Tire revs per mile rating

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JHZR2

JHZR2

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As I've mentioned on another thread, I'm looking to go from 195/65r14 to 195/60r14 in my application because of tire availability.

My car has an exhaust harmonic at 3450 rpm that I want to avoid in highway driving.

I know my speed (speedo and actual), my diff is 4.10, trans is 0.81, but I cannot get reasonable tire revolution per mile values. My current tires when new were supposedly 870 revs per mile. I'm getting more like 793 when the math is done, on falken ze512 tires with about 50% tread. Rims are BMW 14" basketweaves that iirc are 6.5" wide. Now granted Michelin as an example uses a 6" rim for their studies on 195 tires... So there will be difference due to that I guess.

How is the revs per mile determined for a tire spec, and what is a good derating factor based upon loading, rim width, etc?

This harmonic is on the ragged edge of my cruising speed for some of the longer segments of driving that I do, and given that different tires with the same "size" seem to have different circumferences by a fairly decent amount, I want to try to be as exact as I can...

Thanks!
 
3450 rpm on the highway ... Man! I'd wish to fit a 70 series under there if you can. Small engine car, I might guess? Revs per mile would be related directly to tyre circumference of course, in fractional feet. 5280 ft/mi / Circumference ft/rev = revs/mi. Do you not wish to calculate, rather, engine revs per minute based on tyre diameter circumference?

update: I now recall Tirerack shows tyre circumference or revs / mile for most all tyre they sell.
 
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Tire calculator is not accurate - threre are subtle differences such that a 195 isn't necessarily 195, 65 isn't necessarily 65, etc.

Ditto for the circumference/revs per mile value... As mentioned above, using real values based upon speed, rpm and gearing, I know that the numbers aren't accurate, at least for my rim width, pressure, etc. Since I know speed and gearing, I know true revolutions, and they dont match up...
 
The rev/mi figure is a published specification from the tire manufacturer. It seems to be about 3% higher than what you would get by figuring
(5280 ft/mi) * (12 in/ft) / (circumference in)

As far as the gear math, it really should work out for a manual transmission or automatic with TCC engaged. Choosing a Michelin 195/60R14 with 895 rev/mi,
(3450 rev/min) * (60 min/hr) / 0.81 / 4.1 / (895 rev/mi) = 69.6 mi/hr

For the 870 rev/mi tire, that should be 71.6 mi/hr.

If it's not working out, one or both of your speed gauges is not accurate. I would actually be surprised if either one of them is accurate.

In order to avoid going insane with instrument calibration, you could just figure that the 895 tire leads to an engine speed that's 2.9% faster than the 870 tire for a given vehicle speed. Then ask if that change in engine speed would put the 3450 rev/min resonance into your typical driving speed.
 
Originally Posted By: Stu_Rock

If it's not working out, one or both of your speed gauges is not accurate. I would actually be surprised if either one of them is accurate.

In order to avoid going insane with instrument calibration, you could just figure that the 895 tire leads to an engine speed that's 2.9% faster than the 870 tire for a given vehicle speed. Then ask if that change in engine speed would put the 3450 rev/min resonance into your typical driving speed.


Stu,

The logic you mentioned there regarding how it falls into my typical driving speed is exactly what I desire to do. The math is not hard, but it just strikes me that the rev/mile is so far off from what is published.

FYI, Im using a GPS and my speedometer. I would not believe the speedometer to be correct, but the GPS should be...
 
I was thinking of speedometer and tachometer accuracy--I didn't realize there was GPS involved. Dashboard tachometers often have poor accuracy.

I should caution that people often put too much trust their GPS units. The manufacturer's specification for speed accuracy is probably for a situation where there's clear line-of-sight to the sky at all elevations above 20° in all compass directions. Which is to say, not in your car and not in New Jersey. For typical conditions, my best guess would be about 0.5 mi/hr uncertainty on the GPS reading at highway speeds, which is probably a lot better than the speedometer.

Using the GPS speed and published tire rev/mi, you can calculate your actual engine RPM. You could try to use a corrected tire rev/mi to account for the worn tread. For 10/32 of diameter lost on a 23 inch tire, you get 1.4% more rev/mi. So 880 rev/mi now.
 
Record the engine at that RPM, isolate the frequency of the drone. Then calculate the 1/4 wave length of the frequency and have a dead end pipe welded onto the exhaust of that length. Done|
 
I think there may be a disconnect here, but to the original question on revs per mile - take a tape measure and measure the drive wheel circumference directly. There are no other related factor other than possibly a minor tyre circumference growth factor at speed. That tyre must travel its circumference once every revolution, Period. NO GPS, NO speedo error. Kudos to colt45ws for getting to the heart of the issue rather than talking around its "circumference".
smile.gif
 
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Originally Posted By: Colt45ws
Record the engine at that RPM, isolate the frequency of the drone. Then calculate the 1/4 wave length of the frequency and have a dead end pipe welded onto the exhaust of that length. Done|


That only works if the resonance is from the entire exhaust system that has no leak. IMO if the resonance bother you that much something is leaking or broken (CAT honeycomb crack), and welding something to change the length doesn't do it.

I had the same problem and I sprayed soap water along the exhaust to find the leak, after fixing the leaks it is quiet. Adjusting tire diameter will means you have to go much different to really have a difference. How much difference? easy to calculate: new tire rev/mi divide by old tire rev/mi, then multiply the whole thing to the current rpm for the target speed. You'll probably have to go 1 size taller or shorter and that'll be much bigger change than any minor difference in rev/mi within the same size
 
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Originally Posted By: PandaBear
Originally Posted By: Colt45ws
Record the engine at that RPM, isolate the frequency of the drone. Then calculate the 1/4 wave length of the frequency and have a dead end pipe welded onto the exhaust of that length. Done|


That only works if the resonance is from the entire exhaust system that has no leak. IMO if the resonance bother you that much something is leaking or broken (CAT honeycomb crack), and welding something to change the length doesn't do it.

I had the same problem and I sprayed soap water along the exhaust to find the leak, after fixing the leaks it is quiet. Adjusting tire diameter will means you have to go much different to really have a difference. How much difference? easy to calculate: new tire rev/mi divide by old tire rev/mi, then multiply the whole thing to the current rpm for the target speed. You'll probably have to go 1 size taller or shorter and that'll be much bigger change than any minor difference in rev/mi within the same size

Yes, but I was guessing he had already checked it out.
Something else Ive heard of working; if the whole exhaust system is vibrating some weight can be added to the pipe.
 
I had to reset my gear ratio to get my speedometer to read right and used the tire calculator from my tire store. Plugging in the revs per mile I see that the factory is off by 2 mph fast to keep me under the speed limit.
 
JHZR2,

You mentioned the engine speed where the vehicle resonates, and while you mentioned that the resonance occurs NEAR your normal cruising speed, you didn't mention what that speed was! If I did my math right, I got the same speed as Stu - 71.6 mph.

You should be aware that vehicle suspensions resonate between 50 to 70 mph. Going to a smaller diameter tires is going to make your exhaust resonace occur within that speed range - which I don't think is a good thing!

I'm thinking you need to find a way to get the 3450 rpm to occur much above 70 mph - so I'm thinking you need LARGER diameter tires!
 
Yeah so the resonance shows up at what looks to me to be 3450ish, and is also 77-79mph.

Doing the math, the tire rpm doesn't match, that's the problem.
 
Got my 195/60r14 exaltos put on iplace of the 195/65 falken ze 512. Went from about 2 mph off to 3 mph off in the 65-70 mph range. With the new tires, at True 65, 2950+ RPM.
 
Why not just get a different muffler..or add a small resonator into an existing section of pipe...probably get one for less than $40, then you could get whatever tire you wanted without the headache...
 
Don't worry, no headache. Revs difference so minor for all intents and purposes the same.

I like the tires, all that matters. The math is still interesting/funny and unanswered... And that was the real crux of my question to start.
 
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