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2 blowouts at 51 PSI -- so I did the math and made a spreadsheet


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4 replies to this topic

#1 OFFLINE   J.C.

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Posted 05 January 2019 - 02:01 PM

My tires had around 35,000 miles on them when I decided to over- (or max) inflate them. Not sure how much life I would get out of the tires, but being positive that they were over 50% done, I figured it was worth a try. 

I did not notice substantial gains in mpg, although I did notice a bump. (YMMV) 

But I did get two blowouts from hitting curbs (one I contacted and one I flat out hit--I figure that one is on me) which has NEVER happened to me before, and I drive/have driven a LOT. 

First blowout was around 40k and the second at 45k, so I got 5k and 10k miles of mpg savings, respectively.

 

But that's just anecdotal evidence, so I made a spreadsheet and did some math.

 

https://docs.google....8g6_dLm4HeNhmQ0

My general conclusion is, it really comes down to your personal belief:
If you think that better tires (or higher pressures) will get you significantly better mileage, then you should always go for it. 
If you think they don't, then you shouldn't.

The problem is, for buying tires, there's not really any way to know what the mpg difference will be, that's why I say it's about belief.

 

With inflation, obviously you can just try various PSIs to see what your savings is--assuming you are measuring PSI and MPG accurately, and assuming you drive the same way/routes when you measure.
But after all that it comes down to your belief in how much your risk has increased from higher inflation. Whether you believe you have no increased chance of blowout/tire loss, or a much greater one; because we can't measure that.

[I'm now in the greater risk camp. ;) ]

 

Also currently (as of Jan 2019) gas prices are pretty low, so it's much harder to generate savings. If we go back to $3.00/gallon, these numbers will change.
I used $2.50 when I messed with the sheet.

Feel free to try out the sheet for yourself.
As the directions say, you have to Make A Copy of the sheet in order to manipulate the numbers.

One thing this sheet does not do is calculate tire savings based on differences in tire durability. That extra factor (durability x mpg) increases complexity exponentially, so I omitted it.

But you can assume that if your low-MPG tire will last longer than your low rolling resistance tire, then you need even more mpg savings to justify that purchase.









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#2 OFFLINE   homestead

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Posted 05 January 2019 - 02:21 PM

What mpg's were you getting with the overinflated tires?



#3 OFFLINE   ptjones

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Posted 05 January 2019 - 05:28 PM

Over inflation by definition is either when the center of the tire wears faster than the edges or using above 51 psi which is Michelin's max pressure for OEM tire.  38 psi is FORD's recommended tire pressure and they say not to go over Tire Manufacturers (Michelin) Max pressure on the tire.  I got 78k miles out of my last set of fronts and currently have 83k miles on backs which are on the front now for 5k miles. I have been using 50 psi for 200k+ miles and got 64k mile out of the first two sets which weren't worn out, but I was going to be driving a lot of miles in the snow so I wanted to have max tread.  I have never had a blowout in 215k miles, but two punctures that were fixed and one in the the sidewall at 78k miles so replaced two tires. Michelin's Energy Savers are warrantied for 55k miles.  It would appear that Michelin Energy Savers 93V last longer at 50 psi than 38 psi recommended by FORD. :)

 

Paul


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#4 OFFLINE   fbov

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Posted 08 January 2019 - 12:14 AM

My tires had around 35,000 miles on them when I decided to over- (or max) inflate them. ...

What was your inflation pressure? I don't see any numbers, and without them, there's not much to say. 

 

I have used sidewall ratings exclusively for setting tire pressures since the Carter administration. This way, tires running at their maximum load capacity, so their actual load is the smallest possible fraction of maximum capacity. Put another way, I want as head room as I can get. Tires run cooler, last longer, handle more predictably. 

 

I have had a blow-out. A granite curb had a lot to do with it, tire pressure was not a factor. 

 

As to the effectiveness improving mileage, unless you are getting very high mileage already, improvements are invisible. No-ethanol fuel is a good example. 

Attached File  Octane Tire Pressure.jpg   81.07KB   0 downloads

 

The first period is 87E10, and average is 54.8 MPG. Second period is 91E0, and average rises to 58.6 MPG. The third period averages 56.3 MPG, and it's curious. The change occurred after braking very hard over very rough roadway (construction). I later found out that my tire pressures had dropped from the 50 PSI setting. Best I can figure, air leaked out the bead when I hit the bumps. But it gives you some idea how tire pressures can affect mileage. 

 

Note that this data is for a known rural route, driven for maximum mileage within speed limit constraints. You'll see things when you're averaging 55 MPG that you don't see at 35 MPG. 

 

Have fun,

Frank


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#5 OFFLINE   pianewman

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Posted 09 January 2019 - 10:44 PM

I'm not sure that making a spreadsheet of your single experience means your information is NOT anecdotal! ;)  I would also offer the opinion that BOTH blowouts were "on you"!! :lol:


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