You're probably right. Looking at this elevation map of Kansas it is super flat with a steady decline from west to east with no major sudden elevation changes. A bit over 8 feet per mile averaged out. Heading north to south or vice versa is going to be substantially less from the looks of it.
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Lots of boring drives in the midwest, but apparently there are like 20 other states with lesser elevation changes than Kansas. Florida is the lowest having only like 350ft between it's highest and lowest point, with Kansas being around 3,400ft. Not that it equates to how "flat" the land is. Also, just for extra boring to add to this post; Colorado's lowest point is higher than about 20 other states' highest points.