Speedometer wrong speed

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Jul 10, 2016 | 09:03 PM

Phil Fleming

Thread Starter

Cross-Country

Joined: Jun 2016

Posts: 67

Likes: 5

From: Mount Forest Ontario

Speedometer wrong speed

The PO of my 79 F150 converted the truck from a 351 automatic to a 300 4spd manual (T18). Today I was checking my speed against my wife's car and the speedo is displaying low about 20km/hr. My speed shows 60 km/hr (37mph) actual is 80 km/hr (50mph). Any thoughts on how to correct this?

Jul 10, 2016 | 10:18 PM

1320stang

Laughing Gas

Joined: Jul 2003

Posts: 765

Likes: 13

From: Edmond, OK

Change the tire height or the speedo gear on the end of the cable where it enters the trans.

Jul 11, 2016 | 01:46 AM

Phil Fleming

Thread Starter

Cross-Country

Joined: Jun 2016

Posts: 67

Likes: 5

From: Mount Forest Ontario

The tires are OEM size, are there different speedo gears based on transmission type? Does anyone know the required gear for the T18 4spd transmission? The truck is 2WD.

Jul 11, 2016 | 07:30 PM

palmrose2

Tuned

Joined: Sep 2003

Posts: 348

Likes: 2

From: Northern Michigan

37x2=74 50x2=100 Your speedometer is reading 74% true. What you need is a speedometer gear that spins the speedometer 36% faster. You can do this by using a speedometer gear that has 36% less teeth.

Let's say your gear has 21 teeth. You would need to replace it with a 16 tooth gear. 21/16=.762 or 76% Close enough.

I would use GPS to check my speedometer. Who knows if your wife's car is perfect?

I think that the T18 and C6 speedo gears are radically different. I would pull the cable from the trans and see what I'm dealing with and compare to available gears.