Shoofin

Since changing the tires/wheels to CL-S wheels and 225/45/17 rubber, I knew that the speedo would be off....but by how much was my question.... So the other day I was driving, thought I would time 1 mile and how long it would take... Cruise control on at 60mph on the nose.. Turned on my stopwatch as the 1/10th meter changed, and timed how long it would take to go 10/10th, or 1 mile. The time came out to 70 seconds it took to go 1 mile at 60 mph...Can somebody figure out how fast I was really going? How far off is my speedo at 60mph?
Ed

Red Hot TL

I think you are actually going at 51.43 MPH.


1mile | 60sec | 60min
-------------------------------- = 51.43
70sec | 1min | 1hr


your overall circumference is 199.27 cm (225/45/17) compared to stock 204.96 cm (205/60/16).

Nate

You are 2.774% too fast. So if you think you are going 60 you are really going 58.3356 MPH.

Fiddler

<b>EDIT- This post is incorrect - read correction below!</b>

but then i suppose wind resistance, rolling resistance, grade of the road etc could have an effect.. as well as human error (no offense but u know )
in any case.. to find out the difference, just find out the diameter difference between the stock wheels and these wheels

205/60/16

16"=406mm
60% of 205mm is 123mm

overall diameter=529mm

-------------
225/45/17

17"=432mm
45% of 225mm is 102mm

overall diameter=534mm

that makes up for a diameter difference of only 5mm (about 1/5")
overall circumference is Pi*D so
stock circ = 1661mm = 1.66 meters = 5.45 feet
new circ = 1677mm = 1.67 meters = 5.5 feet

this means that for every revolution of the wheel, with the new wheels you'll go about a centimeter and a half further (figure half an inch)
so if i didnt make some horrible mistake above, if your speedo is showing 60, you should actually be going slightly faster than 60


lets see........ a mile is 5280 ft
on the old wheels, to go a mile, you'd need 968.8 revolutions
with the new wheels, you'd need 960 revolutions

if your speedo is configured to your old wheels, that means that 968.8 revolutions per minute = 60mph

with the new wheels, you'll actually cover 5328.4 feet which is 1.01 miles. 1.01 miles per minute= 60.5 mph

so there u have it (of course i welcome everyone to check my math please )

Red Hot TL

fiddler:
hehe, there are two sidewalls...

Red Hot TL

ed, from your info, you went one whole mile in exactly 70 seconds. so basically, distance/time = speed. based on the formula, you are going 51.43 MPH according to your info. but if you're going 58 MPH according to Nate, I am pretty sure there's some human errors on timing and etc.

NSXNEXT

iTrader: (2)

Hey Ed,

You also might consider that your speedo was off with the stock wheels and tires.

Fiddler

<b>CORRECTION! </b> as pointed out by red hot .. there are 2 sidewalls (duh).. hehhe oops therefore:
205/60/16

16"=406mm
60% of 205mm is 123mm

overall diameter=652mm

-------------
225/45/17

17"=432mm
45% of 225mm is 102mm

overall diameter=636mm

that makes up for a diameter difference of 16mm (about 5/8")
overall circumference is Pi*D so
stock circ = 2047=2.04 meters = 6.71 feet
new circ = 1997=1.99 meters = 6.55 feet
this means that for every revolution of the wheel, with the new wheels you'll go about 5m further (figure 2 inche)
so if i didnt make some horrible mistake above, if your speedo is showing 60, you should actually be going slightly faster than 60


lets see........ a mile is 5280 ft
on the old wheels, to go a mile, you'd need 787 revolutions
with the new wheels, you'd need 806 revolutions

if your speedo is configured to your old wheels, that means that 787 revolutions per minute = 60mph

with the new wheels, you'll actually cover 5155 feet which is 0.97 miles. 0.97 miles per minute=~ 58.6mph

Red Hot TL

anyway, I'm pretty sure you're not off by 8 or 9 MPH but only 1 or 2. so I bet 1)you didn't go exact one mile 2)you didn't go for exact 70 seconds.

NSX4dr

But when the tires wear down...............

Here's the real formula!

Section width (x) aspect ratio (x) 2 (/) 25.4 (+) wheel diameter

Therefore:

Diameter
205/60/16 = 25.685039"
225/45/17 = 24.972441"

3% decrease

Therefore:

Circumference
205/60/16 = 80.65" - 982.02 rev/mi
225/45/17 = 78.41" - 1010.08 rev/mi

3% increase

3% higher speedometer meaning 60mph actually is 61.8mph

Shoofin

Well, I was on this stretch of road for about 20 miles, and for about 10 of it I kept timing over and over and over... Each 10th took 7 seconds, I kept recounting it...
Ed

NSX4dr

Eddie,

Did I help or confuse you more?.........

speed

Over many miles, the odometer will show that he put on more miles than he actually did. Can't the speedometer be calibrated for the new tires and wheels?

MikeS

Yes, you can have your speedo recalibrated....

Shoofin

Nuh-uhhhh not possible....prove me wrong...PLEAAAASE so I can do that..my mileage is hurting bigtime..
Ed

bluefrank

Ok, thanks guys now I have a headache....

AHHHHH