Just realized I was sold a 3.0-18, or a 90-90-18 tube. Is this too small for the rear tire?

 

You can even use the front one in the rear tire when you're stuck.
It should work no problem.

 

Thanks! I've heard that before.

Although I'm sure this tube will work, are there any potentially negative downfalls to running it?

This is my first tire swap so I want to make sure I get it done right.

Thanks

 

Get the right sized tube. A 3.00-18 tube is going to be a bit thin stretched to 4.60-18 tire.

Using a front tube in the back tire is an emergency measure.

 

Take it back, unless you did it on-line.

Fact is it will stretch and odds are it won't be any big deal considering some tubes range over an inch in fit according to labeling, going from 4.00"-5.10". Like this IRC tube on ebay. A puncture will be a puncture regardless.