I've spoken about this effect before on this forum, but I never really worked up numbers. This time, I have.
Specifically, I've always wondered what the climate would be like in northern Delaware if there was no tilt. That is, if the Sun shone directly over the equator every day of the year. For starters, there would be 12 hours of daylight and 12 of night -- from pole to pole. A 6:00 AM sunrise and a 6:00 PM sunset I could live with. Perhaps we could adjust it to 6:30/6:30.
When considering the average daily high and low in a world where the Sun is over the equator, a good place to start is with our Spring and Autumn equinoxes. Figuring in seasonal lag, it is best to look at the average temperature one month after each equinox. This is where it really got fun! The average high/low for April 20 is 65F/44F. For October 20, 65F/45F. An amazing bullseye match!
Another approach to the question is to simply average out the hottest and coldest times of the year. You should, theoretically, get similar figures. And you do. I took July 20's & January 20's average high/lows, 86F/68F and 40F/24F and got 63F/46F.
So, it would seem that Wilmington, Delaware would "enjoy" daytime highs in the 58F to 70F range and nightly lows between 39F and 51F.
On the one hand, we might need to use our furnaces periodically all year round, but we could throw away our air-conditioners for good!
Many other questions come to mind. What kinds of crops could this region grow in such a climate? What about all the other plants and trees in our landscape that have evolved to deal with our hot summers and cold winters? And how would plants grow here with equal light and darkness rather than the 9 hours of light in December / 15 hours of light in June? Lot of fun "what ifs."
I'd be interested to see some of the other members here work up the figures like I did for their own locations.