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The only assumption I'm going to make here is that 2600rpm represents a governed limit and you can set the engine on that speed "forever".
A lot of discussion has been posted here, but no one has ever broached the subject of Math. Here's what you need:
* Overall wheel diameter (from the top of the tread to the bottom)
* Maximum RPM
* Desired highway speed
* Rear end gear ratio (can be derived)
The formula for figuring speed based on RPM and rear end ratio:
MPH = [(pi)x(Tire diameter in inches)x(60)x(RPM)] Divided by [(12)x(5280)x(rear end ratio)]
Let's say you're stuck at 55 MPH, engine is limited to 2600RPM and your wheels are 28" in diameter. Solving for Gear ratio would show that you have (roughly) a 3.94:1 rear end ratio. Let's say you want it to cruise at 65MPH with the same set up. This would require the rear end ratio to change to (roughly) 3.34:1. Solving for tire size and keeping the 3.94 ratio would mean you need tires of 33" overall diameter to make 65MPH at 2600 RPM. I don't want to bore you further with Algebra; I can hear eyeballs glazing already.
This is a good estimate: a lot of the ratios you figure will not be available commercially, so you'll have to make due with an approximation. I also used 3.146 as a value for pi (easier math) because I am making an estimate.
Torque is the ability to do work. Horsepower is work per unit time. More horsepower, more work in less time. Here's the second half: Higher ratios (lower numerically) mean less mechanical advantage. This means you need *MORE* horsepower to do the *SAME* amount of work in the *SAME* amount of time. Conversely, the *SAME* amount of horsepower with less mechanical advantage will perform *LESS* work in the *SAME* amount of time. This means you *COULD* travel at 65MPH, but the motor won't move you that fast due to the load increase from giving up mechanical advantage. How much horsepower you need is based on factors such as weight, rolling friction and drag (wind resistance). Needless to say, if eyes glazed during ratios, they'll fall out if I go into horsepower. Suffice to say, if you gear correctly and you don't get to speed, you need to increase your motor's power output.
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