And won't you take a gas mileage hit just because the tires are new?
Yes, new tires have a higher rolling resistance than worn tires, because of the increased amount of tread. I've estimated that the difference would be bout 5% on vehicle fuel usage.
Would I be right in thinking that all terrain tyres would generally fall into the grades with higher rolling resistance. I wonder how the OP determined that one tyre had a higher rolling resistance than the other. Even with no grading system yet in place, figures for the two tyres coefficient of rolling resistance in Kg per tonne could be translated in an effect on fuel economy.
Yes, for 2 reasons: Increased mass of tread rubber and the tread rubber composition. Tires designed for any sort of off-road usage will use rubber compounds that will be worse for RR.
I too wonder where the OP got the RRC values.
So allow me to explain the problem with translating RRC into actual fuel economy: Rolling resistance is distance dependent. To calculate the effect RRC has on fuel economy, it is proportional to the distance. But for steady-state driving the effect the car has tends to dominate things.
HOWEVER, there is a small speed effect - faster is worse. That's due to the additional distortion speed does to a tire.
But the effect the weight of a tire has on fuel economy is related to acceleration - frequent speed changes increases fuel consumption. In other words, weight has more effect on fuel economy in city driving, and less in country driving. But the weight of tires is relatively small compared to the weight of the vehicle, so the effect of tire weight gets overwhelmed.
So it is difficult to accurately estimate what change in RR has on actual fuel economy. Yes, better RR will result in better fuel economy, but how much? In my opinion, removing stuff from the car would have more effect.
