Speedo adjust after gear/tire change

[Creig]

Some of you may have seen the '87 SuperCab 4x4 I built for my daughter in the garage section. Well, it's now time for me to start fixing all the little things that I didn't have time to do during the main build.

First on my list is to get the speedo corrected. I've read the posts regarding speedo gear changes but am not sure if I somehow missed something. Here's the background:

I believe I had originally had 3.45 gears with the stock 27" tires. I went to 4.10 gears and 32" tires. What's weird, is that I thought I had calculated it out before I started and the speedometer should have been close to actual after the gear/tires swap. At 62 mph, going from the 3.45 to 4.10 should have made a speedometer read 11 miles per hour fast while moving from 27" tires to 32" tires should have made the speedo read 8 miles per hour slower. It should have been pretty much a wash. But instead, the speedo will read 45 mph while an onboard GPS reads 62 mph. So I pulled the speedo cable out of the transfer case and it's the yellow gear, which should be the 18 tooth.

Following the formula for calculating the error percentage:

Error = (62 mph - 45 mph) / 62 mph = 0.274 x 100 = 27.4% fast... Still confused. I don't see how it's possible that the speedo is that far off.

So going by that:

Yellow 18 tooth gear x (1 - 0.274) = 18 x .726 = 13.068 which is a 13 tooth gear. Which I don't see on the list. The lowest gear count is the maroon 16 tooth.

Just to double-check, I went to rocky-road.com and used their online calculator. Entering a 27" stock tire, 32" new tire and original gear ratio of 3.45 yields a proposed gear ratio of 4.089, which is close enough to the 4.10 I installed.

So what gives? Am I missing something? How can I get the speedo to read correctly again?

 

[85_Ranger4x4]

Did your build include a different t-case?

There can be different drive gears inside the t-case that would mess up your calculations.

I used the formula and info in the following thread when I picked my gear. It was a tooth off of ideal but was as close as I could get. As near as I can tell it is pretty close.

http://www.therangerstation.com/forums/showthread.php?t=113620&highlight=correction

 

[Creig]

Nope. I haven't touched the drivetrain other than swapping the 3.45s for 4.10s and going from an open diff rear to limited slip.

 

[85_Ranger4x4]

According to my calculations:

If you have a 6 tooth (dark red) drive gear you need a 16 tooth (maroon) driven gear.

If you have a 7 tooth (white plastic coatied in red ATF) drive gear you need a 18 tooth (yellow) driven gear... which is what you already have.

(using the Yokohama Geolandar A/T-S 659 revs per mile, it was the middle of the three in your size on Tirerack, I couldn't find that spec for your exact tire anywhere)

Stock with 3.45's and 27" tires if you have a 6 tooth you should have had a 16 tooth, 7 tooth equals 18 tooth driven gear so you are correct, it should have been a wash.

If everything else lines up it sounds like you either got jipped on your gears (wrong ratio) or you have a problem with your cluster. I don't know if there is anything that could go wrong with your drive and/or cable but that may a possiblity also. One other thing to point out, since I see you have overlays on your guages (which look awesome BTW ) it could be that the hole is too small or off center very slightly creating drag on the shaft the needle sits on. My brother works at a Ford dealership and has a couple come in for that problem.

Nice "Torch Red" truck