I read alot of numbers on here about gear ratios, tranny's, tire sizes and fuel economy. Just not real sure how it all fits together or what ratios are better for running certain loads or pulling different trailers. One website I found explains things to experienced drivers..not newbies like me trying to figure it all out. So. Keeping rpm's low obviousley saves fuel depending on weight and terrain but does bigger tires = better fuel economy? (plus on spare could fit all according to a post i read here somewere)
10 spd vs 13 spd ??
Lower gear numbers = ??
Higher gear numbers = ??
power bands = ?
Just want to learn more about gear ratios and what to consider. And how to apply the technology to different applications. Weights, tranny speeds, gear ratios, power bands, tire sizes, terrain, fuel economy, performance exceeding 70 mph sometimes if you need it but not the norm.

Does anyone know of a website to spec trucks or a book or website where to find this info and explain it on newbie terms ? Like gear jammin for dumma$$e$ or whatever.

 

search for cummins powerspec, its a software tool that calculates all the different specs for you.

The gear ratio works like this:

If the number is 3.73, the driveshaft spins 3.73 times for every time the axle/wheel turns.

This number is VERY important, it tells you how much torque at the drive shaft gets multiplied. A higher number means more multiplication, but lower cruising speeds. A higher number also means it will go up a hill better, with narrower "drops" in between gears.

There is another number that is important, final gear of the transmission. Most are .73, some are 1. That means that the engine will spin .73 times every time the driveshaft spins once. This is called "over drive" because the engine is spinning slower then the driveshaft.

Another final number that is important is "final drive ratio". This is simply the gear ratio multiplied by the final gear transmission ratio. In our example of 3.73 gear ratio and .73 final transmission ratio we'd have a final drive ratio of 2.7229.

Gear ratio * final gear transmission ratio = final drive ratio

This number represents how many times the engine will spin for one rotation of the tires.

Tire size is also important, and is a very easy way to make a truck specced a little off, to "right on". Tires are rated in RPM(Revolutions Per Mile) and as you can imagine, a higher number means the tire spins more per mile.

Typical "RPM" numbers are from 490 to 530. We'll use 500 for our example.

At 60 mph, we are running exactly at one mile per minute.

Take that 500 "revolutions per mile" and multiply it by the final drive ratio, and we get a number of 1361. This is how many RPMs the engine is turning at 60 mph.

Different engines have different requirements. Generally the rule is that larger displacement engines(15L monsters such as the c15, and ISX) like to be spun lower, and smaller engines higher.

For our example, a small 11 litre engine would be very unhappy at those rpms, but a cat c15 would fit the bill perfectly.

Mostly specs are for how much you weigh, how fast you want to go, and where you want to run.

I know of a local company who hauls b-trains and specs cat c15s totally different from everyone else because they go into the mountains with them. They spin over 1500 rpms at speed. They need to, they're well over 130,000 lbs.

Say we took our example, and put shorter tires on it(530 rpm tires). 530 * 2.7229 = 1,443. Just so you can see how tires can make a difference.

 

Thanks allan5oh,

Wow, the numbers are making a little more sense now. I will download the cummins program when I get home tomorrow. It will make more sense to me since you broke it down and can relate to the numbers.
I appreciate the help / explainations.

 

No problem, sometimes I'm not very good at explaining things.

If you have another question, fire away.

 

Here is an Excel spreadsheet that may help. If you have any questions on it let me know.

Download Here

 

This link is to mediafire..not a maintenance excel file. Am I suppose to sign up to get the spreadsheet?

 

No there will be a second link inside page that opens. Click on it to download

 

Thanks. Thats a nice spreadsheet.