Tire Diameter, RPM, Speed

[Jack Olsen]

I've got a magnesium 8:31 915/60 transaxle with close-ratio gears. It was pretty easy to input my particular gear sets to be able to see what actual speeds I'm doing at particular RPMs. This is the chart for a set of tires I used to run:



But I'm currently running 275/40x17 rear tires, which will change those numbers because the diameter of the tire is different (and unfortunately, I've lost the Excel spreadsheet that gave me the above graphic. But using the Instant-G spreadsheet, I can still plug in my gears and see numbers for the new tires.

The problem is, I can compare these numbers to speed and RPM data from my car's data logger, and it doesn't quite correspond. In order to get the actual numbers, I have to reduce my tire's diameter from 25.86" to 25.01", which would be a tire sized 275/37x17. At that point, all of my gears and RPMs correspond to actual GPS speeds.

Can anyone tell me why that is?

Here's a snapshot of the gearing when it works correctly (although the RPM and speed numbers are not visible in this screen grab).

[Norm K]

Originally Posted by Jack Olsen

Can anyone tell me why that is?

Since my math doesn't match yours, I cannot tell you why that is. The theoretical heights I get when running the sizes you have listed are 25.66 and 25.34 respectively, a difference of .32", or just under 1.25%. Assuming my "theoretical" calculations are correct; 275 X .40 (or .386) / 12.7 + 17, then perhaps tread-wear accounts, at least in part, for the difference in the readings you're seeing.

[Bill Verburg]

Originally Posted by Jack Olsen

I've got a magnesium 8:31 915/60 transaxle with close-ratio gears. It was pretty easy to input my particular gear sets to be able to see what actual speeds I'm doing at particular RPMs. This is the chart for a set of tires I used to run:



But I'm currently running 275/40x17 rear tires, which will change those numbers because the diameter of the tire is different (and unfortunately, I've lost the Excel spreadsheet that gave me the above graphic. But using the Instant-G spreadsheet, I can still plug in my gears and see numbers for the new tires.

The problem is, I can compare these numbers to speed and RPM data from my car's data logger, and it doesn't quite correspond. In order to get the actual numbers, I have to reduce my tire's diameter from 25.86" to 25.01", which would be a tire sized 275/37x17. At that point, all of my gears and RPMs correspond to actual GPS speeds.

Can anyone tell me why that is?

Here's a snapshot of the gearing when it works correctly (although the RPM and speed numbers are not visible in this screen grab).

tires are round until they are mounted on the car and the car is on the ground, then they flatten on the bottom, the amount of flattening depends on the wheel width, tire pressure, car weight and the construction of the tire and to throw a lasy wrench in the works varies dynamically too.

Additionally tires which may share the same nominal size will have idiosyncratic actual measures.

When I do a speed calculation I try to use a measured roll out of the specific tire involved. If that is not available the manufacturer's revs/ mi is the next best source to start w/.

lastly there is a +/- in even the best data acquisition, I notice discrepancies all the time, all you can do is try to know where they are coming from and make according adjustments to your analysis.

[Jack Olsen]

Thanks.

I measured my actual tire diameter, and it's 24.25". That puts the data pretty close to the Excel numbers.

[moneymanager]

Jack, Here's a useful source of wheel size/gear ratio/speed wisdom. Gear Ratio Calculator - Porsche 993 Turbo
Data entered is 930 related I think but you can put in anything you want. Jim Taylor

[Paulporsche]

Originally Posted by Jack Olsen

Thanks.

I measured my actual tire diameter, and it's 24.25". That puts the data pretty close to the Excel numbers.

Goes to show how manufacturers' sizing are not exact.

[Quicksilver]

Originally Posted by Jack Olsen

Thanks.

I measured my actual tire diameter, and it's 24.25". That puts the data pretty close to the Excel numbers.

I think that you need to look just at the radius of the tire to the bottom where it is distorted. The top isn't distorted so it is skewing the diameter number.


I bet a physics wiz could take the amount of distortion and use it to calculate rolling resistance and heating while rolling straight ahead. Never would have guessed the number would be that high.

[javadog]

Once upon a time I was the tire guy for a team running a 935 at the Daytona 24 hour race. I was responsible for taking the wheels as they came off the car at each pit stop and getting new rubber on them and ready to go. Even though we used the exact same tires each time, you couldn't just assume they were all the same size. Because a 935 runs a solid spool in place of the differential, both rear tires had to have exactly the same circumference. Any difference from one side to the other would cause the car to pull to the side of the smaller tire. So, each tire got measured with a steel tape and I played with the pressures until I got them the same size. When they went on the car, the pressures had to be the same, so this required some fiddling around. Race tires are different from street tires, but this illustrates the point that not all tires are the same. And, it's real common for the theoretical tire sizes on the sidewalls to be off. You gotta measure them, if you want real data.

JR

[Jack Olsen]

Thanks. The reason I'm interested is that I know I'm getting close to the rev limiter at two points on my favorite track, and I'd like to know where it's going to kick in. The easy way to figure this would be to turn on the data logger and hit the rev limiter in 5th on a lonely stretch of highway. But I've been told that engaging the rev limiter on a 911 is pretty hard on the motor. If I'm too close, I'll look into moving up to larger-diamter tires. But if that's not necessary, I'd like to avoid it.

Based on the data I've collected from the track, it looks as though I'll be able to get to 135 on the current set of tires. I think that will be all right, since it's 3 mph more than I'm hitting now.

[wholberg]

In addition to what has already been mentioned, downforce or lifting at speed will change your tire diameter slightly from a static measurement. However, the greatest error might be your GPS. Civilian devices have a resolution of about 7.8 meters (Welcome to GPS.gov) worst case, with about 3 meters being average. 100 mph is about 45 meters/second, and it takes about 0.6 seconds for the signal to travel from the satellite to the device. As you can see, this is a huge error. Military GPS units work on 2 frequencies so they can perform an ionosphere correction (whatever that is) to get better accuracy.

GPS devices compensate for this error by averaging satellite signals, typically 1/second for 10 seconds. If you travel at a set speed for 15-20 seconds on perfectly flat ground you will get a very accurate measure of speed within about 0.5 mph If you are accelerating down a straightaway, the GPS reading is always going to be lagging behind your actual speed because it is averaging the speed you were going 5-10 seconds ago and the signal from the satellite more than a half-second old.

In addition, how accurate is your tachometer? Is it digital and has it been calibrated? Accuracy of +/- 2% means 6000 +/- 120 rpm.

If you propagate the possible error through the tach, gearbox, GPS, etc., I think that the potential error will be much greater than the difference you are seeing from theoretical value.