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Ok I have time now. How do you figure out what your speed is when you only have engine RPM? Well, we go back to high school. Welcome back to either math or science class. This will be something you probably learned and promptly forgot about: Dimensional Analysis
Dimensional Analysis really is the key to like 90% of engineering math problems. You figure out what your units are to start, and then what you want to have for units, and then you step your way through canceling the units out.
In this case, we're starting with Vehicle Speed in miles per hour, and we want to find out engine speed in revolutions per minute.
First step, we have to convert vehicle speed into tire speed. To do that we need the rollout of the tire, or the circumference. On nascar tires that's around 87 inches. But since our vehicle speed is in miles, we need to convert the vehicle speed to inches per hour
Step your way through this. 1 Mile to 5280 feet, 1 foot to 12 inches. Break it down. Now you can see that our vehicle speed unit is inches per hour.
So now we convert that inches per hour into tire revolutions per hour by dividing by the rollout of the tire. We can also add in another unit conversion here to convert 1 hour into 60 minutes. Now we have tire revolutions per minute.
This is where most of your street cars that have wheel speed sensors stop. We know how many tire revolutions make what vehicle speed. We're going to calculate this out at 45 mph for ease of use from here.
Now is when we get into the gear train. To work our way to engine speed. Tire speed is the same as axle speed. It is reduced from driveshaft speed by the rear gear. The driveshaft speed is reduced by the transmission gear ratio.
This can all be added into this by again just looking at your units.
So let's say we are going to run pit road in second gear. At this particular track let's say we have a 1.80 second gear and a 3.70 rear gear. So 1.8 engine revolutions will make 1 driveshaft revolution, and 3.7 driveshaft revolutions will make one axle/ tire revolution.
So for what we calculated before at 45 mph, we need 546 tire revolutions per minute. To get 546 tire revolutions, we now know we need an engine speed of 3636 rpm. That's how vehicle speed can be had from engine rpm.
This applies to pretty much any car at any speed, too. Older vehicles had the vehicle speed sensor off the transmission, so changing the final drive or changing the tire size will change the vehicle's mechanical speed calculations.
This also means that knowing the parts that are on your car means you can pretty easily figure out your vehicle speed using only engine speed. My truck used to be like this before I fixed the speedometer, where 70 mph was about 2000 rpm in 4th gear.
Hopefully this all made sense. I'm an engineer not a teacher or a scribe. Normally I'd just punch this all into Excell and it spits the numbers out at the end
I'll also put that this is how a lot of race series set their pit speed limiters. It's just an rpm cap that's manually set in the ecu and not an actual speed sensor
