tiers size calibration

[1977cherokeechief]

doe anyone know of a site that will do a metric to an inch calibration for a speedo?

i have a couple of cars that have oversized tires on it and was wondering how far off they were...

one is my 77 chero which has 33"

my REALLY REALLY BIG POS:banghead: is a 88 ford bronco which has 31"

it would be nice to know exactly how fast i am going so i can be prepard next time i see a black and white...

thanks
nate

[Radio Flyer]

doe anyone know of a site that will do a metric to an inch calibration for a speedo?

it would be nice to know exactly how fast i am going so i can be prepard next time i see a black and white...

thanks
nate
http://www.1010tires.com/TireSizeCalculator.asp

This one tells you how much your speedo is off.

[joe]

The speedo/odo will be off the same % as the tire size change from stock. If you increase tire size 10% the speedo/odo will read 10% slow.

[1977cherokeechief]

i may be dumb but i cant get a speed that i want...

wondering if anyone had a site where you could put MM to Inches and the speed... i know this is a BIG:rolleyes: boat to fill...

[malodin]

i may be dumb but i cant get a speed that i want...

wondering if anyone had a site where you could put MM to Inches and the speed... i know this is a BIG:rolleyes: boat to fill...

i was trying to do something very similar the other day and didnt come accross anything that would tell you a 265/55/16 is the same as a 31x10.50x15 i only found that info out from one website that someone had said it, no actuall websites that do the conversion i have found yet.

sorry

[joe]

i may be dumb but i cant get a speed that i want...

wondering if anyone had a site where you could put MM to Inches and the speed... i know this is a BIG:rolleyes: boat to fill...
This a cut-n-paste from a file I have (I'm a slooow typist). Hope it shows up ok. If not email me and I'll send it as a text file.
---------
How to Convert Metric Tire Sizes
Metric tire sizes are made up of four components: prefix, width, aspect ratio, and wheel size. It takes three of
these components to calculate the diameter of the tire. It sure is not a very straight forward sizing convention.
The prefix is used to indicate general use of the tire. The tire input field trys to ignore the prefix and if your
prefix causes an error, just don't input it. The Gear and Tire Applet currently will successfully ignore 'P' for
passenger car and 'LT' for light truck. When in doubt, drop the prefix.
The width simply the width of the tire in mm. For example the tire size P215/75R15 has a width of 215mm.
The next component is the aspect ratio. The aspect ratio is the percentage of the width that makes up the sidewall
of the tire. In the above example the aspect ratio is 75 or 75%, so the sidewall height is 75% of 215 or 161.25mm.
If you want the diameter of the tire, you need to take into acount that the diameter of the tire passes two
sections of the side wall. This means the sidewall takes up 161.25*2 or 322.5mm of the diameter.
The next component is the wheel size. In the above example the wheel size is 15". If you convert the total
sidewall into inches you get 322.5mm*0.03937in/mm or about 12.6". Add that to wheel size and you get the diameter
15"+12.6" is 27.6". So a P215/75R15 tire has a diameter of about 27.6".
Example:
ok, the 245 is the width (mm) and the 75 is the profile, a % of the width.
hence a 245/75 on a 16" rim is a s follows
245 * 0.75 = 183.75mm - this is the tyre hieght from the rim to the tread.
convert to in = 183.75/ 25.4 = 7.24 "
now double that + the dia of the wheel 7.24 * 2 = 14.46 add dia , 16 =
30.46" in height
rounded off is 30.5 inches in height
-------------------------------
Actual speed: When changed to new tires and you need the actual speed withhout changing speedo drives. Bigger
tires cause speedo to read slower than true speed.
actual speed = (new tire dia. x indicated speed) / old tire dia.
Example: Old tires 28 inch, new tires 35 inch and speedo reading 60 mph

(35 x 60 mph) / 28 = 75 mph true speed

edit: Or just borrow a buds GPS and go for a ride...and take notes. :)

[malodin]

awsome, now someone just needs to put those formulas into a handy calculator, anyone know how to program?

[J4GRAND]

wondering if anyone had a site where you could put MM to Inches and the speed...
That site that Radio Flyer posted did that for you. I did a comparison of my 235/65s that used to be on my '05 Grand Cherokee to my new 255/65s and it showed the diameter in inches and how much of a difference they were: 3.5%. Just use a calculator from there: 70mph x 3.5%= 2.5mph. Add that to the 70mph indicated and the actual speed is 72.5mph.

[malodin]

That site that Radio Flyer posted did that for you. I did a comparison of my 235/65s that used to be on my '05 Grand Cherokee to my new 255/65s and it showed the diameter in inches and how much of a difference they were: 3.5%. Just use a calculator from there: 70mph x 3.5%= 2.5mph. Add that to the 70mph indicated and the actual speed is 72.5mph.

it does show the difference but im pretty sure the question was supposed to be, "site that will do a metric to an inch calibration" i.e......255/75/16 was my old tire and now i am running 33x10.50x15 whats the difference between the two....

that was explained in joe's calculations.

[shepherdskeep]

Try these pages:

http://oly4x4.com/metrictireconv.htm
http://oly4x4.com/4x4calculators.htm

If you're lucky and have local gestapo that use a stationary trailered radar that doesn't record (like my local police) - it says "Your speed is _____" that will help you get actual speedo differences for your rig. As soon as it clocks your rig take a glance at the speedometer and see how far off it is. I used that method, and having my wife ride alongside me while we talked on the cell phones. She would let me know when we kit key speeds (50, 55, 65, etc). I now know that when my TJ's speedo says 38mph, I'm actually doing 50mph :cool: