Another Weight Type Question?

Larryzv7 wrote:
I did not want to tag onto that other thread about weights, but I do have a few questions.

1) Does the rear axle gear ratio of your tow vehicle have anything to do with your vehicles towing capacity?

If you read the tow guides you will see that the lower the gear ratio the more you can tow. Example a 4.10:1 can tow more than a 3.42:1 there are other variable like 4 speed or 6 speed transmission and diesel vs gas. The weight carrying ability of the truck (GVWR) (what you put in bed/cab)will normally remain constant between the different rear axle gear ratios. There is what you can carry and what you can pull.

2) A trucker told me that the towing capacity of their semi-truck, 18-wheels, etc., is determined by the size of the tires on the truck. Have you ever heard this before?

I'm a trucker. My gross or how much I can carry is limited by what I'm plated (registered) to carry and the number of axles and weight on each axle is covered under "bridge laws" so I don't break the road or the bridge as I drive over it. The standard class 8 tractor can pull 80,000# or 140,000# it's more do with the limits on the roads and the tax man. More axles (tires) more load and more taxes. See the 8 axle trailers they pull in MI.

I read somewhere that smaller tires will give you a higher engine rpm but more towing power, and larger tires will decrease the engine rpm and towing power.

I can see it in my odometer if I have new or old tires (might log 2 miles less in the same route between new and old tire) but the effect of gearing makes this a mote change in power. Changing tire/rim size I.E. 17" to 19" can effect the effective gearing of the truck but then you are also changing the bed ride height and other issues with the truck.

Fifth wheel trailers generally have small tires while semi-truck trailers, 18-wheels, etc., usually have the same size tires on both the truck and trailer.

Tire size is not an issue here. All that matters is weight rating of the tire and weight of axle on the pavement. We run drop deck/van trailers with small rear tires. Still limited to 34,000# on a tandem axle. The only real difference is small tires cost more, wear out faster and want to turn corners faster so you have to swing a little wider.

3) Last question; is it wise to use overdrive when towing or is overdrive just necessary when gears seems to be struggling?

Your best fuel mileage will be in overdrive since your transmission gear ration is less than 1:1. If the load on the engine permits you to stay in overdrive great otherwise if the transmission starts shifting in and out or the engine "lugs" you are best to downshift. Normally if you use the tow haul mode it will automatically stop the transmission from doing this.

Thanks for your answers, and the education.
Larry:)

Larryzv7 wrote:
I did not want to tag onto that other thread about weights, but I do have a few questions.

1) Does the rear axle gear ratio of your tow vehicle have anything to do with your vehicles towing capacity?
I can only speak from my experience and knowledge - I'm not a mechanical engineer. The rear axle ratio effects the torque available when you start pulling but not the amount you can pull. Take a 1:1 rear axle ratio and try to start off pulling 10K lbs. You can imagine slipping the clutch or transmission converter A LOT to get moving. Now imagine that same load with a 10:1 rear axle reduction, it will start pulling very easy. However the available HP & torque qill be the same so both situations will pull the same weight once moving.

2) A trucker told me that the towing capacity of their semi-truck, 18-wheels, etc., is determined by the size of the tires on the truck. Have you ever heard this before?

As are your trucks tires what determine SOME of it's carrying capacity. You have to read the tire manufacturers specs on a particular tire to see what it is rated. There are 16" tires rated for 3.5K lbs of load and then there are those that maybe rated for 4K lbs. of load(each). So in the first instance you can carry 7K lbs. and in the second 8K lbs. But the weight limit on the truck, the rear axle weight limit and the tires will all eventually determine what you can haul (in the bed).

I read somewhere that smaller tires will give you a higher engine rpm but more towing power, and larger tires will decrease the engine rpm and towing power.

Fifth wheel trailers generally have small tires while semi-truck trailers, 18-wheels, etc., usually have the same size tires on both the truck and trailer.

3) Last question; is it wise to use overdrive when towing or is overdrive just necessary when gears seems to be struggling?

Over drive is a function of available power, engine speed an the load on the truck. With my 4.88 rear end, 650 ft. lbs. torque, I can run overdrive with no problems. The truck will not hunt for a gear at anytime, downshifts to go up steep hills and increase my MPG by about 1 verses not using OD.
Thanks for your answers, and the education.
Larry:)

Larryzv7 wrote:
I did not want to tag onto that other thread about weights, but I do have a few questions.

1) Does the rear axle gear ratio of your tow vehicle have anything to do with your vehicles towing capacity?

It increases the pulling power, but not the weight capacity. It also changing the gear ratio also can lower fuel milage as the engine is turning faster.

2) A trucker told me that the towing capacity of their semi-truck, 18-wheels, etc., is determined by the size of the tires on the truck. Have you ever heard this before?

Semi-trucks have to keep their weights per axle in line with capacities. They will often have added tag axles to increase the trucks weight capabilities. There's a lot more to it than that though. It's a totally different ball game. You can put major heavy items in a freight trailer, and they are regularly weighed to keep them in specs by the DOT.

I read somewhere that smaller tires will give you a higher engine rpm but more towing power, and larger tires will decrease the engine rpm and towing power.

Yes this is true, but gearing has a bigger effect.

Fifth wheel trailers generally have small tires while semi-truck trailers, 18-wheels, etc., usually have the same size tires on both the truck and trailer.

As said before, semi tractor trailers are a different ball game, and play by different rules.

3) Last question; is it wise to use overdrive when towing or is overdrive just necessary when gears seems to be struggling?

In my opinion, if you have to take your vehicle out of overdrive all the time to tow what you have, then you are over taxing the vehicle. (Been there, done that. Wore out engines. Not again.) On my truck, I take it out of overdrive on steeper hills, but for general cruise, my truck engine has more than enough power to stay in overdrive with out lugging the engine hard. This also depends on other factors. I have a diesel, designed to have the torque for pulling heavy loads. It's max torque is at 1850 RPM. Gas engines have max power at higher RPM's and so have to rev up higher to do the same work. It takes a toll on them and so they don't last as long, nor is the power as much when they do their work. Translation, you have a gas engine, yes you will have to kick it out of overdrive when the engine is struggling.

You can feel it when you need to. When pulling, if you feel adding more throttle gives you no increase in power or speed, you need to kick it out of overdrive. Also, don't push the pedal to the floor and hold it there hoping it will go faster. That is a good way to at least warp and crack exhaust manifolds. When hill climbing, take it out of overdrive. When you reach the place where it is slowing down, don't keep pressing the throttle down farther, getting nothing. Hold it at the place where lifting it reduces speed, but adding it gets nothing. Then if it's still struggling, use your gears to manually to drop a gear. It's much easier on the engine and transmission, and you'll get many more miles on them both.

Thanks for your answers, and the education.
Larry:)

Larryzv7 wrote:
I did not want to tag onto that other thread about weights, but I do have a few questions.

1) Does the rear axle gear ratio of your tow vehicle have anything to do with your vehicles towing capacity? and

2) A trucker told me that the towing capacity of their semi-truck, 18-wheels, etc., is determined by the size of the tires on the truck. Have you ever heard this before? Changing drive axle tire diameter has a similar effect as changing axle gear ratio's.

I read somewhere that smaller tires will give you a higher engine rpm but more towing power, and larger tires will decrease the engine rpm and towing power.

Fifth wheel trailers generally have small tires while semi-truck trailers, 18-wheels, etc., usually have the same size tires on both the truck and trailer. Not a big issue, but some tires like those with steel ply carcasses have lower rolling resistance.

3) Last question; is it wise to use overdrive when towing or is overdrive just necessary when gears seems to be struggling?Lock out overdrive when the tranny is hunting between gears.

Thanks for your answers, and the education.
Larry:)