Most folks don't realize that some belts are measured on the inside circumference. A B C and D classical or industrial belts and some are measured on the outside circumference. FHP 3L 4L and 5L belts and there is a 2" difference between an A belt and a 4L belt and a 3" difference between a B belt and a 5L belt. Plus the fact that some OEM belts that list the length may be for an A B C industrial belt and some may be in FHP 3L 4L or 5L. JD does this a lot depending on The application. Most mowers use belts in clutching applications which are usually FHP belts but some nonclutching application they use an industrial A B C belt. Then you have the turd manufacturers who purposely design machines to use odd size belts with no adjustment so you really need OEM belts or exact equivalent.
You give them too much credit
The original designs used standard size belts , then when the tip speed rules change or the decks get bigger etc they just change the parts then plug the details into the belt calculator so it spits out sizes like 217+ 25/32"
However +/- 3% makes next to no difference to most other than a manual PTO that requires belt to slip .
The problem with belt measuring tools is belt tension variations are doubled because you are reading 2 runs
When I took over the business I struggled with getting my head around belts
I spent hours measuring belts old & new all sorts of ways & finally went to the meter long steel rule and a white marker around the back.
Gates usually list belts by outside length & effective length ( ~ 1/3 the distance back to front )
some belts can not be fiddled with but most applications with a spring tensioner & electric PTOs going to a standard length will make no operational difference .
Then there is pulley wear particularly on the engine pulleys so after a few years of heavy use the pulley is worn smaller so the belt sits deeper thus a shorter belt is needed
Vari drives of course are a big exception they must be the exact same length but in most cases +/- an inch or so makes little difference
A touch loose and at worst the blades spin a little slower in very heavy cutting but has the benefit of slipping easier when the customer hits tree roots & other such obsticles .
Too tight & you risk snapping the belt or premature bearing failure.
In the USA it is probably not a problem but down here a lot of factory belts go better than $ 300
To satisify my curosity I have cut loads of factory belts in half to see the layout of the reinforcement cords & their sizes ( worn out ones of course ) and comparred these to the OEM spec belts I get from my suppliers and in most cases the belts are the same .
The reinforcement in the std size belts is usually thicker rope with fewer strands but in use it seems to make little to no difference and if the customer goes through a belt every 3 years or so the $ 100 cheaper is not that much of a saving but if they do 4 belts a year like the commercial customers do then that is a big difference
On average deck belts seem to have thicker reinforcement strands than drive belts do
So when ever I get a mower in that I have not worked on before one of the first things is to go to Gates & look up the belt
Then go to the wholesalers "belts by length" listing and see what is available around that size
A 1/4" on a belt under 100" will make absolutely no difference , in fact 1" will make no difference in anything over 50"
