th400 speedometer gear help

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.

wes104

Junior Member
Joined
Sep 8, 2015
Posts
12
Reaction score
0
Location
oklahoma
First Name
wesley
Truck Year
1989
Truck Model
r3500
Engine Size
454
i have a 89 gmc r3500 with a th400 i just rebuilt and noticed that the speedometer is accurate to about 55 than at about 60 its 2-3 miles slower and at 65 its about 3 to 5mph slower. it currently has a 16 tooth drive gear and 40 tooth driven gear in it.i was wondering if going to a 34-39 housing and 39 tooth gear fix that?
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,121
Reaction score
6,038
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
i have a 89 gmc r3500 with a th400 i just rebuilt and noticed that the speedometer is accurate to about 55 than at about 60 its 2-3 miles slower and at 65 its about 3 to 5mph slower. it currently has a 16 tooth drive gear and 40 tooth driven gear in it.i was wondering if going to a 34-39 housing and 39 tooth gear fix that?
sounds like a speedo or cable issue unless the gear is worn visibly, i would inspect it and your cable. if its accurate TOO 55 that tells me something is working right enough for quite a bit
 

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
2,338
Reaction score
3,569
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
I would agree. The speedo is calibrated with a specific drive/driven gear depending on tire diameter and rear end ratio. It should be accurate at any speed if the gears are correct. Sound like the speedo itself is the problem.
 

Forum statistics

Threads
41,856
Posts
903,586
Members
33,368
Latest member
TheDuke
Top