Tire RPM-Eng. RPM-MPH?

Low Class · June 22, 2012, 03:28:07 PM

Does anyone know how the tire manufacturers calculate RPM? I have always used the loaded radius, but this does not jive with the manufacturers numbers.

Next topic is calculating mph versus engine rpm. There are several calculators on the internet for this purpose. While the formulas may be correct, every one I have seen says to enter the tire diameter. This is guaranteed to give the wrong answer. The saying "junk in=junk out". Would not the correct way be to double the loaded radius for the rolling diameter?

Many years ago I found the following formula in an automotive magazine. It seems to be 100 per cent correct. multiply the loaded tire radius by engine rpm and divide by the overall gear ratio multiplyed by 168

LR x RPM / ratio x 168

Oonrahnjay · June 22, 2012, 03:33:41 PM

Quote from: Low Class on June 22, 2012, 03:28:07 PM
Does anyone know how the tire manufacturers calculate RPM? I have always used the loaded radius, but this does not jive with the manufacturers numbers.

Next topic is calculating mph versus engine rpm. There are several calculators on the internet for this purpose. While the formulas may be correct, every one I have seen says to enter the tire diameter. This is guaranteed to give the wrong answer. The saying "junk in=junk out". Would not the correct way be to double the loaded radius for the rolling diameter?

Many years ago I found the following formula in an automotive magazine. It seems to be 100 per cent correct. multiply the loaded tire radius by engine rpm and divide by the overall gear ratio multiplyed by 168

LR x RPM / ratio x 168

I haven't tried your formula in detail but I have always had good luck with formulae that use "Tire Revs per Mile". If yours are accurate and these are, they're probably equivalent.

HB of CJ · June 22, 2012, 04:56:12 PM

I think there is a relationship between revs per mile and loaded radius or diameter. Perhaps the tires expand due to centrifugal force? Interesting theory.

There are many ways to figure out rpm at different road speeds while in different gears. An example would be a 750 hp Cummins Big Cam 2

Running thru a RTO-14510 10 speed with a .80 10th gear rolling 11.5 x 24.5 tires twisting a 3.08 axle gear running flat out at 2500 rpm would work out...

To about 126 mph give or take. Enjoy. HB of CJ (old coot)

Low Class · June 22, 2012, 07:17:38 PM

I ran the numbers on your big cummins and came up with only 122 mph. I couldn't find the specs for a 11.5-24.5 so I used a 11-24.5. Also, I did not allow for centrifical expansion or wheel spin.

Seriously, has anyone ever witnessed a class 8 truck doing 120 mph? Back when I used to ride (motorcycle) a lot, I have run with trucks doing in the mid eighties, but don't recall ever encountering one doing 90 or above. I have encountered several drivers that claimed their truck would do 120. I apoligize for getting off bus topic.

luvrbus · June 22, 2012, 07:25:03 PM

My pal Sonnie has a 3406 Cat in a Eagle with 2:94 rear gear and a 740 1:1 it likes 90 mph all day long

good luck

Sean · June 22, 2012, 09:17:24 PM

Quote from: Low Class on June 22, 2012, 03:28:07 PM
Does anyone know how the tire manufacturers calculate RPM? I have always used the loaded radius, but this does not jive with the manufacturers numbers. ...

It is not calculated, it is measured. Remember that the "Overall Diameter" and the "Loaded Radius" are both static numbers. The former is measured with the tire inflated but unloaded. The latter is measured with the tire inflated and the rated load applied at rest.

The revolutions per mile, however, is a dynamic measurement, measured with the tire inflated and loaded but also spinning at a designated speed. The dynamic overall diameter of the tire is different than either the published "overall diameter" or twice the "loaded radius."

This is the reason why tire manufacturers publish all three numbers. If there was a static formula to relate all three of them to one another, they would only need to publish one of them.

When figuring MPH from driveline RPM, use the published revs per mile, not a formula based on loaded radius.

-Sean
http://OurOdyssey.BlogSpot.com

Low Class · June 23, 2012, 06:44:58 AM

Luvrurbus,
I drove 450 miles to BK's ralley mainly because I heard that Sonny was attending. I met him. He is a fine person. A real gentleman. His bus is awesome.
All the people at the ralley were nice. Don Fairchild, Uncle Ned (already knew him) and last but certainly not least BK

Sean,
That is a great explanation. I have learned something new today. I have been doing it wrong all these years. Now I need to go remove the counter from the hay baler. Got to keep check of those maunfacturers and keep them honest.

TomC · June 23, 2012, 07:06:04 AM

Sonnie running his 3406 at 90mph works out (if he has 12R-22.5) 2140rpm-which isn't to fast for the 3406, but above its' fuel efficient range. Now running at 65mph-he'd be at 1545rpm which would get him some great mileage! Good Luck, TomC

Don Fairchild · June 23, 2012, 12:22:52 PM

you guy's drive to fast for me. My bus with 4:56 gears 22.5 tires and the double over drive HD 4060 will only top out at 88 mph. LOL

Don

usbusin · June 23, 2012, 02:00:57 PM

Yea, the little 3126b Kitty in my Freightliner will only do 88 mph. That is with 3.91 rear axle, an overdrive 6th gear and 255/80R 22.5 tires. Guess that Don Fairchild and I are about even on top speed. You other guys will blow right on by!!

60 MPH @ 1625 RPM, 70 MPH @ 1900 RPM, 80 MPH @ 2150 RPM

Top Speed: 88 mph (governed @ 2400 RPM).

HB of CJ · June 23, 2012, 07:24:53 PM

I meant 11Rx24.5 tires. The application was mythical and kinda was a 1974 Crown Super Coach 10 wheeler. Tire circumference was 132 inches. I don't know what the revs per mile is..best guess around 478 or so. To go faster the rear driver ratios would be lower. (lower number) HB of CJ (old coot)

Low Class · June 24, 2012, 05:36:20 AM

HB
I knew it was a typo. I was just having a little fun with you. Now that Sean has explained where I was going wrong, it looks like the joke is on me.

HB of CJ · June 24, 2012, 02:37:27 PM

RPM--divided by final drive ratio--(don't forget any overdrives)--times tire circumference (in inches)--divided by 12--divided by 528--times 6--equals mph. This seems to work with any size tire and any rpm and gear ratio. Strange, but it kinda true.

FWIW, us dumm firemen years ago used this home made formula to figure out how fast our DD 353 and 453 Ford 1-ton and pickups would go with different tires, trannys and rear axle gears sometimes employing Spicer 5831C auxillary boxes. HB of CJ (old coot)

AN EXAMPLE.....with a Spicer 70 rear end at 4.11 gears with 7.50 x 16 tube tires on split rims at 2800 rpm our mighty cool but slow but fun 353N Ford 1-ton flatbed farm truck would go a whoppping 63 mph. Had a Jake too. NP 540. Long ago in Bakersfield.