R38 vs r42 speed difference

[daveydiesel1]

On a 40k tractor, they mostly do more than 40k but lets assume it does 40k on the dot, if its running 650/65r38, if that was changed to 650/65r42 what would the difference in top speed be?

 

[glasshouse]

On a 40k tractor, they mostly do more than 40k but lets assume it does 40k on the dot, if its running 650/65r38, if that was changed to 650/65r42 what would the difference in top speed be?

Change it to 650/85-38, same height as 42
If 38 tyre is 6ft and 42 is 4 inches highr,
That is 7% increase so same incease in speed

 

[clbarclay]

42.4 km/h for the r42 at the same gear and revs.

That's based on the difference in rolling circumference of one brand.

 

[Chae1]

On a 40k tractor, they mostly do more than 40k but lets assume it does 40k on the dot, if its running 650/65r38, if that was changed to 650/65r42 what would the difference in top speed be?

Most modern tractors think the speed is limited by electronics. It will have top speed in settings and stop accelerating when it reaches that speed.

We changed from 38s to 42s on our t7 new Holland and top speed just stayed the same.

 

[clbarclay]

How are you measuring the difference?

If the tractor only has a sensor counting different teeth then it will still be reading the same speed if you change tyre sizes, but not be traveling at the indicated speed.

If it as a radar or GPS input then it could automatically compensate for changes in tyre size.

 

[daveydiesel1]

Change it to 650/85-38, same height as 42
If 38 tyre is 6ft and 42 is 4 inches highr,
That is 7% increase so same incease in speed

The 85 profile tyre is a serious amount more expensive compared to the 42 rim though

 

[Chae1]

The 85 profile tyre is a serious amount more expensive compared to the 42 rim though

Not worth it in my opinion. Prefer 42s.

 

[daveydiesel1]

Not worth it in my opinion. Prefer 42s.

Same especially if ye happen to cut a tyre off

 

[michael N123]

Most modern tractors think the speed is limited by electronics. It will have top speed in settings and stop accelerating when it reaches that speed.

We changed from 38s to 42s on our t7 new Holland and top speed just stayed the same.

Speed be read by radar so tyres won’t make a difference

on the older newHollands you could tell the computer you where on smaller wheels for more speed

 

[onesiedale]

don't want to sound thick, but if you change the size of the back tyres, do you have to change the front ones too? Surely they will work against each other when in 4wd.. ?

 

[clbarclay]

They fight most of the time anyway. Standard practice is to have front tyres leading 1 to 5 percent faster than the rears. Good reason not to leave 4wd engaged on the road, though most tractors engage it when the brake peddles are pressed.

If your change the rears then you will almost certainly need to change the fronts to suit.

 

[ccp contracting]

I think a 650/85x 38 is bigger than a 650/65 x 42 , i think you would need a 650/75x38

 

[Chuckie]

Would it not be that if you were doing 38k on 38", you would be doing 42k after changing to 42"? Assuming they were the same width and profile

 

[clbarclay]

Would it not be that if you were doing 38k on 38", you would be doing 42k after changing to 42"? Assuming they were the same width and profile

Nope, because you haven't taken into account the size of the rubber around the wheels.

The difference is still 4 inches, but it's 4 inches between 72" and 76" overall diameter of the wheel and tyre.

 

[snarling bee]

Would it not be that if you were doing 38k on 38", you would be doing 42k after changing to 42"? Assuming they were the same width and profile

No because the rim size is not the same as the overall diameter.
You really need to check the manufacturer's rolling circumference

 

[Blue.]

I think a 650/85x 38 is bigger than a 650/65 x 42 , i think you would need a 650/75x38

2071mm diameter versus 1931mm for the 42”,the 650/75r38 has a diameter of 1941mm

 

[Drillman]

We swapped from Michelin 520/85/38s which always looked small to BKT 650/70/42s (and BKT are known for been on the large side) with matching fronts on our Vario NH and it now does same top speed cos the computer rules but at 300rpm less

 

[daveydiesel1]

don't want to sound thick, but if you change the size of the back tyres, do you have to change the front ones too? Surely they will work against each other when in 4wd.. ?

Yes would have to go 540 65 30 fronts

 

[Farmer_Joe]

We swapped from Michelin 520/85/38s which always looked small to BKT 650/70/42s (and BKT are known for been on the large side) with matching fronts on our Vario NH and it now does same top speed cos the computer rules but at 300rpm less

Just think of the fuel saving

 

[Drillman]

Just think of the fuel saving

Ironically that tractor spends most of its time either on the drill or the baler. it Hardly ever does a non PTO job.