speedometer wrong

[idman]

So this may sound dumb to some of you. My speedometer is wrong, it appears to be reading high.  It was reading about 70mph when a buddy behind me said we were only going 50mph.  What would cause this?  I dont think either of the previous owners changed the gear ratio on the axle, and it has 238/75/15 tires. What else could be causing the speedometer to be off so much, and is there anything I can do to fix it?  Thanks in advance.

D.

 

[idman]

Sorry, forgot to mention, it is 1984 with 302 auto trans.

D.

 

[Bully Bob]

Well, for sure one of the two vehicles is giving a false reading.

RU sure it's yours..??    That would be the 1st. step. To, in fact, be sure it's yours.

Does the other veh. have "large" tires..? That would slow his speedo down.

On your rig, does the speedo needle shake/wiggle..?  This usually indicates a worn/binding speedo cable.

 

[miesk5]

yo,

As BOB Advised;

and you can use state/county/local road mile marker poles to check your speed; do steady 60 MPH and your odometer will or should show exactly one mile between markers and time will be a mile a minute.

Gear Ratio Determination; "...remove the differential cover or the 3rd member and count the number of teeth on the gears. Unfortunately, this can get messy. One somewhat easy yet very reliable way is to jack up the wheels, disconnect the driveshaft, rotate one wheel, and count how many revolutions the pinion yoke/****** makes. If you turn one tire & the other one either stays still while the pinion rotates, or the other one rotates backward while the pinion stays still, or something in between, then you've got an open diff or a very weak LSD (like a Trac-Lok with too many miles on it). In this case, you need to secure one wheel (perhaps by lowering the tire onto the ground), then you can figure your gear ratio by rotating the airborne tire TWO full rotations and counting the number of times the pinion rotates. 3.73 turns means 3.73 gears. If you only rotate the tire once, then 1.865 turns means 3.73 gears. If you turn one tire & the other one turns the same number of turns in the same direction, then you've got a good LSD or locker. In this case, you can just rotate the tires ONE full rotation and figure your gear ratio by counting the number of times your pinion rotates. 3.73 turns means 3.73 gears..." Read More
Source: by Obi-Wan

http://www.jedi.com/obiwan/jeep/misc/gearratio.html

 

[idman]

So my friend has stock tires, so his shouldn't be wrong, plus it is a lot newer, so I would assume it is more likely to be more accurate, not that it is necessarily correct.  I will try timing the mile markers. The needle doesn't really shake or wiggle, but at lower speeds it seems to climb really fast.  What else would cause the speedometer to read faster than I am actually going?  Because bigger tires cause it to read slower, don't they?  If the gear ratio was changed, which way would cause it to read faster?

 

[miesk5]

yo ID,

Speedo Calculator with Tire Dia Change

http://www.csgnetwork.com/speedocalibcalc.html

Correct; bigger tires cause speedo to read slower

Actual Speed Change From Ring Gear And Pinion Change Calculator

 http://www.csgnetwork.com/rgpspeedcalc.html

 

[idman]

Thanks miesk, I checked both of those calculators, it looks like one of the previous owners may have either went with like a 4.10 gear or had it corrected for like 31" tires. I will do some more checking, hopefully I can get this fixed, cause it is obnoxious as heck.  Thanks guys, I will let you know what I find out.