Citation needed.
According to Wikipedia, and the UNESCO Institute for Statistics, the only country in the entire world with a literacy rate (for people over 15 years of age) below 30% - and therefore an illiteracy rate over 70% - is Niger.
Every other country - including the countries with the largest populations of Muslims - has a higher than 30% literacy rate; Indonesia, the world's largest Islamic nation, has a literacy rate of 93.9%. The lowest literacy rate outside Africa is Afghanistan, where illiteracy affects only 61.8% of the population (and only 48% of the male population).
Some other majority Islamic nations, and their overall literacy rates (that is, including both male and female literacy) are:
Saudi Arabia - 94.7
Iran - 86.8
Iraq - 79.7
Syria - 86.4
Pakistan - 58.7
Bangladesh - 61.5
Libya - 91.0
Egypt - 75.2
So basically, the UNESCO data appear to contradict your assertion, which it seems was sourced intra-rectally. Even in Bangladesh, more than twice as many of the population are literate as you claim for the average of all Muslims worldwide. I wonder whether you invented this stupidly easy to refute bullshit, or whether you just swallowed it whole when someone passed it on to you. Either way, you should know better than to try to pass this kind of readily checked 'information' off as fact on Talk Freethought.
When making up statistics, you need to ensure that they are at least close enough to reality to be difficult to refute.