Gear swap 1/2 done. What drives speedo ? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 10-29-2003, 12:47 PM
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Gear swap 1/2 done. What drives speedo ?

I am switching out gears from 3.73 to 4.10. As some of you are aware, I bought a complete 14 bolt sf rear end with new 4.10's in it. I haven't done the front yet, still has the 3.73. Need to wait for the next paycheck [img]images/graemlins/burnout.gif[/img] At 1900 RPM with the 3.73, the speedo used to be at 65 mph. Got on the highway this morning fully expecting the speedo to register different with the 4.10s in the rear but the speedo still said 65 mph at 1900 rpm.

Can someone explain to me why?

Thanks,
Harmsie
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post #2 of (permalink) Old 10-29-2003, 01:07 PM
 
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Re: Gear swap 1/2 done. What drives speedo ?

yuor speedometer doesnt actually measure your vehicle speed. it measures that indirectly. your speedo runs off of a sensor that is on the output of your tranny. it essentially measures the angular velocit of your driveshaft. if you take this number and then devide by the geear ratio you'll have the axles angular velocity. multiply that by the rolling radius and you've got your linear velocity relative to the ground...i.e. speed.
if you give me your initial gear size, and tire size, and also your final tire and gear size i can figure it out for you.
or you can go to www.4lo.com and play with their gear and tire size calculators.
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post #3 of (permalink) Old 10-29-2003, 01:57 PM
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Re: Gear swap 1/2 done. What drives speedo ?

Thanks, I've been beating myself up on this. I'm starting to doubt my decision to go with the 4.10 gears and think maybe I should have gone with the 4.56. I believe that I have read on the forum here that 4.10 was the way to go.

When I bought the truck it had 245/75R15 with 3.73 gears. Then I lifted it and was running 35's with the 3.73 gears. Now I have the 35's and 4.10 gears installed in the rear axle.

Thanks
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post #4 of (permalink) Old 10-29-2003, 04:34 PM
 
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Re: Gear swap 1/2 done. What drives speedo ?

i dont have time to grind out all the calculations sicne i have an exam tomorrow, but it looks like you'll be off by about 10%
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post #5 of (permalink) Old 10-29-2003, 07:07 PM
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Re: Gear swap 1/2 done. What drives speedo ?

Your gonna be reading over by only 3.3%
speed says 60, you are doing 62
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post #6 of (permalink) Old 10-31-2003, 07:41 AM
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Re: Gear swap 1/2 done. What drives speedo ?

reading under what actual speed is, like mine speedo says 75 Im actualy doing 85
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