:
We selected the middle ratio for us...

The 4:10 give you most towing weights and least fuel mileage
The 3:73 gives the best of both worlds for towing weights and fuel mileage
The 3:55 gives least towing weights and best fuel mileage.

Those were the options I have on my 2010 FORD TRUCK. I notice the lowest 3:55 is not offered on the newer trucks now.

Roy Ken

This makes some invalid assumptions, namely that the around town driving is going to improve with taller gears, which it will not, and that when towing the taller gears will provide better fuel economy and they will not. The second point may be less obvious but big rigs that fight to gain an extra few percent in MPG will use transmissions that provide 1:1 gears when carrying a load. The overdrive helps MPG only when the truck has an empty bed or trailer(s).

A lot of assumptions are based on the old 4-speed transmissions which had a 1:1 gearing for 4th speed. Most tow vehicles today have 6-speed or even 8-speed transmissions and so have 2 or even 3 overdrive gears.

The very tall gearing does help with the EPA fuel economy highway estimate figures which are based on testing done indoors. This is what matters the most to the truck and SUV manufacturers who need to meet federal CAFE standards while selling as many large, heavy, fuel inefficient vehicles as possible as these are also far and away the most profitable. The average truck/SUV sold in the USA provides a profit to the manufacturer of more than $10,000. Only the drug and tobacco companies do better than that.

The taller the gears the more load is placed on the engine's main bearings, on the transmission, and on the drive shaft and related bearings. This is why the gears affect the towing capacity of a vehicle.

Another consideration that gets overlooked is the 20 to 55 MPH time for a vehicle with even 50% of its rated tow load. That number is going to change considerably with taller gears and it will take more time and more "runway" to get up to the speed needed to safely merge with highway traffic. For this reason alone I would never buy a vehicle to tow with a gas engine unless it had 4.10 or lower gears (as with the Toyota trucks and the Ford F-450 pickup).

Sport45 wrote:
I'm just trying to call it what it is. The power doesn't change. You just swap rpm for torque.

That's how the 400hp turbo-diesel climbs the hill at 50mph in 4th while the 150hp hot water six in can only make 20 mph in 1st. Gearing down won't give the diesel more power, it would just slow it down.

Simple example you need to torque a bolt to 1000 you use a 10 to 1 multiplier so you only have to exert 100 ft lbs to get that torque. If you go to a 5 to 1 multiplier you would need 200 ft lbs to gain the same same torque.

The lower gears affect every gear all the time. Diesels have to be geared different than a gas motor because of their rpm operating range, gears that work for diesels aren't the best for gas motors just like gears for gas aren't the best for diesels.

Denny