Why is gear ratio so important

[TOO LONG]

Thanks for all the replies. Interesting thoughts

[Pianotuna]

Originally Posted by TOO LONG

Thanks for all the replies. Interesting thoughts

Hi,


I put on taller tires which gave me an effectively lower differential gear ratio. It moved me down from 4.56 to about 4.22. I did get a slight improvement on fuel economy. It gave me 2 inches more ground clearance.



My class C is still quite happy in the mountains.

[Ryqster]

Back in the days when we were limited to 3-4 gears, and torque were limited lower (higher numerically) gears were needed. Now with transmissions with double overdrives, 8-10 speed transmissions etc. it is much less of a factor until you get to the very heavy coaches. I was impressed in how well my RAM did pulling my Arctic Fox, weighing out at about 15K all over the Pacific Northwest with 3.43:1 axle ratio, the Aisin trans and the torque monster under the hood. If I was climbing. just locked it into direct (4th) and up the hills I went, at virtually 55mph in most cases. Drop the trailer left it in 6th and got excellent fuel economy. To me this is the best of both worlds. If you pull something in the 20k range me personally I would opt for at least 3.73-4.10:1. Much better and the overdrive of todays transmissions still give your hard working engine a much need break when not towing! Todays trucks are amazing compared to those say manufactured back 5-8 years ago.

[Elisavaet]

Originally Posted by TOO LONG

I see where gear ratio is discussed on several threads. What difference does gear ratio make? Is it for the start of the pull or for highway speeds. Is it for gas milage or for power? Please educate me.

Thanks

Too Long,
Every engine has a different torque and horsepower band. The rule of thumb is that; "Torque gets you up to speed, power keeps you there."

Many modern vehicles have a very high ratio (low number) to get better fuel economy, but bay need a lower ratio (higher number) to actually get the engine into the appropriate area of the power band when towing. (Cummins curves).

If you've added larger diameter tires you have effectively made the gear ratio higher (lower number) and may need to change the gear ratio to get back to factory. But again, since the factory may have used a higher ratio (lower number) to get better fuel economy, you may need to change the gear ratio anyway to get into a good spot on the curve for towing.

If you look at the towing packages on HD trucks you'll see the lower ratio (higher number) gives the better towing capacity. The Ram Trucks towing guide gives you an easy tool to play around with and see that, for example, with a Crew Cab Dually 4x4 Cummins HO you get 25,070 pounds of towing capacity with the 3.73 gears but 33,620 pounds of towing capacity with the 4.10 gears. You can use this tool with the 1500 and 2500 series to see similar differences with lighter duty trucks.

I lower ratio (higher number) will get you up and into the power band quicker off the start, so it typically gives better acceleration, but, your engine will be running at higher rpms all the time so you get lower fuel economy. The exception to this would be where you have a massively powerful engine that can pull the higher ratio (lower number) gears, but this is typically a racing only scenario.

A classic question is do you get the truck first, or the trailer first? The reasoning behind getting the trailer first is so you know how to spec the truck to be able to comfortably tow the trailer.

Gearing is a tradeoff. If you have an existing out of warranty truck you can increase engine power and torque with modifications and tuning. If you have an in warranty truck or are going to buy a new truck engine modifications and tuning may not be a good idea as you'll likely void your warranty. In this case you try to get the truck with the engine/transmission/gear ratio combination you need, but if you add larger tires or need to increase the towing capacity (within the spec of the mfr) you can change out the gears without voiding your powertrain warranty (probably) as the least intrusive option.

I hope this helped!

Kate

[LargeMarge]

Usually, the ratio of gears inside the rear axle is a compromise between 'startability' and cruise speed.

Startability is the ability of the engine / transmission to get the vehicle plus cargo moving from a stop on a theoretical grade, usually about eight percent.

Cruise is the best fuel economy at the best speed to destination.

A half-century ago, hotrods raced stoplight-to-stoplight.
Acceleration was the goal.
Cruise speed was not a factor.
A direct-drive transmission fed a rear-axle ratio as low as 4.88 to 1, usually abbreviated as '4.88'.
A 4.88 required quick shifts of the four-speed transmission.
Halfway across an intersection might be third gear!
That ratio limited top speed to a snailish 80 or 90mph.
As you can imagine, engine RPM was screaming at freeway speeds.

For better fuel economy, a ratio of 3.30 to 1 was standard on many sedans.
That ratio means that grocery-getter was a 'dog' off-the-line.

As you might expect, vehicle weight is an additional factor in moving a vehicle and keeping it moving.
Seeking lightweight, old-time hotrodders went extreme, removing the heater, insulation, passenger seat, and spare tire!

At around 50mph, wind resistance adds another factor.
My 1953 Austin Healey weighed about 1,700#.
After I replaced the slug four-banger with a Chevy V-8, I was pleasantly surprised with the difference in acceleration.
Pleasantly.

Unfortunately, my 1953 had the aerodynamics of a shoebox.
Above 130mph, it tended to float, losing directional stability.
That was perfect because I designed it to compete on tracks and in rallies at 30mph to 90mph.

Beginning in the 1990s, overdrive transmissions used a fifth or sixth gear to lower cruise RPM while maintaining good startability.

Late-2020, I doubt any road vehicle has a direct-drive transmission.
My 1997 ExpeditionVehicle has an Allison 3060 with a direct-drive fourth gear, plus two overdrive gears.
Its axle ratio is 5.88, unheard of a few decades ago.
If I interfere with its standard shift, limiting it to fourth gear, top speed would be around 42mph.

So, my answer to your question is 'maybe'.

[TwoStik]

Had a gear vendors.
If you plan on using engine braking for descending hills the gear vendors transmission must be in direct drive.
If its in overdrive and engine braking is used severe damage to its over running clutch is a real possibility.
once that occurs there is no power through the transmission.
Been there.
Got rid of it and had the transmission rebuilt into a T19A, added a Spicer 7231B behind it with a 4.10 differential.
Now with 12 speeds available first low was over 6:1 and 4th over was .86.
With torque peak at 1600 and rpm peak hp at 2200 rpm i had the best of both worlds.
My shift splits were 300 rpm if I went through all gears. Worked really well for towing the Sierra Nevada mountains.
Long winded but the point is, the solution will be a compromise.
Gross weight, power band (torque peak rpm to hp rpm), transmission low gear ratio and top 2-3 gear ratios (low for starting the load, top gear for cruising, next gear or two down for hills), maximum grade in percent (starting load and pulling hills), desired performance (average road speed and reasonable average mpg.
Its easier now as the power bands are wider and flatter, 8-10 speed transmissions with lock up torque converters are common.
Only choice may be final drive ratio and that may be limited to two choices depending on vehicle. Half ton pickup or large SUV may only have two options. 3/4 ton and one ton 3 or more options. Many years ago GMC salesman told me, if your GCVW gross combination vehicle(s) weight is less than 75% of vehicle rated capacity stay with standard final drive ratio. 75% or more drop final drive ratios accordingly.
For example standard ratio 3.23, 3.54 for more weight, 3.90 heavier still. I'm not sure what lower final drive ratios are a available but at one time it was 4.10 due to the differential case size.
If you travel US numbered highways the compensated grade limit is 6%. Compensated means overall percentage for a given grade. If you run hills like US numbered highways then your 75% less 6% for hills now becomes 69%.
Good luck and always, safe travels !

[krusty]

One other item you that may not be a part of your particular case, but affected me was the tire size. I have a truck with 4.10 gearing, and I got a lift kit got larger tires - went from about 30" to 33". The RPMs dropped and when towing at 55-60 was too low for my gas truck. I moved up to 4.56 and that solved the problem in my case. Here's a nice example of the affect of gearing and tire size:

https://auto.howstuffworks.com/auto-...size-chart.htm

Krusty

[Goodspike]

Originally Posted by Ryqster

Back in the days when we were limited to 3-4 gears, and torque were limited lower (higher numerically) gears were needed. Now with transmissions with double overdrives, 8-10 speed transmissions etc. it is much less of a factor until you get to the very heavy coaches..

I haven't read all the answers in this thread, but this is the answer I was going to give--although I probably wouldn't have mentioned heavy coaches. It's much less important now, which is why I think it's maybe GM that doesn't even offer a choice on many pickups.

The main thing affected now with heavier loads or more powerful engines would be the driveshaft. The engine should be allowed to stay in its proper powerband.

[WillT]

As pointed out there is much more to it than just the ratio. I have a 19 year old 1 ton gas old truck with a 3.73 and a 3 or 4 speed transmission, I think. It is really slow down pulling up long mountain passes. It will do it all day but at 35 to 40 mph and I added an oversized transmission cooler to keep temps down. On the other hand I have talked to people at campgrounds with similar sized TTs as me and they are towing with a new gas 1/2 or 3/4 truck and go up whatever they want at whatever speed they want. But I would not want to tow our TT with a 1/2 ton even if it would pull it fine.

[TOO LONG]

Originally Posted by Ryqster

Back in the days when we were limited to 3-4 gears, and torque were limited lower (higher numerically) gears were needed. Now with transmissions with double overdrives, 8-10 speed transmissions etc. it is much less of a factor until you get to the very heavy coaches. I was impressed in how well my RAM did pulling my Arctic Fox, weighing out at about 15K all over the Pacific Northwest with 3.43:1 axle ratio, the Aisin trans and the torque monster under the hood. If I was climbing. just locked it into direct (4th) and up the hills I went, at virtually 55mph in most cases. Drop the trailer left it in 6th and got excellent fuel economy. To me this is the best of both worlds. If you pull something in the 20k range me personally I would opt for at least 3.73-4.10:1. Much better and the overdrive of todays transmissions still give your hard working engine a much need break when not towing! Todays trucks are amazing compared to those say manufactured back 5-8 years ago.

Thanks for the info

[TOO LONG]

Originally Posted by Elisavaet

Too Long,
Every engine has a different torque and horsepower band. The rule of thumb is that; "Torque gets you up to speed, power keeps you there."

Many modern vehicles have a very high ratio (low number) to get better fuel economy, but bay need a lower ratio (higher number) to actually get the engine into the appropriate area of the power band when towing. (Cummins curves).

If you've added larger diameter tires you have effectively made the gear ratio higher (lower number) and may need to change the gear ratio to get back to factory. But again, since the factory may have used a higher ratio (lower number) to get better fuel economy, you may need to change the gear ratio anyway to get into a good spot on the curve for towing.

If you look at the towing packages on HD trucks you'll see the lower ratio (higher number) gives the better towing capacity. The Ram Trucks towing guide gives you an easy tool to play around with and see that, for example, with a Crew Cab Dually 4x4 Cummins HO you get 25,070 pounds of towing capacity with the 3.73 gears but 33,620 pounds of towing capacity with the 4.10 gears. You can use this tool with the 1500 and 2500 series to see similar differences with lighter duty trucks.

I lower ratio (higher number) will get you up and into the power band quicker off the start, so it typically gives better acceleration, but, your engine will be running at higher rpms all the time so you get lower fuel economy. The exception to this would be where you have a massively powerful engine that can pull the higher ratio (lower number) gears, but this is typically a racing only scenario.

A classic question is do you get the truck first, or the trailer first? The reasoning behind getting the trailer first is so you know how to spec the truck to be able to comfortably tow the trailer.

Gearing is a tradeoff. If you have an existing out of warranty truck you can increase engine power and torque with modifications and tuning. If you have an in warranty truck or are going to buy a new truck engine modifications and tuning may not be a good idea as you'll likely void your warranty. In this case you try to get the truck with the engine/transmission/gear ratio combination you need, but if you add larger tires or need to increase the towing capacity (within the spec of the mfr) you can change out the gears without voiding your powertrain warranty (probably) as the least intrusive option.

I hope this helped!

Kate

Wow that for the detailed help

[TOO LONG]

Originally Posted by LargeMarge

Usually, the ratio of gears inside the rear axle is a compromise between 'startability' and cruise speed.

Startability is the ability of the engine / transmission to get the vehicle plus cargo moving from a stop on a theoretical grade, usually about eight percent.

Cruise is the best fuel economy at the best speed to destination.

A half-century ago, hotrods raced stoplight-to-stoplight.
Acceleration was the goal.
Cruise speed was not a factor.
A direct-drive transmission fed a rear-axle ratio as low as 4.88 to 1, usually abbreviated as '4.88'.
A 4.88 required quick shifts of the four-speed transmission.
Halfway across an intersection might be third gear!
That ratio limited top speed to a snailish 80 or 90mph.
As you can imagine, engine RPM was screaming at freeway speeds.

For better fuel economy, a ratio of 3.30 to 1 was standard on many sedans.
That ratio means that grocery-getter was a 'dog' off-the-line.

As you might expect, vehicle weight is an additional factor in moving a vehicle and keeping it moving.
Seeking lightweight, old-time hotrodders went extreme, removing the heater, insulation, passenger seat, and spare tire!

At around 50mph, wind resistance adds another factor.
My 1953 Austin Healey weighed about 1,700#.
After I replaced the slug four-banger with a Chevy V-8, I was pleasantly surprised with the difference in acceleration.
Pleasantly.

Unfortunately, my 1953 had the aerodynamics of a shoebox.
Above 130mph, it tended to float, losing directional stability.
That was perfect because I designed it to compete on tracks and in rallies at 30mph to 90mph.

Beginning in the 1990s, overdrive transmissions used a fifth or sixth gear to lower cruise RPM while maintaining good startability.

Late-2020, I doubt any road vehicle has a direct-drive transmission.
My 1997 ExpeditionVehicle has an Allison 3060 with a direct-drive fourth gear, plus two overdrive gears.
Its axle ratio is 5.88, unheard of a few decades ago.
If I interfere with its standard shift, limiting it to fourth gear, top speed would be around 42mph.

So, my answer to your question is 'maybe'.

Thanks for the reply

[TOO LONG]

Originally Posted by TwoStik

Had a gear vendors.
If you plan on using engine braking for descending hills the gear vendors transmission must be in direct drive.
If its in overdrive and engine braking is used severe damage to its over running clutch is a real possibility.
once that occurs there is no power through the transmission.
Been there.
Got rid of it and had the transmission rebuilt into a T19A, added a Spicer 7231B behind it with a 4.10 differential.
Now with 12 speeds available first low was over 6:1 and 4th over was .86.
With torque peak at 1600 and rpm peak hp at 2200 rpm i had the best of both worlds.
My shift splits were 300 rpm if I went through all gears. Worked really well for towing the Sierra Nevada mountains.
Long winded but the point is, the solution will be a compromise.
Gross weight, power band (torque peak rpm to hp rpm), transmission low gear ratio and top 2-3 gear ratios (low for starting the load, top gear for cruising, next gear or two down for hills), maximum grade in percent (starting load and pulling hills), desired performance (average road speed and reasonable average mpg.
Its easier now as the power bands are wider and flatter, 8-10 speed transmissions with lock up torque converters are common.
Only choice may be final drive ratio and that may be limited to two choices depending on vehicle. Half ton pickup or large SUV may only have two options. 3/4 ton and one ton 3 or more options. Many years ago GMC salesman told me, if your GCVW gross combination vehicle(s) weight is less than 75% of vehicle rated capacity stay with standard final drive ratio. 75% or more drop final drive ratios accordingly.
For example standard ratio 3.23, 3.54 for more weight, 3.90 heavier still. I'm not sure what lower final drive ratios are a available but at one time it was 4.10 due to the differential case size.
If you travel US numbered highways the compensated grade limit is 6%. Compensated means overall percentage for a given grade. If you run hills like US numbered highways then your 75% less 6% for hills now becomes 69%.
Good luck and always, safe travels !

Good info Thanks

[TOO LONG]

Originally Posted by krusty

One other item you that may not be a part of your particular case, but affected me was the tire size. I have a truck with 4.10 gearing, and I got a lift kit got larger tires - went from about 30" to 33". The RPMs dropped and when towing at 55-60 was too low for my gas truck. I moved up to 4.56 and that solved the problem in my case. Here's a nice example of the affect of gearing and tire size:

https://auto.howstuffworks.com/auto-...size-chart.htm

Krusty

Thanks for the info