Due to an argument on another forum, I want you fine people's input. I say a generic calculation isn't 100% accurate, it comes down to, two big variables. Number 1. Advertised tire height vs actual tire height. A few of us have found BFG Radial TA's advertised height taller than the actual height. The 245/60R14, I measured were a 1/2" short. Fun71 found one 1.85" short. That alone makes rpm nearly 100 rpm higher than calculated with the advertised height. I may have found my BFG 225/70R14 current tires short as well, I can't remember. Between shortage in height and brown letters, I can see why people are going with different tires. Who knows if other manufacturers aren't the same and I bet any out of spec, will be short on height, not taller, if they are wrong. Companies spending 5 cents less rubber on every tire makes the Shareholder's happy. Number 2 is slippage, it isn't 5% on every car made, it varies. The first article I found mentioned 3 to 5 percent as the norm but as high as 8% on a mild Street Converter. Step into a cheapy 4000 stall, is it going to be 5%? I think not. Are tachometers 100% accurate, probably not but how far off are they? On GM factory tachs to Autometer, I have seen a 250 rpm drop for most factory 1500 to 1600 flash stall lock up torque converters and around 300 rpm for mild, cheap mid 2000 flash stall converters, locked up. My tachometer may be not 100% accurate, is it that far off? What is the allowed variance? I don't 100% rely on the speedometer for speed, I always check with GPS, it eliminates one more variable, Speedometer inaccuracy. Even the calculator I use says the number calculated varies 100 to 200 rpm. I can hit that number bang on with my converter locked up, otherwise it is 250 to 300 rpm lower.