[84brian]

hey guys, im curious how the speedo is hooked up in my 84, I believe there are some small gears in the tail shaft of my tranny that run it? no?

any how, here goes:
my stock tires were 255/50-16 and measured 11.83 for a loaded tire radius
my new tires are 275/40-18 and measure 13.0 loaded tire radius,

so im obviously getting a lower mph readout on the dash than my actual speed is, because my roll out is now longer for each revolution of the tire.

to add complication to this equasion, I just bought a new rear end case with some 3.07 gears in it, ( too good a deal to pass up ) to replace my 2.73 gears in the car now. so id like to factor the gear reduction in to the new equasion when I get it installed.

im thinking I need to find a local cop, and ask him to radar me driving the speedlimit on a road, and see what my actual mph is from the radar gun and compair it to my digital readout on the dash, maby do a couple diffrent speeds, like 35mph and 55 mph to establish a pattern in the amount my mph is off by,

do you think there is a easier way to do this?

 

[WIT911]

bump

 

[raidmagic]

Without the gear change in the equation your speedo reads 3mph faster than you are going. If your speedo reads 60 you are really going 57.:thumbsup:

 

[84brian]

no sir raid, if my new tires are larger they will travel more ground for each revolution of the wheel. lets just say going 55 mph my wheel is spinning 50 times per minute, ( I know its wrong but you get the idea ) so my new wheels are now adding an extra little bit of length to each revolution, times that by 50 and im going to be covering alot more ground, wich would equal increased mph ? but the car is still only registering the stock tire size and giving me the lower mph read out on the dash than id actually be going.

man, this is even confusing me. :smack

I hope I find one of those darn poliece speed check things on the side of a road somewhers, that post your mph your going, and ill have to bust a u and make a couple of passes at it and jot down actual speed vs dash read out. and try and establish some kind of pattern from it.

 

[Grim-Reaper]

One of these may help...

http://www.corvetteactioncenter.com/tech/tools/

 

[dongle]

Handy website:

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

Plugging in your numbers comes up with the speedometer reading 2.4% too slow. So when your speedometer reads 60MPH, you're doing 61.4MPH.