You might have to go back to first principles.
1. I don't know what type of transmission you have....but the basic procedure for any gear-driven four wheel drive machine is the same. Start by engaging 4wd and then jacking up one side of the tractor so that both wheels on one side are clear of the ground. Block the other tires. Now rig up some sort of very accurate pointer. A C-clamp and coat hanger will do. One for each tire. And make a chalk mark on the tires under the pointer. Now VERY SLOWLY and gently rotate the front tire while keeping an accurate count of how many revolutions of front is required to rotate the rear wheel one complete turn. You may have to go around more than once turn on the rear tire to get the marks to line up exactly as they were to start. That's OK, just keep counting. Repeat it a few times. This ratio shouldn't vary at all if repeated. This is your basic gear ratio between the front and the rear. It was necessary to turn the tire slowly and carefully so as to avoid any engagement of any limited slip or differential action.
I don't know why manufacturers don't provide that information, but few do.
You are now a third of the way through the process.
2. Next you go to the manufacturer's site for your ORIGINAL tires and hopefully you can find the spec called: "Rolling Circumference". Because loaded tire distort when moving you will want the manufacturer's original spec for rolling circumference rather than measuring it for yourself. In fact, if they have a spec for "loaded rolling circumference " that's even better.
Get the specs for front and rear tires and compare that ratio with the ratio you measured in the first part above.
Note: some manufacturers quote a "loaded diameter"....I'm not sure what that is....some sort of average?
Anyway, if you absolutely cannot find the manufacturer's specs for your original tires all is not lost; you can measure their rolling circumference directly for yourself by finding a clean level straight road, putting a spotch of wet paint on the F & R tires (not in line with each other!
) and driving the course. Measure the distance between paint marks on the road and that is your rolling circumference. Sounds easy, but actually you'll be amazed at how the slightest change of course will cause the distances to vary. Do it several times. Ignore kibitzers or enlist them. Take an average.
3. Almost done.
Hopefully you will find that the front to rear ratios of tires and gearing match within a few percent. Rarely do they differ by more than 5 or 10%, but I have seen that happen. When the ratio is off more than about 5% it probably means means that the manufacturer designed the tractor for one type of tire - say an R1, but also offered an option for say R4s or turfs. Even thought those tires might measure exactly the same diameter when mounted statically, they can have very different rolling diameters. That is because they change shape when loaded and rolling. High difference ratios also mean that the tractor may be difficult to get out of 4wd sometimes.
4. Last step. Check the spec for rolling circumference on the tires that you are thinking of buying. Compare their ratio with what you have been using.
5. I once did this and found a set of tires that gave me exactly zero percent slip. I thought that was perfection and perhaps for some purposes it would be. But in use, I found that the tire differenctial or slippage was also used by the manufacturer to keep the manual 4wd lever engaged. True, with a perfect ratio match I could slip into and out of 4wd on the fly with never a grind - but had to keep my leg against the lever to keep the weight of the lever from shifting it into and out of 4wd on rough roads!
Good luck. Enjoy Sweden! Let me know if you have questions.
rScotty
