Mathematicians, Help me Formulate my tire PSI! - Fuelly Forums

Click here to see important news regarding the aCar App

Reply
 
Thread Tools Display Modes
 
Old 12-24-2009, 08:13 AM   #1
Banned
 
Join Date: Jul 2009
Posts: 427
Country: United States
Mathematicians, Help me Formulate my tire PSI!

Ok, here's some ideas ive been teetering with for the last few days,
considering the snow me having to let air out of my tires for saferty.. lol

Does anyone know the formula that can calculate tire height vs trip odometer mileage..

because

one thing that can cause your odometer to really rack up the miles is smaller tires(more spins per mile)

so having bigger/taller tires, ect can cause your mpg numbers to go down due to the less spins needed per mile..

yet adding psi will increase your mpg by a certain said amount( this varies by car)
so by adding psi are you not really getting exact mileage unless you've thrown in a math formula to account for higher tire height?

meaning that you might be getting even better mileage(providing you have a control mpg using stock psi/height that matches your car's computer's recognition as what a mile is) than you're odometer says you are..

wouldnt you think on some cars that this unaccounted for error could actually be a 1-2mpg difference, well I would need the formula to even hypothesize this, someone here has to know it...
__________________

spotaneagle is offline   Reply With Quote
Old 12-24-2009, 09:36 AM   #2
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Good question. I've never considered the inflation-induced diameter change to be significant, but I haven't measured it.
__________________

__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 12-24-2009, 10:00 AM   #3
Banned
 
Join Date: Jul 2009
Posts: 427
Country: United States
Quote:
Originally Posted by spotaneagle View Post
Does anyone know the formula that can calculate tire height vs trip odometer mileage..
i think ive actually seen a formula for this before, but it wouldnt surprise me if you needed to make one for every different car
spotaneagle is offline   Reply With Quote
Old 12-24-2009, 10:03 AM   #4
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Well, that issue can be done by many gear calculators. Height is only useful for determining circumference. Get the circumference and you have the distance the car will go with one tire revolution. You can feed size, diameter, or revolutions per mile into this gear calculator:
http://f-body.org/gears

Also feed in your diferential and at least one transmission gear and you'll get your answer.
__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Old 12-24-2009, 11:05 AM   #5
Registered Member
 
Join Date: Oct 2005
Posts: 682
Country: United States
Trie diameter doesn't hange much with air pressure. Unlike a ballon, it has steel cords in the tread that don't stertch much.
__________________
Capitalism: The cream rises. Socialism: The scum rises.
Sludgy is offline   Reply With Quote
Old 12-24-2009, 01:05 PM   #6
Registered Member
 
Join Date: Dec 2007
Posts: 170
Country: United States
As above and remember too IF you are going to this length you might want to find / build an equation to take the effect of centrifugal forces into account as well.

Personally I think the differences would be so slight they would be lost in background "noise" and effects like road materials and surface finishes would also be an influence.

Pete.
GasSavers_Pete is offline   Reply With Quote
Old 12-24-2009, 02:12 PM   #7
Banned
 
Join Date: Jul 2009
Posts: 427
Country: United States
Quote:
Originally Posted by Sludgy View Post
Trie diameter doesn't hange much with air pressure. Unlike a ballon, it has steel cords in the tread that don't stertch much.
yea.. well an inch on my tires on my car is the difference from 29psi to38psi, I mean how tall the tire is while on the car.. and could potentially mean alot in terms of long term results measuring mpg, to the point where this has its own calculator

all you can be doing is giving your self a higher mpg readout and more accurate.. whats wrong with that?
spotaneagle is offline   Reply With Quote
Old 12-24-2009, 06:21 PM   #8
DRW
Registered Member
 
Join Date: Oct 2006
Posts: 615
Country: United States
The easiest way to measure this is to use a ruler or tape measure and measure the height of the tire from the ground to the bottom of the rim at it's lowest point. While you're doing that, let out ten psi and compare the change. Use that measurement to calculate effective diameter/circumference. It'll be different for front and rear tires since they carry different ammounts of weight.
__________________
Dave W.
DRW is offline   Reply With Quote
Old 12-26-2009, 08:14 AM   #9
Registered Member
 
JanGeo's Avatar
 
Join Date: Jan 2006
Posts: 2,444
Country: United States
Location: Tiverton, RI
Send a message via Yahoo to JanGeo
That doesn't work because the tire CIRCUMFERENCE is NOT changing with different air pressure only the about of SAG in the sidewall on the bottom of the tire. You still travel the exact same distance per revolution no matter what the tire pressure.

Now for some bad news . . . apparently the tire rolling resistance increases with taller tires so putting taller tires may lower the engine RPM and effectively change the final gear ratio and Odometer reading but it also apparently increases rolling resistance too.
JanGeo is offline   Reply With Quote
Old 12-26-2009, 04:15 PM   #10
Registered Member
 
theholycow's Avatar
 
Join Date: Apr 2008
Posts: 6,624
Country: United States
Send a message via ICQ to theholycow Send a message via AIM to theholycow Send a message via MSN to theholycow Send a message via Yahoo to theholycow
Where did you get the idea that taller tires increase RR? As far as I can tell, shorter tires increase RR...but if you have some more concrete theory or some actual data, I'd be happy to learn.
__________________

__________________
This sig may return, some day.
theholycow is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
'95 Honda Civic DX hatchback w/ VX tranny pinging cems70 General Maintenance and Repair 8 02-09-2011 02:31 AM
Anyone bought the gold VX in NH yesterday? XyKo General Fuel Topics 1 07-06-2008 03:39 PM
More Consumers, Workers Shoplift as Economy Slows HondaTorneoSiR Automotive News, Articles and Products 2 06-19-2008 03:20 PM
Beating the Baro... MAP reading trickery? GasSavers_RoadWarrior General Fuel Topics 4 03-06-2008 01:43 AM
Performance Underdrive Pulleys? cfg83 General Maintenance and Repair 10 09-08-2007 08:04 PM

» Fuelly iOS Apps
No Threads to Display.
Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 10:09 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.