speedo check?
 

i've been trying to find a place to check my actual speed. i've got a different gear ratio now and need to figure out how off the speedo is. the tires are the same size so i need a calculator to do the ratios. thanks, Steve

Try this.
Todd

the easy way to do a rough check is go out on an interstate that has mile markers with a stopwatch. with your speedo at a steady 60mph your time between mile markers should be 1minute

Get a buddy that has a GPS and go for a ride

GPS is the way to go. Takes all of the guess work out, no math. It will even give you a reason to buy one. Then it is a tool, not a toy!

If you are not all that worried about if it was right in the first place, all you will need to do is divide your new ratio by your old ratio.

4.10/3.73=1.10 A 4.10 ratio is 110% of the 3.73 ratio. 3,000 rpms with a 3.73 will be 3,300 rpms with a 4.10 ratio. Your speedo would be off by 10%. 60mph would now be at 66mph on the speedo.

3.08/4.10=.75 if your 4.10 gear was 3300 rpms at 60mph a 3.08 would be 75% of the old 4.10 gear or 2475 rpms. your speedo would be off by 25% You would now be doing 60mph when the speedo says it is doing 45mph.

hopefully I didn't turn any of the numbers around, but it should be correct.

Here are a couple of other sites I have used to help figure this out before.
http://www.novaresource.org/speedo.htm
http://www.bgsoflex.com/speedo1.html

You guys are way overcomplicating this--Cdowns has it right.
If you are really picky, take the deviation in seconds from 1 minute to travel measured mile (+ 0r - seconds) divide that by 60 for the percentage it deviates--that's the percentage your speedo is off.
IE: 58 sec. = 2 seconds deviation, div. by 60 = 3% fast or 61.8 actual mph. Close enough for government work and you never have to get out of cab or buy a $200+ piece of equipment! :D

alright thanks for the help guys according to that site i am suppose to be going 80 at 3000 rpms with the 3.08 gears but with 3.73 i'm only goin 69.9. thanks, Steve

I checked mine today , parked my crown vic. beside the road and showed the wife how to lock the radar. Then I drove by at 55 mph. she locked it when I flashed my headlights. Found that the 95 mustang gt is off by 5 mph and the 72 c-10 is off 4 mph
55 on the speedo = 50 mph.
My suggestion is to just ask an officer to check you. I do it for people... sometimes when they ask sometimes not! LOL