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HELP

9

98 dually

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HELP replaced ring n pinion, no other changes . original 10x41 tooth (4.10 ) with 11x45 tooth (4.09) NAPA / RICHMOND GEAR part listed as 4.10 ratio . I had a 2mph difference before and now have 14 mph difference , 74 on speedo / 60 on gps . Replaced speed sensor and still have same numbers . 1998 chev c3500 dually 7.4 auto trans Any ideas
 
Hello and welcome aboard. you'll have to have the PCM updated with the new gear ratio to have the spedo read correctly. there are a few guys here that can help you get it done, but I think it will require someone with a tech 2 or some kind of programmer on the obd2 system. for pre 96 obd1 models there is a small box under the PCM behind the glove box that can have the configuration changed without any programming.
 
Sorry I didn't explain my self clearly . I didn't change ratios. I replaced worn gear set with same numerical ratio .the differance being 4.10 to 4.09 , .01 change . they were replacements
Thanks
 
Ok, you lost me! so you pulled the ring and pinion which was a 41 tooth ring gear and 10 tooth pinion and installed a 45 tooth ring with 11 tooth pinion? that is a huge change which will explain the speedo being off. your RPM will be higher also. say where you used to get 2200 rpm at 70mph in overdrive, youll be seeing somewhere close to 2700-3k at the same speed.
 
Yeah, did you verify that what you installed is what you thought went in? From your narrative, you did the install yourself, right? With all the supply chain/mfr issues the past year+ it is very much more than possible what you installed wasn't what was ordered/marked on box. Have had it happen twice now that what I ordered was NOT what came in!

Sounds almost like it now has a 4.88 ring and pinion set in it to get that fast of speedo reading over GPS vs the 4.10!

First, is your tach reading higher than before at highway speed, by about 400-600 rpm? Second, confirm your new rear ratio by jacking up the rear with both tires off the ground, in Neutral, mark a rear U-joint cap and slowly turn one rear wheel one complete revolution while counting the number of revolutions the marked cap makes. Almost five - 4.88. Just past 4½ - 4.56. Just past 4 - 4.10/4.09.

If indeed you did actually swap the worn 4.10 for the Richmond 4.09 you would see no noticeable difference in either speedometer or tachometer readings over the OEM gears.

The only way that you can see this sudden huge difference in speedometer reading without a corresponding difference in tachometer RPMs would be with a fault in the VSS or ECM - which would have to be corrected.
 
dbrannon79 said:
Ok, you lost me! so you pulled the ring and pinion which was a 41 tooth ring gear and 10 tooth pinion and installed a 45 tooth ring with 11 tooth pinion? that is a huge change which will explain the speedo being off. your RPM will be higher also. say where you used to get 2200 rpm at 70mph in overdrive, youll be seeing somewhere close to 2700-3k at the same speed.
Click to expand...
Actually, virtually NO difference. 41/10=4.10, 45/11=4.09.
 
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