Hi, I have a 91 Dodge D250 Cummins. Can I use LT215/85R/16? I currently have LT235's, but I thought maybe might get better diesel mileage with the 215's. I'm strictly on highway miles. Safety is also important. Any input is welcomed!!! Thanks.
-Jeff[

 

As I recall a 215 is a bit shorter in height than a 235. Shorter means speedo could read 75, but you are really going 70. That also means less fuel economy.

 

What rear end ratio do you have, sometime a larger tire diameter will give you better fuel millage and put the engine in a better rpm range. I had a d250 with 265-75/r16 tires and got great fuel economy. that was with a 3.73 rear end. you could also go with something tall and thin liek a 255/85/r16 but tire choices will be limited.

 

What is the aspect ratio of your 235s? 75 or 85? What is your gear ratio? What speed do you travel on the highway?

--Eric

 

http://www.miata.net/garage/tirecalc.html here you go try this