I wouldn't worry about it for picking your rims. Top speed is the last thing you need to think about when selecting wheels for a 60. Your fuel mileage is going to be affected more by tire size and gearing. It is more a matter of what kind of durability you need in your wheels. If you plan on wheeling a lot, then stick with some steel wheels.
A heavy wheel/tire combo will affect how fast the vehicle will accellerate/brake. I installed a set of 36" Swamper TSL's with inflatable beadlocks on steel wheels on my FJ40. The brakes had been "enough" for my previous tire combo's were now "not enough" to stop the heavy tires from highway speeds. "Not enough" in this case being as fast as I wanted to stop. Sure the brakes stopped the truck but the flywheel effect of the heavier wheel/tire combo made stoping distances longer.
Adding weight in general means more fuel to get it all moving regardless.
Heavy steel rims will take a toll on available engine horse power. But to calculate that you'd have to do some calculus and related rates problems. I'd be willing to bet money that you could tell the difference between a 15x8 steel and a 15x8 aluminum rim.
The effect on gas mileage wouldn't be as noticeable.
I know that in professional cycling, mere grams shaved from wheels is always preferred to saving the same amount or even more weight from the frame. So in a theoretical sense you might have a case. In real terms, I'm gonna go with what the others have already stated.
However; using the bicycle example again, I think tire width has more to do with efficiency than wheel weight. I run very narrow tires (255/85/R16) on steel wheels and get pretty good fuel economy (13 MPG in all conditions). Matching the correct gearing to your tire size, running a narrower tire, and having a properly tuned engine is probably your best place to start for both top speed and fuel economy.
As noted the wheel & tire's weight will affect trying to change the speed (up or down) of the vehicle more than it will affect the vehicle when driven at a steady pace.
I went from 15x8 steel "Rockcrawlers" to 15x7 alloy wheels (same tire size & brand) on my '84 Mini and the weight per wheel was very noticeable both when moving the wheels around and in driving the truck.
The Rule of Thumb in road racing is that one pound shed off the unsprung suspension & wheel/tire combo is the same as ten pounds off the rest of the car.
You probably won't notice the difference in wheel weight going slow on dirt trails and roads. You will notice the difference if you log a lot of highway miles. Those rockcrawlers are now on a J-10 driven by a 13 year old because he won't be able to hurt them for a while....
I went from a cheapo 15x8 steel wheel to a 16x8 steel Stockton Explorer 72 and from 31x10.5 BFG ATs (measured 30.4") to Nitto Terra Grapplers in a 265/75R16 (measure 31.5"). The Stockton/Nitto combo is significantly heavier and I noticed an impact on MPG (-1 MPG) and acceleration (MPG corrected for tire size difference). Don't know if this is strictly the larger tire size or if it is also the weight.
That was great information. I am quite clear now about the issue.
Here i would also like the comments and expert opinion of everyone about my exact problem.
When i purchased my land cruiser, it had its original stock 16" 5.5" wide steel rims which were very heavy and had very small tyres something like 215 65 R 16. So i changed them and purchased brand new tyres 31 X 10.50 R 15, and kabli alloy rims of 15" 8.5" wide. Remember, the weight of my cruiser is around 2500 KGs.
The result was as follows
1. Nice looking overall
2. Light weight, giving good speed, torque, comfort and less wear and tear of the suspension
1. Not giving a stock look, remember my cruiser is 100% stock in original form.
2. Might be the rims are not as strong as the steel ones were (remember 2500 KGs)
3. The tyres are being stretched and since the rims are 8.5" wide so there might be some hardening of steering.
4. The tyres throw mud on doors as they are a bit out from the body.
Now keeping in view the above, please advise me as
1. Should i keep the existing set.
2. Should i go for bigger tyres with the original 16" steel rims.
3. Should i just change the rims only and buy alloy rims of about 7" or 7.5" wide with the existing tyres.
Remember, beauty is not at all important. Also see the below pics.