The Stovebolt Site Forums The Shop Area General Truck Talk Speedometer Adjustment

Looking for some help here....My odometer registers about 1.25 miles compared to each mile marker on the interstate. How would you interpert that when looking for a speedometer ratio adjustment adapter? Would that mean I need to reduce my speed by 25%? I think that is right, but just trying to be sure. The closest adjustment I can find is minus 26.7%, but I think that should work. Any thoughts?
Thanks,
Joe

http://www.speedometershop.com/ratio.html

I used Method 1, except I took 10-mile measurements at about 60 mph (4 sets of measurement on a marked highway - 20 miles in one direction and 20 miles back). The adapter that they sent me is accurate to within 1% at highway speeds.

1 mile actual / 1.25 shown = .8 or 80% -> 1 mile actual should be 80% of your shown mileage. Reduce by 20% and you're good to go.

I agree with 54 Tow. You want a .80 reduction which is 20 %.

Beautiful truck Joe.

Your math is correct. Your speedo is reading 125% of actual, you need to reduce by 25%.

Since you have a T5, this is a simple and inexpensive fix. Just pull the driven gear out of the tranny (disconnect the speedo cable from tranny, undo the holder, and it'll slide right out. Call the speedometershop guys in the link above and they'll tell you what you need. Should be under $15 bucks.

75% (or .75 of 1.25) is 0.9375
80% (or .80 of 1.25) is 1.0

If he wants it 1 - 1 he will need a 1/5 (or 20%) decrease.

You be right 54. I'll plead innocent by means of a CA "education". At any rate he's gonna need a BIGGER driven gear in order to slow that puppy down.

I think g.m. had a color code system for each driven gear, each color was a different number of teeth, each tooth changed speed about 3 mph?

Joe, Let us know what your final fix was. I need to fix my spedo also. If I can change out the gear in my T5 that would be an easy one.

George

According to what I understand, the gear in the transmission has to be remove by disassembling the transmission or while the transmission is disassembled. I've already installed the transmission and I'm not deinstalling to change a speedometer gear. I have found some speedometer gear ratio adapters on eBay, but I need to be sure of which one I need. I found one that slows down the speedometer 26.7% which fits close to one of the suggestions. I can't find one for 20%. Still looking. I hate doing math problems. Everytime I do one I get a different answer.
Joe

6cyl;
There are 2 gears that control speedo rotation: the drive gear, which is internal to the tranny and requires a tear-down to change out: and the driven gear.

The driven gear is easy to remove, takes about 2 minutes. It's a little plastic thing and is colored coded as to the number of teeth it has. You can count the number of teeth yours has, add 20% and that's the gear you need. Example: yours has 10 teeth. 20% of 10 is 2 (better check my math on this). You'll need a 12 tooth gear. BUT, it's possible they don't make a gear as big as you need, in which case the drive gear would need to be changed (I think there are 3 different drive gears).

Costwise, changing a driven gear is the way to go, it's cheap and easy. If that's not possible, rather than tearing into your T5, I'd look at the adapter you mentioned.

Joe,

As stated the driven gear comes right out. But if your T5 is like mine. It has the red drive gear which is 6 teeth with the grey driven gear which is 22 teeth. That is the lowest ratio there is for that size speedometer set.

Please correct my math if I am wrong, but I believe the options are for 6, 7 or 8 tooth drive gears and 16, 17, 18, 19, 20, 21 and 22 tooth driven gears.

Joe,

I may have been wrong about the options for driven gears.

It looks like you could go all the way up to 25 teeth on the driven gear. I thought you needed something else to go that high.

If you take out the driven gear and look at the drive gear inside the transmission you can tell the number of teeth by the color of the gear.

Let us know what you have and we can do the math as a group project.

I'll get on that tomorrow (Friday).

Thanks, Bill.

Joe

Bill-
I removed the speedometer cable and the gear right inside the transmission is a blue gear. I didn't go any further because I wasn't sure what else I should really be looking for. There is a retainer tab that holds the small housing that has the blue gear in it, or maybe it holds the blue gear. Anyhow, that's as far as I got with it today. Thought I would get to it Friday, but other stuff keeps coming up. Nothing is as important as truck stuff, but I have to act like other stuff is more important.
Joe

Joe,

I founnd this info on the web. If you have the blue driven gear, that would have 20 teeth. From your earlier post you need to cut the speed by 20%, so you would need to go to 24 tooth driven gear to achieve that. From this data, I don't think you can do that with out changing the drive gear. That can only be done by removing it from the shaft inside the rear housing.

You could try to call someone like Hanlon Motorsports: http://www.hanlonmotorsports.com/

or theses guys: http://www.5speeds.com/ to see if they make other driven gears.

Here is the info:

SPEEDOMETER DRIVEN GEAR

New OEM AC Delco gears are used in Muncie, GM T10, GM Super T10, Saginaw 3 & 4 speed, GM T5, Powerglide, 200 & 350 transmissions.

18 tooth brown dimension -1.75 x .305 x .810
19 tooth white dimension -1.75 x .305 x .810
20 tooth blue dimension -1.75 x .305 x .810
21 tooth red dimension -1.75 x .305 x .810
22 tooth grey dimension -1.75 x .305 x .810
18-22T .810 diameter gears must be used with a 1.84 OD speedo drive gear.
22 tooth green dimension -1.75 x .305 x .870
23 tooth black dimension -1.75 x .305 x .870
24 tooth yellow dimension -1.75 x .305 x .870
25 tooth orange dimension -1.75 x .305 x .870
22-25T .870 diameter gears must be used with a 1.76 OD speedo drive gear.

I have a grey 22 tooth gear in a spare transmission in the basement, I could mail it to you if you want to see what a 10% reduction would get you.

waldo53, thank you for the complement on my truck.

Bill -
If I have to disassemble the transmission to change the speedometer gear, I'm thinking more in terms of waiting until I have to pull it for an additional reason. I can't see pulling the tranny to replace the speedometer gear.
I thought maybe I could pull the housing which surrounds the driven gear and change the blue gear to a yellow gear. Since you have messed with this, what kind of wrench or tool do I need to remove that housing around the driven gear. I appreciate the help! If I can get that housing off, I'll be looking for a yellow gear.
Thanks,
Joe

Can anyone tell me how to remove the retainer that holds the driven gear at the tail of a T-5 transmission?
Thanks,
Joe

Joe,

Check with these guys -

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

They have the gears and they also make ratio adapters.
Since they're just over in Bossier City, they have pretty quick response time, too.
I got a 10% ration box from them a month or so back.

regards,
Leon

Remove the retainer and the speedometer cable.
Pull straight out (you may have to use a pair
of pliers). It's a tight fit and has an "O"
ring oil seal.

waldo53 -
Thanks! I just about figured that it had to be a push in fit after I removed the retainer and was stopped in my tracks as to any other way to go. I'm sure this is also a good way to drain the transmission fluid.
Joe

So...the one I had made by the Speedometer shop is wrong too. I guess I just need to start over with this

Hi Dogface,

I followed Cmayna's suggestion on a previous post and called Jim at PATC, 888-877-1008. He wanted speedo reading when the true speed was 40, 50 and 60. Had to get Speedbox app or my iPhone, which showed correct speed. Adaptor cost $79, but easy to install and speedo is right on now.

I initially got the wrong one, too! Didnt have to do calculations, but I still got it wrong. I had given him true speeds when the speedo read 40, 50 and 60. Sent it back and he rebuilt it. Only additional cost was shipping. Sure is nice to have speedo showing correct speed.

Bob

Bob,

I called him the other day. Then got the measurements backwards the same as you... LOL...Now will get it right. I have GPS Speed ap for my iPhone and it works like a charm..

Mark,

Yeah...even though it was actually backward of how they want it, seemed more logical to get true speeds when the speedo is on the 40, 50 and 60.

By the way, the door art on your truck is fantastic.

Bob