If you used a mountain bike on the road and push it how fast could you go if you had a 21 speed highest gear?

Question

50t in frount and 13t wheel

Answer 1

Depending on the tires 22mph would be easy, and average over 18 if there is not too much climbing.

EDIT: Over 30 to 40 miles your average speed will most likely be in the 13 to 16 range. When riding the biggest gear is not always the fastest, find a gear that will keep your cadence high and you will go faster, and get some tires that are intended for road use as knobbies are speed killers.

Answer 2

Matthew H, if this guy can sustain that average speed on a MTB, he should immediately, get a $10,000 14lbs road bike and send his resume to every Pro-Tour team! he'll also become the most tested athlete, for banned substances! I don't question the math, it's just not plausible for a non athlete to sustain that cadence at that gear ratio, over varied terrain, especially over the Col du Tourmalet!

Answer 3

well, speed is based on RPMs and gear inches.

Take your ratio of front teeth to rear teeth (50/13 = 3.85) and multiply that by the circumference of your rear wheel.  

a 26 inch wheel (diameter) will have a circumference of appx 81 inches.

So, in your hardest gear you will travel inches for every turn of the cranks.

now, lets say you can pedal at 80RPM, we'll add it all up

80RPM * 81inches (circumference) * 3.85 (gear ratio)  / 12 (feet per inch) * 60 (minutes per hour) / 5280 (feet per mile) = 23.625 MPH