Calibrating speedometer.

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

fast_n_low86

fast_n_low86

Junior Member
Joined
Feb 17, 2023
Posts
26
Reaction score
32
Location
Sturgeon County, Ab
First Name
Daniel
Truck Year
1986
Truck Model
c1500
Engine Size
7.4L
I recently replaced my 3.08 gearing to 3.42 on my 8.5" and since then my speedo is out to lunch when I'm trying to do highway speed (100km/h). Needle is buried when I'm only doing 109km/h.

How could I recalibrate this. Also have my winter tires on and the are 235/75r15. Also will have to adjust for my summer tires which are 235/70r15 rear and 225/55r15 front. Transmission is th350c.
 

Attachments

  • 20230207_191127.jpg
    20230207_191127.jpg
    150.9 KB · Views: 32
  • 20221229_155737.jpg
    20221229_155737.jpg
    173.1 KB · Views: 30
WebMonkey

WebMonkey

Full Access Member
Joined
Jan 3, 2020
Posts
686
Reaction score
1,402
Location
Missouri Ozarks
First Name
Chris
Truck Year
1985
Truck Model
K20
Engine Size
350
you have to change the speedo drive gear.
in the transmission if you're talking about the 2 wheel drive truck in your profile.
 
fast_n_low86

fast_n_low86

Junior Member
Joined
Feb 17, 2023
Posts
26
Reaction score
32
Location
Sturgeon County, Ab
First Name
Daniel
Truck Year
1986
Truck Model
c1500
Engine Size
7.4L
Yes. That is the one. I've never done any work in a transmission before so I'm not sure how to go about that or anything
 
R

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,066
Reaction score
5,914
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Look up something that has the same gear ratio,transmission,and approximately the same size tires order transmission speedometer gears for that,pull the driveshaft and the transmission tail shaft. It is totally self explanatory from there.
 
I

idahovette

Full Access Member
Joined
Aug 7, 2016
Posts
6,556
Reaction score
14,083
Location
Weiser Idaho
First Name
Perry
Truck Year
1975-1979
Truck Model
K20-K10
Engine Size
350
Should be able to just change the driven gear and get satisfactory results??......unless it was not correct from the start.
 
R

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,066
Reaction score
5,914
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Well things just keep getting easier. Now there is this handy calculator that figures everything for you.

TCI Auto Speedometer Gear Calculator - TCI® Auto

Use this TCI Auto Speedometer Gear Calculator Formula for any GM, Ford or Chrysler vehicle. This is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line!
www.tciauto.com www.tciauto.com
 
75gmck25

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
2,038
Reaction score
1,833
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
I’ve never changed one myself, but aren’t the gears color coded so it’s easier to tell which one you have?
 
B

Bennyt

Full Access Member
Joined
May 17, 2019
Posts
1,041
Reaction score
1,457
Location
Surprise
First Name
Ben
Truck Year
1977
Truck Model
C10
Engine Size
350
Ricko1966 said:
Well things just keep getting easier. Now there is this handy calculator that figures everything for you.

TCI Auto Speedometer Gear Calculator - TCI® Auto

Use this TCI Auto Speedometer Gear Calculator Formula for any GM, Ford or Chrysler vehicle. This is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line!
www.tciauto.com www.tciauto.com
Click to expand...
This is great. Exactly what I've been looking for!
 
Matt69olds

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
2,352
Reaction score
3,600
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
75gmck25 said:
I’ve never changed one myself, but aren’t the gears color coded so it’s easier to tell which one you have?
Click to expand...


They are color coded, but unless they are new gears it’s best to count teeth. They tend to discolor after 30 years in trans fluid.
 
You must log in or register to reply here.

Similar threads

AyWoSch Motors
How much does tire size effect speedometer?
Replies
13
Views
2K
78 Jimmy 2WD
78 Jimmy 2WD
T
How to diagnose speedometer head issue
Replies
3
Views
940
SirRobyn0
S
S
GM 8.5" 10 Bolt Rebuild / Knowledge, Opinions, and Advice Requested
Replies
6
Views
2K
Curt
C
mibars
Solving gauge mysteries
Replies
3
Views
845
mibars
mibars
7
Question on tire size for an '85 K20...
Replies
12
Views
4K
Frankenchevy
Frankenchevy

Latest posts

Forum statistics

Threads
42,126
Posts
909,663
Members
33,618
Latest member
1975K20
Top