18" wheels on a SRW might be better for heavy towing

[Les-in-AK]

I towed a travel trailer weighing about 9000 lbs from Spokane to Anchorage. With my wheel size of 20" I found the shift down from 6th to 5th a bit early as the engine felt like it had plenty left.

This got me to thinking of going back to 18" wheels for the ratio improvement with heavy towing. Either having a second set for heavy towing or swap somebody with 18's on their new truck who wants the larger wheels.

Does anyone know on the 2011's F350 if the speedo can be changed program wise to reflect an 18" tire. My local dealer is not yet familiar with the truck, no stock or training yet.

I need to find out if this speedo change to tire size is doable before looking into 18" wheels purchase/swap.

The other incentive for 18" wheels is I am thinking of selling the travel trailer and going to a 5th wheel trialer. The 20" wheels look great but elevate the truck height to the point a new 5th will have to be raised 2 - 3 inches for bed clearance according to measurements taken of the truck and the 5th by the RV dealer. These trucks are tall!

[xjeeper]

Tire diameter, not wheel diameter is what you need to focus on, also , a height reduction of 2"-3" inches at the bed rail would require a tire height reduction of 4"-6" inches. Shorter blocks above the rear axle to lower the truck, and air bags to bring it back to level under load would be a better solution.

[53 willy]

going from the factory 20s with 65 series tires to factory 18s with 75 series tires prob wont reduce the diametere more than an inch if that, wont do much for the gearing or the bed height either one.

[S&W 460 XVR]

Correct me if I'm wrong but I think both are 34" tall!

[Les-in-AK]

Found the data on a tire site.

Looks like about 2.78% circumference difference between the 20" and 18" wheel/tire combo's used by Ford on this truck. Rev's per mile: 627.4/610.5 = 1.0276 factor or 2.76% difference.

Height difference:
Radius 275/65R20 @ 17.03" - 275/70R18 @ 16.57" = .46" difference

[ErnieT]

Answer to your question is yes. I asked my dealer if they can recalibrate my speedo after my lift kit and wheel package and they said yes. Its $150 which I thought was reasonable.

[hammett88]

unless it feels like it's lugging too hard.. i wouldn't worry.. if so.. keeps those nice rims and fix it with a programmer when they come out