Hi folks, I am new to this forum and recently bought my first deisel. Hopefully I am putting this post in the right place.
Can anyone explain why putting a larger diameter tire on a CTD equipted Ram, or any other truck for that matter, results in lower MPGs. I just finished reviewing the CTD fuel milage thread and it seems everyone who installed larger tires lost fuel milage. I have an 06 CTD Ram 2500, auto, 373's with the 265 tires on it. Because I do a lot of off road traveling , often in those deep ruts I want to go to a 35 inch dia tire to get those differentials a couple of inches higher off the ground.
A novice like me thinks 10% larger dia tire equals 10% lower RPM should equal better milage not worse. Surely the extra weight of the tires can not be the reason for worse milage. Not on the mighty CTD. Is it rolling resistance? Any objective facts on this would be great. Opinions and general comments are also welcome.

Thanks, Ridgeman

 

I think it is because of the added weight of the tire plus with more surface area on the road the friction working against the truck is greater. It isn't a sign of the engine being weak. REally with any rig when you add more weight the engine has to work a slight bit harder resulting in a mpg loss. For example when towing most people i talk to get around 15-18 mpg with a heavy load but get 20-23 when they are just cruising along