Tachometer And Speedometer Error...
Senior Member
Joined: Jan 2005
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From: Suwanee, GA & Montana
i just moved from 33's to 35's two days ago- the tach used to read 2k @ 70mph- now while my speedo is off for sure now, i now read 2k at roughly 74-76mph i suppose the tach could remain constant as it is an engine based component rather than transmission? (ie, speedo, odometer) do you have a tuner? an easy way to check would be to update the tire size in the tuner, and reload to see if things change
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Joined: Mar 2006
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From: Peoria, AZ
I wouldn't think tach would be affected by tire size because it should be read right off the motor. Your speed would be different at the same RPM because of tire size, but an RPM is an RPM. But I have been drinking today! 
And learning to use the smileys! 
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Joined: Aug 2005
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From: South Carolina
Originally Posted by BalogUK
i just moved from 33's to 35's two days ago- the tach used to read 2k @ 70mph- now while my speedo is off for sure now, i now read 2k at roughly 74-76mph i suppose the tach could remain constant as it is an engine based component rather than transmission? (ie, speedo, odometer) do you have a tuner? an easy way to check would be to update the tire size in the tuner, and reload to see if things change
Assuming your speedometer hasn't been corrected, you can calculate your correct speed with the taller tires by using the following equation: (new tire size)/(old tire size)*(current speedometer reading). In this case, When your speedo reads 70 MPH, your true speed is 35/33*70 = 74.2 MPH. So, with 33s at 70 MPH, your tach reads 2K. With the 35s at an uncorrected speedo reading of 70 MPH, you should actually be going 74.2 MPH, and the tach should read 2K. (It sounds as if the speed you reported above with the taller tires is from a GPS?)
Hope this makes sense.
Last edited by C-17 Pilot; Jun 16, 2006 at 07:25 PM.
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Joined: Jan 2005
Posts: 2,436
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From: Suwanee, GA & Montana
Originally Posted by C-17 Pilot
Taller tires will affect speedo and odometer; the tach is unaffected.
Assuming your speedometer hasn't been corrected, you can calculate your correct speed with the taller tires by using the following equation: (new tire size)/(old tire size)*(current speedometer reading). In this case, When your speedo reads 70 MPH, your true speed is 35/33*70 = 74.2 MPH. So, with 33s at 70 MPH, your tach reads 2K. With the 35s at an uncorrected speedo reading of 70 MPH, you should actually be going 74.2 MPH, and the tach should read 2K. (It sounds as if the speed you reported above with the taller tires is from a GPS?)
Hope this makes sense.
Assuming your speedometer hasn't been corrected, you can calculate your correct speed with the taller tires by using the following equation: (new tire size)/(old tire size)*(current speedometer reading). In this case, When your speedo reads 70 MPH, your true speed is 35/33*70 = 74.2 MPH. So, with 33s at 70 MPH, your tach reads 2K. With the 35s at an uncorrected speedo reading of 70 MPH, you should actually be going 74.2 MPH, and the tach should read 2K. (It sounds as if the speed you reported above with the taller tires is from a GPS?)
Hope this makes sense.
i used the same equation above to figure out my 74-76mph at 2k rpm... was on a 2 hr drive earlier, and needed to figure out how fast i was going to avoid the montana state troopers in blacked out dodge chargers...
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Joined: Mar 1998
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From: Virginia
Changing either gears or tires will affect the # of rpms the engine turns for a given sped & gear - that's the proper way to describe it.
Changing gears or tires alters the speedometer, odometer and the automatic transmission's shift points as well.
Changing gears or tires alters the speedometer, odometer and the automatic transmission's shift points as well.