I know your in the US, but Royal Distributing here offers an 8" billet big wheel kit for a fairly reasonable price.
http://www.royaldistributing.com/main/index.php?option=com_flippingbook&view=book&id=3&Itemid=419&firstPageNumber=142
(the link will not show the direct page, so enter 185 into the page search bar above the current page and it will take you to the information about the big wheel kit)

Larger idler wheels lessen the sharpness of the angle at the rear of the track (around the stock idler wheels) which in turn reduces the rolling resistance, allowing the track to spin around those rear idler wheels a lot easier. This frees up (does not add) 2-3HP as it is now easier for the motor to turn the track. I am unsure about disadvantages, but it is possible that the larger wheels will cause more wear on the front of the hifax due to that the larger wheels may raise the rear of the skid putting more pressure on the front of the rails.

 

I don't know for sure, but I don't think you want to run that large of a wheel on a 121, not sure about a 136 could work, but I ran a larger diameter wheel kit on mine, it was like 7" or so, I didn't notice much difference if any over the stock wheels.

 

I know your in the US, but Royal Distributing here offers an 8" billet big wheel kit for a fairly reasonable price.
http://www.royaldistributing.com/main/index.php?option=com_flippingbook&view=book&id=3&Itemid=419&firstPageNumber=142
(the link will not show the direct page, so enter 185 into the page search bar above the current page and it will take you to the information about the big wheel kit)

Larger idler wheels lessen the sharpness of the angle at the rear of the track (around the stock idler wheels) which in turn reduces the rolling resistance, allowing the track to spin around those rear idler wheels a lot easier. This frees up (does not add) 2-3HP as it is now easier for the motor to turn the track.

You are absolutely correct here.

I am unsure about disadvantages, but it is possible that the larger wheels will cause more wear on the front of the hifax due to that the larger wheels may raise the rear of the skid putting more pressure on the front of the rails.

This is the part you are missing.
You need the off set axle with the big wheel kit.
It raises and sets the axle inward to make it work right.
These are a definate MUST DO or you will break your big wheels when you hit bumps because they would hang down.

 

hiperf.com, dennis kirk and royale all sell junk, been there done that. tricked toys is the only way to go for about the same money.
if i had to do it all over again i would go with the 9" rear wheels and there offset axle.

its pure bling on a trail sled, dont expect to feel any difference in performance or see a gain in mpg. on a sled setup for racing and you are looking for a thousanths of a second or a hundreth of a mph they can help.

 

hmmmm, maybe it isen't worth it then... i cant see spending 2or $300 on looks alone.

 

I'm running the 8 inch plastics from avid, cheap and good wheels. I think Erick is running the same on his sleds. https://www.avid-products.com/store/store.cfm?DeptID=9&Grp=6

 

Yup, just looks. Tricked toys or Mountain machines both make great Billet wheels. But Tricked toys has much better customer service and you might see the wheels before the snows flies!

 

I know your in the US, but Royal Distributing here offers an 8" billet big wheel kit for a fairly reasonable price.
http://www.royaldistributing.com/main/index.php?option=com_flippingbook&view=book&id=3&Itemid=419&firstPageNumber=142
(the link will not show the direct page, so enter 185 into the page search bar above the current page and it will take you to the information about the big wheel kit)

Larger idler wheels lessen the sharpness of the angle at the rear of the track (around the stock idler wheels) which in turn reduces the rolling resistance, allowing the track to spin around those rear idler wheels a lot easier. This frees up (does not add) 2-3HP as it is now easier for the motor to turn the track.

You are absolutely correct here.

I am unsure about disadvantages, but it is possible that the larger wheels will cause more wear on the front of the hifax due to that the larger wheels may raise the rear of the skid putting more pressure on the front of the rails.

This is the part you are missing.
You need the off set axle with the big wheel kit.
It raises and sets the axle inward to make it work right.
These are a definate MUST DO or you will break your big wheels when you hit bumps because they would hang down.

Oh ok, I was unclear to as why exactly you needed an offset axle in the first place. Thanks for the info.