calculating ground speed

[defed]

my tractor lists the ground speeds in various gears and ranges using 11.2-24 r1 tires. i have 15-19.5 r4's. is the ground speed a simple ratio of the circumferences? the rolling circumferences are not that different (130 for r1, 117 for r4), but just curious how different the actual ground speed is.

thanks.

 

[Dmace]

If those circumferences are correct, then you should be able to multiply the given speed by 0.9 (90%) and that will give you your true speed with the smaller tires.

 

[Redneck in training]

If those circumferences are correct, then you should be able to multiply the given speed by 0.9 (90%) and that will give you your true speed with the smaller tires.

Then get GPS and measure it. Easy and fun project for HS kids.

 

[Tarheeler]

Then get GPS and measure it. Easy and fun project for HS kids.

Yep those GPS are neat

 

[AV8R62]

if you like this sort of thing , go buy a small digital bycycle seedometer they under $20
an in the set up instructions their is a formula for the size of tire .
Glue the magnet to the rim and mount the sensor where it will read the magnet when it passes and you are done. now you have more fun facts than you know what to do with like
total mowing time,average speed, instant speed and time of day

 

[Raspy]

defed,

Yes, it's a simple ratio of the circumferences. Or the diameters. Whichever you prefer. Measuring the diameters might be easier, or you might get them from the manufacturer's site.

 

[RoyJackson]

defed,

Yes, it's a simple ratio of the circumferences. Or the diameters. Whichever you prefer. Measuring the diameters might be easier, or you might get them from the manufacturer's site.

I'm curious how you figure the diameters come in to this (other then to calculate the circumference). Doesn't seem like just the diameters would help...

 

[MikeD74T]

I'm curious how you figure the diameters come in to this (other then to calculate the circumference). Doesn't seem like just the diameters would help...

He's just saying that comparing diameters is a linear relationship, double the diameter - double the circumference, etc. MikeD74T

 

[Raspy]

The ratio of the diameters is the same as the ratio of the circumferences. It's all a linear relationship and circuference is just twice diameter.

Sometimes tire manufacturers give tire info in diameter and sometimes in revolutions per mile. (5280 X 12) divided by circumference in inches, is revolutions per mile. Or, for diameter to revolutions per mile, (5280 X 12) divided by (diameter in inches X 2)

Either way, once you decide on diameter or circumference, and get the numbers for both tires, you just divide the new into the old. This gives you a multiplier such as 1.08 for instance. Next, multiply your speed by that multiplier (ratio). If the new tires are bigger you'll go faster by the percentage difference. In this case the old speed X 1.08, just as an example.

If the new tires are larger, your ratio will be more than 1. Smaller will give you less than 1.

 

[tjc1989]

From my old HS days that I use often. C = pi D or circumference equals pi Diameter. If diameter is say 40 inches, multiply 40 x 3.1414 to get the circumference of 125.66 inches. I guess you could put a piece of tape on the tire, and time the amount of time the tire takes to make a complete revolution or several revolutions, then calculate according to how many feet you traveled in that time.