Author Topic: speedometer gear  (Read 3677 times)

Offline Smitty31

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speedometer gear
« on: October 13, 2017, 06:35:34 PM »
My truck is a 1986 custom deluxe 4.3L 4 BBL 400 turbo trans. 2wd. My speedometer gear is a medium green i believe it had 42 teeth on it. Because of tire size my speedometer is about 5 mph off. So when it shows say 45 its actually running 50. My question is there a gear that will correct that ? I know you can get different gears just not sure which one.
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Offline bd

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Re: speedometer gear
« Reply #1 on: October 13, 2017, 06:44:17 PM »
What's the tire size and axle ratio?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Smitty31

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Re: speedometer gear
« Reply #2 on: October 13, 2017, 06:50:53 PM »
275/60 15 in. i dont know about the axle ratio and don't know how to find out.
IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.

Offline bd

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Re: speedometer gear
« Reply #3 on: October 13, 2017, 07:08:42 PM »
Look at the SPID label in the glovebox.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Smitty31

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Re: speedometer gear
« Reply #4 on: October 13, 2017, 07:34:02 PM »
2.75 ratio Is that a good ratio for gas mileage can you tell me how it stacks up against other ratios in rear-ins? I know less than than nothing about rear-ins lol.
IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.

Offline Smitty31

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Re: speedometer gear
« Reply #5 on: October 14, 2017, 10:48:15 AM »
Is that the info you was looking for Bd
IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.

Offline Smitty31

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Re: speedometer gear
« Reply #6 on: October 15, 2017, 02:21:20 PM »
I misread the SPID label the other day the gear ratio is not 2.75 but is 2.73 sorry guys for the wrong info. Also it saids its a 1Gu2 model if that helps at all. So again tire size 275/60, ratio 2.73 and I have the medium green speedometer gear. Presently my speedometer is off 5 mph at 45mph it is really 50mph due to tire size. So I was wondering if there was a different speedometer gear that would correct this.
IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.

Offline Smitty31

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Re: speedometer gear
« Reply #7 on: October 19, 2017, 05:06:00 PM »
This is over my head big time any help would be great guys.
IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.

Offline zieg85

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Re: speedometer gear
« Reply #8 on: October 19, 2017, 05:13:00 PM »
here is a chart based on some know parameters
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline bd

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Re: speedometer gear
« Reply #9 on: October 19, 2017, 06:12:26 PM »
I've been looking at that chart for several days off-and-on in spare time....



The gear combination you currently have in the truck suggests the original tire size was P195/75R15 (26.5" diameter).  The P275/60R15 tire you presently are running has a diameter of 28".  Using the gear combination specified for a P225/75R15 (28.5" diameter) would shift the speedometer error to ~1.5 MPH fast, instead of the current 5 MPH slow.  Or, take the vehicle to a local speedometer shop for retrofit of a ratio adapter that will calibrate your speedometer exactly.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Smitty31

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Re: speedometer gear
« Reply #10 on: October 19, 2017, 06:27:11 PM »
I may have misspoke it is running 5 mph faster than it shows according to the gps so at 50 your really going 55
IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.

Offline bd

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Re: speedometer gear
« Reply #11 on: October 19, 2017, 07:57:46 PM »
Correct.  Your speedometer is indicating 5 MPH slow @ 55 MPH actual, running a tire that is 1.5" oversized for the original factory speedometer calibration.  So, if you change the factory calibration to a tire diameter that is 28.5" tall compared to the 28" tall P275/60R15, the speedometer will be slightly off in the opposite direction using the new gearing.  This is because, after recalibration, the P275/60R15 will be 0.5" undersized for the new speedometer gear combination.  The recalibrated speedometer reading should be ~1.5 MPH too fast, indicating 56.5 MPH @ 55 MPH actual.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)