I know that once my 3.73's are in that my speedo will be off, but I was told that this will affect my odometer as well, is this true. For one thing my speedo has always been off by about 5-7 mph since I got the car and wondered if it was messing with my milage.

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'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
FLOWMASTER, NO CAT
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
160* Stat
2:73 GEARS
Rebuilt 700R4, with manual valve body
B&M Megashifter, 5" Autometer Tach w/shift lite
Soon to be installed
3:73 Richmond Gears and 3" Hooker Aero Chamber
Corvette Servo & .500 boost valve
15.61@ 95.2mph G-Teched, 1-2 shift slippage, and just replaced a burnt plug wire
"There is nothing more addicting than speed"

 

Speedometers can be off even if the odometer is correct and vise versa, the speedo needle is driven magnetically and the odometer is driven by gear. I usually test the odometer by getting out on a highway with mile markers and compare, the tenths on the odometer should be at the same number at each mile marker you pass, if it's off the odometer will adventually read a tenth lower or higher than where it started.
Mile markers aren't always exact either but they're somewhat close.

 

Yes the deeper 3.73's will affect the odometer as well as the speedometer. The speedo will show faster than you are actually going and the odometer will register mileage faster than it should.

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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, current red metallic paint. Lots of other stuff...
82camaro