Close enough for government work

So, my new-to-me/old little Smart Car has what i can only guess is a very European design, and as a consequence is always surprising me with odd features (you can only remove the key when the car is in reverse) and oddly placed control buttons (the dome light control is a rocker switch next to the cigarette lighter). The latest quirk revealed itself as the weather got cooler here in B'ham: whenever the temperature drops to within a few degrees 0f freezing, the odometer display automatically transforms for sixty seconds into a gauge showing the exterior temperature. Viz:

When it first changes, there's even a little snowflake icon!  All that to say "Hey! It might get a little slick out there... be careful!"

Of course, since the car was never intended for import into the U.S., the temperature is only displayed in Centigrade - there's no way to toggle it to Fahrenheit. When I first pointed this feature out to Coco, she asked how to do the degree conversion. I gave her my simple formula: double the degrees and add thirty.

Yes. I know, that's not exactly it. We all learned (C * 9/5) + 32 = F  in grade school. But really - who can or would multiple by nine and then divide by five (or multiply by 1.8) and then add 32 in their head? Doubling comes easy - we do that more or less naturally all the time. Adding a round number is easier - you only ever have to deal with the tens column. The formula is easier to remember, and it's close enough.

Or is it close? I always figured it was, but I had never really checked it out.

Well, now I have: I made a spreadsheet. I'll spare you the table and just show you the chart:

At typical temperatures, the simplified method ranges from 13% under to 5% over the more formally calculated Fahrenheit equivalent of a Centigrade temperature. At the extremes: -5º C should be 23º F but shorthands to 20º, and 36º C calculates to 97º F, but the approximation works out to102º.

That's as far off as you'll ever be: between the freezing point of 0º C/32º F and a balmy 22º C/72º F, the easy approximation is two degrees or less from the complicated calculation. And between 46º and 54º F, the two calculations are within a half a degree.

I'm not saying you should translate recipes or chemical formulae using this method, but when you hear the weather forecast from a Canadian station - or drive a car that will not relinquish its European identity - it's quite good enough.