Q I have a 1986 Chevrolet Camaro that I want to take to the dragstrip occasionally. I’d like to put drag radials on the car. The original tyre size is 215/65R15, and the drag radials are 235/60R15. A little maths reveals that the drag tyres will rotate two fewer revolutions in a kilometre (480) than the stock tyres (482). I’d like to know how much this will affect my speedometer.
A You’ve picked just about the perfect car for this kind of thing. Camaros and Mustangs were practically built for the strip, and aftermarket parts are so plentiful you can almost build the car from scratch. Your choice to go with drag radials for racing is a wise one; these tyres have much better grip than standard tyres, owing to a softer rubber compound. Great for racing, bad for daily driving.
The drag radials are slightly larger in diameter, which will have a tiny effect on the accuracy of your speedometer, and you’ve already done the hard calculations to figure out how much. The revolutions per kilometre of the drag tyres are a percentage of the original tyres’ and thus will result in a speedometer reading lower by the same percentage. How much? Divide the stocks’ revs per kilometre (480) by those of the drag radials (482). That gets you to 1,0042, the amount to multiply to figure out the true speed.
For example, with the drag radials installed and the speedo reading 120 km/h, multiply by 1,0042 to get 120,504 km/h. The error at 120 km/h is a scant 0,504 km/h. Here’s the catch to all of this: your speedometer isn’t accurate anyway. Unless you’re driving an ex-police-spec car with a calibrated speedometer, your factory speedo is off by as much as 5 per cent even when new. That’s much more than the difference your new tyres will make. The takeaway? Just run it at the strip for fun and don’t worry about the speedometer’s accuracy; that’s what the timing lights are for!
GLOSSARY: ETHYLENE GLYCOL
The main chemical in auto antifreeze. Its freezing point is only about 10 degrees lower than that of water, but when the two are mixed 50/50, they stay fluid to minus 30. Antifreeze contains anticorrosion additives. Check your owner’s manual to find which brand plays nice with your engine.