Driving in Belize

Not that many rental companies in Mexico are happy to rent if they know you want to take it inti Belize. and you will have to pay double insurance one for each country should be doable and hopefully some one will come here to tell you how to do it but wether it would work out cheaper in the long run I dont know. Crystal is a local owned rental company they offer discounts for booking on line and give you free loan of a local phone (you add top up minutes at a local grocery store) Also loan you a cool box very useful. Their charges reduce for longer rentals. Also keeping it local is good.
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Yes, rental vehicles in Belize are expensive.  Not all MX rental agencies allow their vehicles to be brought into Belize so you have to have a letter and authorization from them prior to arrival.  MX auto insurance is not valid in Belize so you will be required to purchase auto insurance from a BZ company.  Not having done the comparison of lower rental rates but adding BZ insurance vs renting a BZ vehicle  where insurance is already in place don't know how that compares.  No matter what, gasoline is still going to be expensive.