While replacing my tensioner, I drained the radiator, heater hoses, and whatever came out with the tensioner and cooling elbow. Couldn't have been more than 1½ gallons.

When refilling the radiator, I poured in a gallon of 100% coolant, and then grabbed a jug of water, but the system was already full. The level didn't drop much when the thermostat opened, either. Very strange, but it'* cooling just as well as it was before, so I won't worry.

I'll assume the block and heater core were still full of my old 50% strength mix, so if I lost half my coolant during the repair, I guess I'm running on about a 75% strength mixture right now. That won't cause any problems, will it?

the water does your cooling really so if anything you should err on the other side. there is a cheap tool that tests your protection level, if you drain a little and refill with water, then test, and repeat until you get around -10 to -30, should be good. you should drive it for a while to make sure everything is mixed up.

Water is a better coolant then coolant. Coolant is mainly a antiboil and antifreeze. So being strong will help prevent it from freezing. You may notice a bit more heat in the summer though.

Keep a 50% solution in the system regardless of the ambient temperature.

Antifreeze contains inhibitors to prevent electrolytic erosion, important for any engine with aluminium parts such as head, water pump, manifold or radiator. These break down over time which is why you should flush and refill every 3 years - time rather than mileage is the factor here.

A 75% solution might lead to overheating in the summer months but you shouldn't have any problems at this time of year.

Roger.