Tires: what saves more, ford 250, fuel economy

Question
what saves more fuel on a diesel either a 275/65R20 or a 35*12.50R17 my truck is a 08 ford 250

Answer
Raul,

Tires within a given size can vary widely for rolling resistance - up to 60%.  So there is no definitive answer to your question.

- BUT -

Flotation type tires - the ones whose sizing looks like this:  35X12.50R17LT - are made of materials that are like LT metric type tires - the ones whose sizing looks like this:  LT245/75R16 - and those are well know for not having good RR values - as compared to Metric and P metric type tires - the ones that do NOT have the letters "LT" in the size, like the 275/65R20 you mentioned.

The 275/65R20 is more likely to give better fuel economy than a 35X12.50r17LT, but it is far from a sure type of thing.