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73-87 Chevy _ GMC Trucks
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speedometer gear
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Topic: speedometer gear (Read 3761 times)
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
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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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
bd
Global Moderator
Senior Member
Posts: 6454
Re: speedometer gear
«
Reply #1 on:
October 13, 2017, 06:44:17 PM »
What's the tire size and axle ratio?
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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)
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
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.
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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
bd
Global Moderator
Senior Member
Posts: 6454
Re: speedometer gear
«
Reply #3 on:
October 13, 2017, 07:08:42 PM »
Look at the SPID label in the glovebox.
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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)
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
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.
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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
Re: speedometer gear
«
Reply #5 on:
October 14, 2017, 10:48:15 AM »
Is that the info you was looking for Bd
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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
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.
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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
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.
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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
zieg85
Global Moderator
Senior Member
Posts: 7543
Re: speedometer gear
«
Reply #8 on:
October 19, 2017, 05:13:00 PM »
here is a chart based on some know parameters
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Carl
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/
bd
Global Moderator
Senior Member
Posts: 6454
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.
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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)
Smitty31
Registered Users
Posts: 148
1979 GMC High Siera 4wd & 1986 custom deluxe 2wd
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
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IF ITS WORTH DOING. THEN ITS WORTH DOING RIGHT.
bd
Global Moderator
Senior Member
Posts: 6454
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.
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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)
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speedometer gear