I would like to have a clarification on this: I know that 倍 means "-times, -fold", but I don't understand why 一倍 seems to mean "twice as (fast)" in the following sentence:
开车最少比骑自行车快一倍。
while we have to use 两倍 in the following sentences:
8是4的两倍。 X贵两倍。
I don't know how to tag this question so feel free to change it.
If some words that mean "is" (e.g 是/相当于 ) are used, then it's literal times.
你的速度 是 我的两倍。
Your speed is twice as mine.
Normally you will not say "one times" in this scenario. 你的速度是我的一倍 (Your speed is one times as mine) sounds strange in both English and Chinese. You can just say "你的速度和我的一样快"/ “我们俩一样快" (You are as fast as me).
If other "comparing" words that means "more than"(e.g. A 比 B 大/高/快/etc.) show, then it's literal times + 1. Followed one has the same meaning of previous one.
你的速度 比 我快一倍。
You are one time faster than me.
一倍
Tuttle's Learners
fold, time
这个学校的学生人数比我们学校多一倍。(=这个学校的学生人数是我们学校的两倍。)
Zhège xuéxiào de xuésheng rénshù bǐ wǒmen xuéxiào duō yí bèi. (= Zhège xuéxiào de xuésheng rénshù shì wǒmen xuéxiào de liǎng bèi.) The student number of this school is twice as big as that of our school. (—> The student population of this school is twice of ours.)
多一倍 is translated, first in Chinese as 两倍 so it's easy to see how confusing this phrasing is! But....
两倍
Oxford
② [indicating difference in quantity]
这一带水稻产量比小麦高两倍
The output of rice is three times that of wheat in this area
两倍 has become three times here(!).
but(!) - still from Oxford:
你的工资相当于我的两倍。
Your salary is twice that of mine.
Basically you've really got to look at the context to figure out the meaning!
Your sentence above
开车最少比骑自行车快一倍。 the 快 here has the same effect as 多 so it's really saying it's "one time as fast" which is really "twice as fast"...
Hope that makes sense
Understand that you have an accpeted answer already and it's correct. I am adding this in hope to make it simple to understand:
一倍 : 100%
两倍 : 200%
They are the same. So when you are saying 快一倍, it means 100% faster, or twice the speed, which means 200% of the origin speed - 两倍速度. The confusion comes in because when 一倍 is used, we ADD the value by itself, while 两倍 we MULTIPLE the value by 2. And we all know that A + A (一倍) = 2A (两倍)
I think the more correct way to use 一倍/兩倍 is answered by user3306356 and Caff Huang.
But, in most contexts, 快一倍 and 快兩倍 have just same meaning. Because if we use 快兩倍 as "x * 3", the meaning will be too vague to understand.
If we really have to say something larger than two times of a value, such as "A = B * 3", a more practical way will be "A是B的3倍" instead of "A比B高兩倍".
