Ok, so I replaced the tach on my 86 SE 2M6 awhile ago, and i'm thinking about putting a 0-120 speedometer in there, but I want to know if I have to calibrate it or not. it has a 0-85 as of now, and i kinda want to know how fast i'm going passing someone.

Should be direct fit

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

i know but i want to know if i have to calibrate the speedo to show the correct speed

you do nothing to cal vs old one.

If you're very careful, Move the Odo from Old to New.
This keep Odo accurate for State Inspections, sell a car, etc.

Could try match numbers but can break the wheels/gears. Many are a bit delicate after 30+ years.
Plus these Odo's are made w/ anti tampering features so harder to "roll back" odo for Odo Fraud.
Don't clean much for same reason. Just carefully wipe/blow away dust. many cleaners can wreck old plastic and paint.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Is your speedometer read correct now with the 0-85? If it is the speedometer is receiving the correct amount of pulses from the transaxle and you will have no problem with the 0-120. If it is not reading incorrectly now it is not going to read correctly with the 0-120.

Jack

it reads about 3mph less than it's actually going.

quote Originally posted by 86fierorestoproject: it reads about 3mph less than it's actually going.

Likely wrong size tires for that. Or wrong VSS gear in the trans.
But...
If trying to "match" GPS etc... Don't. GPS system isn't made to calc exact speeds for consumer use. Many GPS receive try to calc speeds but not exact speeds.