RPM Calculator, actual Tire diameter question

-
rustytoolss

rustytoolss

Well-Known Member
Joined
Sep 26, 2013
Messages
972
Reaction score
73
Location
Clinton, Ohio
When using a RPM calculator input 3.23 gear/ tire size 225/70/15 =27.4" at 60 mph I get an RPM of 2377.
But If you measure the tire with the weight of the car on it. The Diameter changes to about 25.5" (thats doubling the radius of the section of the tire that has the weight load).....
So 3.23 gear/tire size 225/70/15 with weight load=25.5" at 60mph I get an RPM of 2554.
So which would be more correct ? the unloaded factory diameter of 27.4"..or the weight loaded tire diameter of 25.5" ??
Just asking ??
 
Did you physically measure the tire without load. Actual sizes very by manufacturer. In other word theoretical vs actual.
 
You have to figure in the amount of centrifugal force that pulls the tire mass tread center outwards as the speed increases.

Inflation PSI of the tire can alter that as well.
 
You need to measure actual tire height. Plus the diameter vs RPM at a speed has more to do with tire roll out.
 
Do an actual tire rollout on the car and reverse calculate the diam. Then only unknown is centrifical growth.
 
This is interesting I have never thought about it like that before.
 
Alaskan_TA said:
You have to figure in the amount of centrifugal force that pulls the tire mass tread center outwards as the speed increases.

Inflation PSI of the tire can alter that as well.
Click to expand...
yes inflation will change the load to center of axle radius of tire.
 
Don't use a measured radius to determine diameter in a case like this. Instead, use a measured circumference. Regardless of pressure, every time the tire rotates, all of the circumference has to come in contact with the ground.
Tire manufacturers often specify revolutions per mile at a specific speed in their spec charts. This is based on circumference, not diameter. If your tires are a readily available popular size, that could be of help. I just looked at BF Goodrich's website and they give the revs per mile at 45 mph. If your size isn't listed, try doing a "roll out" measurement. Mark the tire and move the car until the tire has made about 10 revolutions and divide the distance by number of revs to get an average circumference. Divide by pi to get the effective diameter.
 
Garry in AZ said:
Don't use a measured radius to determine diameter in a case like this. Instead, use a measured circumference. Regardless of pressure, every time the tire rotates, all of the circumference has to come in contact with the ground.
Tire manufacturers often specify revolutions per mile at a specific speed in their spec charts. This is based on circumference, not diameter. If your tires are a readily available popular size, that could be of help. I just looked at BF Goodrich's website and they give the revs per mile at 45 mph. If your size isn't listed, try doing a "roll out" measurement. Mark the tire and move the car until the tire has made about 10 revolutions and divide the distance by number of revs to get an average circumference. Divide by pi to get the effective diameter.
Click to expand...

That aint the way racers do it, just measure the outside distance around the tire in the center , with a flexable tape measure = rollout .
 
famous bob said:
That aint the way racers do it, just measure the outside distance around the tire in the center , with a flexable tape measure = rollout .
Click to expand...

You are absolutely right Bob, that ain't the way racers do it. They use a tape measure, exactly as you said. But they are solving a different problem. Racers are concerned with the circumference of the tire to be exactly the same so the car goes straight down the dragstrip, or for the circumference to be what's required to provide the correct amount of stagger to help the car turn in circle track racing. They are using those measurements to compare one tire to another, not to determine an absolute circumference value. They are looking for the difference between the two tires.
The original post was not concerned with making the car go straight or turn better, he was looking for a way to accurately predict engine RPM at a given speed. I think the suggestion I posted for that problem will get him in the ballpark.
By the way, marking the tire and rolling the car 10 revolutions and then doing the math will result in a number almost identical to the racers method of wrapping a tape around the tire. What changes, is with the car at speed, the circumference can grow slightly, which is why the tire manufacturers post a "revolutions per mile" spec at a stated speed.
 
famous bob said:
That aint the way racers do it, just measure the outside distance around the tire in the center , with a flexable tape measure = rollout .
Click to expand...
That is the way I did it. I had a real thin tape that I would hook into a depth indicator wear hole and roll the tire the hole again.
 
With a non-loc-up automatic, you will never get the at-speed rpm to agree with the calculated rpm......... because the TC is slipping always slipping. It's a fluid coupling not a clutch.
Next time you are out cruising, make note of the tach reading. Then simultaneously feather the brake and gas pedal to maintain your cruise speed and watch the tach. Next increase your speed a couple of mphs then lift off the gas, again watching the tach as the mph comes down across the chosen cruise mph.
Don't be surprised to see 200 to 400rpm difference, or more with a looser TC.

On a sidenote; my experience with radial tires is that at hiway speed they don't grow much; at 60mph, an 85"tire is rolling along at only 5250x12/85=745 rpm. How much is not much, IDK. But if it was 2%, that would be .02x745=15 tire rpm.
On your dashtach with 3.55s , the calculated cruise rpm using the roll-out of 85" is 2646, and 2% would be 53rpm,can you read 2646 on your tach? I bet your needle is more than 53 rpm wide lol.But worse is your speedo error is likely greater than that, on account of 2% of the 60 mph reading is 1.2 mph. I bet your needle is 1.2 mph wide. So how can you resolve this?
IDK either. And to be blunt, why would I care....... jus asking

As to calculating the roll-out from the branded tire size, this is impossible.
Impossible because there is a really large amount of leeway in the branding procedure.
Take for instance a 235/70-15 tire. The 235 is the section width on the specified test rim of 70%. So if you mount this tire on a .70x235/25.4=6.47 wheel, it will measure somewhere close to 235. But we already have the first problem; how many times in your life have you seen a 6.47" rim? Or even 6.5 with rounding?lol. But let's ignore that for a minute.
So now you have this tire mounted on a 6.5, and it measures not 235. Suppose it measures 230 or 240, both of which can legally be rounded to 235. This number, together with the aspect ratio is used to calculate the tire diameter and then the circumference.
So lets say your 235 actually measures somewhere between 230 and 240, and the aspect ratio is 70.
Hang on a sec; aspect ratios are rounded to the nearest 5 on the 5s, so your 70 series tire could actually be legally somewhere between 67.51 and 72.49. You see where this is going?
After you best guess the diameter, along comes Mr. Pi, and says he will multiply your best guess error by 3.1416.
It's impossible, I tellya,lol.
 
Last edited:
AJ/FormS said:
With a non-loc-up automatic, you will never get the at-speed rpm to agree with the calculated rpm......... because the TC is slipping always slipping. It's a fluid coupling not a clutch.
Next time you are out cruising, make note of the tach reading. Then simultaneously feather the brake and gas pedal to maintain your cruise speed and watch the tach. Next increase your speed a couple of mphs then lift off the gas, again watching the tach as the mph comes across the chosen cruise mph.
Don't be surprised to see 400rpm or more difference.
Click to expand...
95% max lockup for an automatic sound correct?
 
Garry in AZ said:
You are absolutely right Bob, that ain't the way racers do it. They use a tape measure, exactly as you said. But they are solving a different problem. Racers are concerned with the circumference of the tire to be exactly the same so the car goes straight down the dragstrip, or for the circumference to be what's required to provide the correct amount of stagger to help the car turn in circle track racing. They are using those measurements to compare one tire to another, not to determine an absolute circumference value. They are looking for the difference between the two tires.
The original post was not concerned with making the car go straight or turn better, he was looking for a way to accurately predict engine RPM at a given speed. I think the suggestion I posted for that problem will get him in the ballpark.
By the way, marking the tire and rolling the car 10 revolutions and then doing the math will result in a number almost identical to the racers method of wrapping a tape around the tire. What changes, is with the car at speed, the circumference can grow slightly, which is why the tire manufacturers post a "revolutions per mile" spec at a stated speed.
Click to expand...
I guess your right. The roll out is the best way to think about this. Thanks
 
You must log in or register to reply here.
-

Back
Top