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Replacing Speedo gear

1K views 14 replies 9 participants last post by  valleyguy 
#1 ·
1998 4.0 5-spd. I just had my differential gear changed from 3:07 to 3:73; now my speedometer reading is off. Amy one know which speedo gear I need to correct this? Stock 15" wheels and tires.

Thanks,

Joe D.
 
#5 ·
If speedo was correct before and all you did was increase axle gear ratio as posted you went to a 21.5% higher gear number so all you have to do is get tooth number off current speedo gear (it is cast into gear and easy to read) and multiply that number by 1.215 then round to nearest whole number and that is the new gear tooth count you need

This is because your new ratio/old ratio =1.215
 
#11 ·
I got the Rugged Ridge speedo gear, works just fine. I would suggest changing the o-ring also at the same time but try and get an OEM Mopar ring. I purchased the Rugged Ridge o-ring and couldn't get it to stop leaking. No, I didn't pinch it when installing (be careful of that by the way). Went to my local dealership and they had the oem ring in stock. Comparing them side by side, the oem is ever so slightly thicker and works like a charm.
 
#12 ·
What about if you know your speedo difference but don't know gear size?

IE, I have 32" tires on 15" rims and by using GPS I know my speedo is about 4~5 mph low. Speedo reading 60 I'm actually doing 64/65 mph.

I don't know what gear ratio I have. Can I calculate what speedo gear I need to correct, or do I need to find out the gear ratio to order the correct one?

Thanks
 
#13 ·
Odds are very high your axle ratio is 3.73 which is what all standard TJs came with when they had the Dana 44 axle option. Only the Rubicon and '97 TJ did not come with 3.73 when they have the Dana 44.

The only reason it wouldn't be a 3.73 is if a previous owner regeared it which is unlikely given its tire size and build.
 
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