Speedo Calc's for all you smarty pants...:)

[festyxfi]

Okay, as Forrest said, "I am not a smart man Jenny..."

What I want to know is, how does one calculate the true miles driven (between fill-ups, using the trip-o-meter) when the tires your running aren't stock size?

I use THIS calculator to figure SPEED adjustment:

http://www.miata.net/garage/tirecalc.html

But how about the formula for miles driven adjustment at fill-up to calc. true MPG's?

Explain as if you were talking to Forrest Gump please, for that's where my math skills dwell...

FX

[JPT]

Assuming 145/80-12 is stock size.

A 145/80-12 tire is 21.12 inches tall

The formula for that is...

Tire width. 145mm.

Convert that to inches by multiplying that by .03937

You get 5.7 inches.

Now, we need the sidewall height. Take 5.7 and multiply that by .8 which is 80%, the second number in the tire size spec (that's the aspect ratio of the tire. How tall the sidewall is in relationship to the width).

This equals 4.56

Multiply this answer twice. Then add the rim diameter.

Total, 21.12 inches.

Now we need to know the size of the tires you're running. Do the same math for your current tires.

Example, 175/70-13.

175mm is 6.9 inches.

70% (the second number) of that is 4.83

Multiply by 2 plus the rim diameter = 22.66 inches.

Take 22.66 and divide that by the stock height (21.12).

You get 1.073 or a little over 7%

Take whatever distance the odometer says and add 7.3% (multiply by 1.073). Then do your MPG calculation.

[festyxfi]

Just a quick kudos to FestivaFan here!!...

He was very helpfull in teaching this math-tard how to calculate true MPG's (via PM)

Thank you, thank you!!

FX

[drddan]

Assuming 145/80-12 is stock size.

A 145/80-12 tire is 21.12 inches tall

The formula for that is...

Tire width. 145mm.

Convert that to inches by multiplying that by .03937

You get 5.7 inches.

Now, we need the sidewall height. Take 5.7 and multiply that by .8 which is 80%, the second number in the tire size spec (that's the aspect ratio of the tire. How tall the sidewall is in relationship to the width).

This equals 4.56

Multiply this answer twice. Then add the rim diameter.

Total, 21.12 inches.

Now we need to know the size of the tires you're running. Do the same math for your current tires.

Example, 175/70-13.

175mm is 6.9 inches.

70% (the second number) of that is 4.83

Multiply by 2 plus the rim diameter = 22.66 inches.

Take 22.66 and divide that by the stock height (21.12).

You get 1.073 or a little over 7%

Take whatever distance the odometer says and add 7.3% (multiply by 1.073). Then do your MPG calculation.

What?