I wasn't aware that there were friction differences. Do cast iron sleeves entirely remove that effect, or is there much friction elsewhere?
There are far too many variables. We would have to take a specific case with well-known properties. How much weight will be lost, how much does the vehicle weigh as a whole, how does power production differ between the two engines, just how much friction loss are we talking about, what type of driving, what is the car's cD, etc.
Simply reducing the weight by 50 pounds or whatever you get out of it is really not going to be effective, except in specific cars that are sensitive to weight loss or in specific cases of aggressive 100% stop-and-go driving. There's a link in my sig to a discussion where people report which cars are and are not sensitive to weight loss. Throw 200 pounds of crap in a Buick Lesabre on a 3000 mile highway trip and you'll never measure a difference; throw the same 200 pounds in a 1992 Civic DX in 500 miles of downtown NYC driving and you'll measure a big difference.
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The Tracker had an aluminum block and aluminum head and the Durango has iron block and iron heads. They both end up at about the same temp by the time I hit the freeway in the mornings. I haven't noticed that one car cools faster than the other.
Even with an all aluminum engine you are looking at a 60-40 split of weight in a front wheel drive car so reducing the weight of that engine up front means a lot.
As for friction, everywhere the crank, pistons touch are the same as in an iron block engine. The cylinder sleeves and main bearings assure that.