Speedometer Correction
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Posting Guru
Joined: Jan 2013
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From: VA
Speedometer Correction
I'm going to be putting larger tires on my truck soon, and I was wondering if there is a way to correct my speedo so that it's accurate (or at least fairly close). I'll be going from 33" tires to 37"s, so it'll be a pretty good difference. It's not necessary to get the speedo right since I could just watch the tach, but it'd be nice to have it working properly.
Is there an adjustment that needs to be made, or additional parts/replacement parts needed to get the speedo to read the correct speed?
Thanks in advance!
Is there an adjustment that needs to be made, or additional parts/replacement parts needed to get the speedo to read the correct speed?
Thanks in advance!
Cargo Master
Joined: Jan 2004
Posts: 3,118
Likes: 149
From: South Dakota
When you get your bigger tires on if you know a stretch of road where you know the mile lines drive the truck at 60 MPH and it should take a minute to go a mile. Or you can go at any speed and use a factor of 3600. For example if you go 40 MPH it should take 90 seconds to go a mile, or at 50 MPH it should take 72 seconds to go a mile.
Figure what percentage your speedometer is off then pull the speedometer cable drive gear out of the transmission or transfer case and count the teeth. Get a gear with whatever percent LESS teeth your speedometer was off and you should be pretty close.
Figure what percentage your speedometer is off then pull the speedometer cable drive gear out of the transmission or transfer case and count the teeth. Get a gear with whatever percent LESS teeth your speedometer was off and you should be pretty close.
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Posting Guru
Joined: Jan 2013
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From: VA
When you get your bigger tires on if you know a stretch of road where you know the mile lines drive the truck at 60 MPH and it should take a minute to go a mile. Or you can go at any speed and use a factor of 3600. For example if you go 40 MPH it should take 90 seconds to go a mile, or at 50 MPH it should take 72 seconds to go a mile.
Figure what percentage your speedometer is off then pull the speedometer cable drive gear out of the transmission or transfer case and count the teeth. Get a gear with whatever percent LESS teeth your speedometer was off and you should be pretty close.
Figure what percentage your speedometer is off then pull the speedometer cable drive gear out of the transmission or transfer case and count the teeth. Get a gear with whatever percent LESS teeth your speedometer was off and you should be pretty close.
Thanks
Logistics Pro
Joined: Mar 2011
Posts: 4,197
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From: Central Wisconsin
To make your speedometer read correctly, you have to change the speedometer gear that is plugged into your transfercase. Whatever is in there now is already wrong based on you having 33" tires.
I've also just had a buddy with a known correct speedometer drive in front of me on the highway and do different speeds, like 5 mph increments from 15 to 70 to figure out about where I need to be to be on the speed limit. I have a pretty good memory though. I guess if you don't have a good memory, you're down to watching the speedometer and having the speeds written on a sticky note on the dash, using a gps with a speedo, or switching the gear.
I've also just had a buddy with a known correct speedometer drive in front of me on the highway and do different speeds, like 5 mph increments from 15 to 70 to figure out about where I need to be to be on the speed limit. I have a pretty good memory though. I guess if you don't have a good memory, you're down to watching the speedometer and having the speeds written on a sticky note on the dash, using a gps with a speedo, or switching the gear.
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Posting Guru
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From: VA
Actually, it's pretty close because rear end used to be 4.10 and it had about 29.5" stock tires. Now with 4.56 and 33"s, it's only off by maybe 1-2 MPH at 60MPH.
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