No go the other way! If your speedo is slow 7mph at 70, then divide the number of miles on the odo by 70, multiply by 7, then add the number of miles driven again, and you have the total miles driven. If anything should make the mpgs a little higher.

 

There two things associated with your tire swap that are affecting your mileage. One is real, the other is not. Both are based on your gauges no longer being accurate.

The first is the odometer. With the larger tires (295 vs 245), you have a fairly significant change in the diameter (33.25" vs 30.5") and a corresponding change in circumference (104.5" vs 95.6"). This will throw off your mileage reading. So for instance, with these numbers, if the ODO reads 250, you have really only gone 230 miles. This will give you a drop in your calculated mileage of about 1mpg.

The second is the speedo. Now the opposite error happens here than the ODO as you are going faster than the speedo says. At an indicated 70mph you are actually going 76.3mph. In my experiments with speed and mileage, this makes quite a difference - which may be more significant than the ODO error - as the Ram has the aerodynamics of a barn door and speed has a huge influence on mileage. This error may introduce up to 2 mpg drop. This might also explain the extra look the state police give you when you go whipping by!

So compensate for the ODO mileage by adding 30 miles to your calculations and slow down by 5 or 6 miles per hour.

Anything left will be your intake. Not sure there is much left though.


Good luck!

 

Ummmm, not quite - BOTH readings will be low. The 245s will turn 661 times/mile, whereas the 295s will turn only 606 times/mile. This will appear as lower distance due to fewer turns of the rear axle. Another effect: City mileage will suffer a bit due to reduced torque.