StealthCivic

Ok, my tire was flat last night, something on the interstate...don't know what it was , well I changed it and put on my little dinkie 14" spare...now, note that I have 15" wheels now, I switched to 15's when I first got the car, so I still have the 14" spare...put it on, my car was even leanin a little it was so small compared to the rest...I drove it and got the tire plugged and patched so everything is cool. The deal is my friend is a honda mechanic , he says it is very bad for the transmission to be driving with a 14" tire with 15" tires on the car. Mainly in the front as it is front wheel drive...He says I should get a 15" spare. I would like to - as much as I get flat tires around here! Anyone have any thoughts on this, or have a 15" spare they want to part with.
Thanks.