How do I guestimate MPH at PTO speed rather than engine rated speed?

[ericm979]

jmc's formula is correct. You may not have been at those rpms, or your tach may be off, or the digital dash tractor's speed reading may be off.

 

[4570Man]

A gear drive tractor will always maintain the same speed per engine rpm assuming the tire doesn’t spin and the tire pressure is the same. Even if the tach isn’t right but maintains the same measurement that shouldn’t matter. I don’t see any reason why the simple math approach wouldn’t work as long as you did it using actual measurements not what it should be based on gear ratio.

 

[MossRoad]

Get your tractor up to the engine RPMs that you want and time yourself in seconds over a measured 100 yard straight strip of land.

100 yards divided by the Time it takes to travel 100 yards times 3600 seconds per hour divided by 1760 yards per mile = mph

100/T x 3600 / 1760 = mph

Let's try it.

Let's say it takes you 96 seconds to travel 100 yards at the engine PRMs that you desire in the gear you desire.

100/96 x 3600 / 1760 = 2.1mph.

Easy peasy....

I think that's right.... someone check the math! :laughing:

 

[BHD]

5.7375 agree

I do not know how much land your trying to fertilize, but a lot of things can throw off the rates applied, the flow rate of the materials, the weight of the materials the moisture of it and so on,

the ground roughness, and the slope, the lap, and even the wind,

unless your plot is small, measure out the size of an acre, and set it the best you can and dump in the pounds of fertilizer you need to cover that much and go spread if you end up with some left over open up the machine a bit if runs out close it down,

charts are just guidelines to get you started,

 

[CobyRupert]

I need to know my forward MPH travel speed with the PTO at 540 RPM to spread fertilizer. Problem is, Kubota (and probably all) list MPH travel speed in the owner's manual at the engine rated speed and that can often be several hundred RPM faster than the speed at 540 PTO. So, if I used those charts I would be going slower than I think and putting down the wrong amounts of fertilizer.

I've never heard of that. Every tractor I've ever seen since beginning of time, where if the tach didn't have a way to show your mph for each gear, had a chart calibrated to 540 rpm.
Why does anybody care about their speed at the engine rated speed (whatever that means)?

 

[WVBill]

For example, a Kubota I have has a 540 PTO engine speed at 2295 RPM
...the MPH speed listed in the owner's manual. At 2600 RPM in 5th gear, it's going 6.5 MPH. ...
Any ideas?

You have the info you need:
2600 ERPM = 6.5 MPH in 5th gear. So the ratio is 2600/6.5 = 400 ERPM/1 MPH. It's a gear tractor so that ratio is fixed.
Each 400 Engine RPM gives you 1 MPH Speed
They've also told you that you need 2295 ERPM to get 540 PTO RPM. That also is fixed

So 2295 / 400 = 5.7375 MPH

 

[joeu235]

Don't know how accurate you want to be, but most tractors are ballasted for ~5% tire slippage when pulling a big load.

 

[SPYDERLK]

jmc's formula is correct. You may not have been at those rpms, or your tach may be off, or the digital dash tractor's speed reading may be off.

Exactly. . . . OP, just lay out a graph on linear graph paper. Rated engine speed and associated rpm are one known point. The other is 0rpm and 0 speed. - - Draw a straight line between the two.

 

[sixdogs]

OK, the math is starting to make sense so I'm now on board and will do some field guestimates and report back. Thank you.

 

[oosik]

sixdogs - with your gear driven tractor - rpm's vs ground speed will be linear. JMC's formula is correct.

Example - - if 1500 rpms gives you a ground speed of 4 mph - then 2250 rpms will give you a ground speed of 6 mph.

50% more rpms equal to 50% more ground speed All other things being equal
Personally - I think you should buy a hand held GPS. Many are available that will give mph readout as you are moving on the tractor.

Weird - my dash on the M6040 gives rpm and mph readouts - - I've never had a real use for either value.