Another Speedo gear Question.

[hopp150]

Okay heres what i know, i have a 17 tooth gear in the transfer case right now as well as 265 75 15 tires, and the tag on the axle says 3:54s. I'm off by 10 miles per hour. Based on GPS speed when i'm doing 30MPH my speedo is reading 20MPH. So, i know i need to go down a gear but from what i understand i can only go to a 16 tooth, does anyone think this is going to actually fix my issue or just make it a little closer? At this point the only way i can see getting it 100% accurate would be going back to a smaller tire.

Also, i'm assuming based on all of this i dont actually have 3:54 gears.

Can someone please help me understand what i'm dealing with and how to fix it. Thanks!

 

[mp]

I would go to the Ford dealership, buy the 6 dollar 15 tooth gear, and spend the 5 minutes to swap it out, then compare you new readings with a GPS. If not perfect, it will be closer to correct.

 

[mp]

I also don't think one tooth will get you down 10mph. Have you measured your wheel circumference and used one of the online calculators?

 

[Broncobowsher]

Drive faster, seriously

I don't think the 20/30 MPH readings are accurate. You are showing a 33% speedometer error on a tire that is real close to stock size. That just doesn't make sense.

Getting deeper into gauge logic there are 3 factors. Gain, offset and linarity.
Gain is controlled by the speedo tooth count. It is a multiplier factor. A tire 10% taller will need a 10% change in gearing to bring back to original.
Offset is where the needle is installed. Basicly a bent needle or a needle put on the wrong place. In this case if it reeds 10MPH slow at 20 MPH it will also read 10 MPH slow at 100 MPH.
Last in linarity, this is an error at specific points. say 1MPH slow at 20, even at 40, 1MPH fast at 60 and even again at 80.

Often you are looking at a mix of all of these being out of whack. Simple gearing and/or tire change will affect only the gain. This is generally what is fixed with a speedo gear swap. But your error of 20=30 is way too severe for the tires you are running. But we only see a single data point so it is impossible to tell if you have a gain error or an offset error (did anyone take the speedo needle off then put it on wrong? Maybe to redo gauges?). and linarity error you are pretty much totally screwed with and either live with it or find a new speedo head and hope it is better.