# Speedometer not reading right



## trainman115 (Jul 31, 2014)

Hi Guys I have this problem with my 69 GTO my speedometer is reading 10 miles a hour to fast I have a Turbo 400 trans with 3:73 gears in the rear I have R225/70/14 tires which are 26.4 " my drive gear has 18 teeth and the driven gear has 38 teeth I look at the calculators and they are telling me I need a 51 tooth driven gear which they dont make any ideas what I do now Thank


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## Pinion head (Jan 3, 2015)

A gray 44 tooth driven gear was correct with Turbo 400, G70-14's, 14x6's, & 3.55's. That is stock driven gear in the original '71 PQ turbo 400 in my HO GT-37. Have built a really HD 3.73 rear for the GT-37, & it will also be running 26.4" tall (G70-14) tires, but car is still in restoration phase, & I have yet to address the driven gear in the PQ. we both may need a specific speedo reducer, but I don't think so. Will try & look late tonight in some old GM master parts books.


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## 64GTOConvertible (Aug 28, 2016)

Speedo error is fun! When you say 10MPH too fast, is it a linear error or 10MPH at a certain speed? How do you know?

Here's a fun way to figure speedo error. Find a measured mile and drive 50MPH, timing the mile from start to finish. Perfect time is 1m12s, or 72 seconds. Find your error and figure the percentage off, or 'factor'. Example. Suppose you ran the mile in 1m22 seconds. (82 seconds). Your error is 10/72, or .1389 (13.89%). Because you are slow, you need to speed up your car by that percentage.

How would you apply this? Well, take tire size and rear gear out of the equation. Your drive/driven gear is 2.11 (38/18) You need it to be 2.40(ish) (2.11 * (1.1389)) In other words, you're slowing your speedo cable down by reducing the driven gears RPM, and speeding up the car relative to the speedometer reading. If you were to keep the same drive gear, you would want a 43 tooth driven gear in this example. If they don't make one, see if you can find a drive/driven gear combo that makes that ratio.

It gets pretty confusing, and I figured the math on the spot, so there may be some errors in logic... Sorry!

BTW, this works with any distance or speed. 1 mile at 30MPH takes 120 seconds, 1.5 miles at 60 takes 90 seconds, etc. The longer the distance and the steadier you can hold your speed the better your calculations will be. And BTW, this is similar to how the Great Race participants figure out their error. Those guys get it PERFECT!


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## 64GTOConvertible (Aug 28, 2016)

On further consideration I changed the direction of my change. I'll probably rethink it again... That's how these things work. If you do the math and it makes things worse, reverse it.


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## PontiacJim (Dec 29, 2012)

Wouldn't it be simpler to drive at 60 MPH than 50 MPH and use the mile marker found on the side of any highway? 60 MPH is 1 mile a minute. There are 60 seconds in 1 minute, therefore in one minute at 60 MPH you should have gone from one mile marker post to the next. If you got to the next post in 60 seconds and your odometer has not registered over 1 mile, you need to speed up the speedo. If you go past the next mile post in 60 seconds and your odometer has registered more than 1 mile, you need to slow down your speedo.

Of course there is a more accurate way. Find a local police officer, or go down to your local police office, and get them to use a radar gun on your car to accurately tell you your MPH. No doubt they would be willing to help you out with this rather than ticket you for driving too fast based on speedo error. 

Me, I'd just drive by a radar cop at an indicated 100 MPH, wait for the cop to catch up with me on the side of the road, then argue with him that the radar gun must be calibrated wrong because my speedometer said I was doing 115 MPH and NOT 100 MPH. :crazy:


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## 1968gto421 (Mar 28, 2014)

Go Jim!! I love it.

Most of the Law Enforcement agencies around here when they notice a "speeding problem" on certain roads or streets, before setting up radar traps, they will use these portable radar units set up on the side of the road as you approach them, they show your speed in large digital numerals which change if you slow down (or speed up if you're ornery). I learned from a cop that they keep these well calibrated, and so they have been very useful for me in figuring speedo error in different cars over the years when I changed rear axle gears, wheels, tire sizes, etc.

TCI Transmission sells trans speedo gears and have been helpful in determining the correct one. Here is their website for speedo calculators and gears available for GM tranny's------- http://www.tciauto.com/tc/speedometer-gear-calculator

Also, Wallace Racing has some excellent speedo calculators for determining what you need--------- Speedo Gear Calculator - Wallace Racing

Hope this is of some help!!


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## 64GTOConvertible (Aug 28, 2016)

PontiacJim said:


> Wouldn't it be simpler to drive at 60 MPH than 50 MPH and use the mile marker found on the side of any highway? 60 MPH is 1 mile a minute. There are 60 seconds in 1 minute, therefore in one minute at 60 MPH you should have gone from one mile marker post to the next. If you got to the next post in 60 seconds and your odometer has not registered over 1 mile, you need to speed up the speedo. If you go past the next mile post in 60 seconds and your odometer has registered more than 1 mile, you need to slow down your speedo.
> 
> Of course there is a more accurate way. Find a local police officer, or go down to your local police office, and get them to use a radar gun on your car to accurately tell you your MPH. No doubt they would be willing to help you out with this rather than ticket you for driving too fast based on speedo error.
> 
> Me, I'd just drive by a radar cop at an indicated 100 MPH, wait for the cop to catch up with me on the side of the road, then argue with him that the radar gun must be calibrated wrong because my speedometer said I was doing 115 MPH and NOT 100 MPH. :crazy:


Not really any simpler, and you can use the mile markers. They just aren't all set up exactly 1 mile apart. Ask me how I know... 

Radar works as well. My method is what we use to calibrate at Time Speed rallies, and it is pretty damned accurate. The 'problem' with cable driven speedos is the error is not usually linear. Accurate at 55 is not necessarily accurate at 40. And granted, close is good enough for normal driving.


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## Pinion head (Jan 3, 2015)

Went through several files I have of receipts to ck what drive & driven gears have used before. also went out & measured a near full tread G70x14 on a 6" rally wheel & an old trunk spare with a full tread F70x14 on a 7" wheel. The latter was fairly close to 26.25" tall. The G70x14 was closer to 26.75" tall. Both were measured as aired up loose tire/wheels. On a tire mounted on a vehicle, you want to take a fabric tape measure, measure the circumference, & then dive by 3.14.

Last, pulled out three early Pontiac master parts books that have charts with: body style (important because of wheel width), axle ratio, tire size, & original speedo driven gear usage, & whether a speedo adaptor was used.
When low ratio gears were used with relatively short tires, or high ratio gears were used with tall tires, the factory used a screw in speedo adaptor. These adaptors came in a bunch of different ratio's, from aprox 1.15 to .875. As originally assenbled by GM Hydramatic, it allows the transmission to use a combination of drive gear & driven gear, with the latter having between 40 & 42 teeth. in trainman's situation, as far as he can go for more teeth on a driven gear is 45 teeth. Another problem is the alum sleeve that the driven gear slides in his current combination, will only take 36-39 teeth driven gears. To use 40 to 45 tooth driven gears, will need to swap out the current sleeve to the 40-45 tooth sleeve #1362293 , then combine the correct drive/driven gear. 

In early dated Pontiac dated master parts books, one of the shortest tire wheel combo's listed with a low ratio gear (3.55's) is for a '67 GTO.
This transmission, a '67 PS code T400, used a 44 tooth driven gear with the relatively short F70-14/6" wheel combination, that should easily be in the 26.25" tire height range. When 3.90's were ordered with same tire/wheel combo, there are not enough teeth to go to with the driven gear. Thus drive & driven gers were changed & an external speedo adaptor had to be used. Thats just how it was across the performance GM lineup in original transmission builds with shorter tire/wheel combo's with factory installed 3.90-4.88 gearing. 

With 3.73's being inbetween 3.55 & 3.90 gearing, it wouldn't hurt to try a 45 tooth driven gear with the correct 40-45 tooth alum sleeve. My '71's PQ does have a different tooth drive gear on the output shaft as the above '67 PS, as the '71's tires are 1/2" taller, but it has the same 44 tooth driven gear with same 3.55 gearing. Having gone to 3.73 gears for PureStock build, am going to try the light blue 45 tooth driven gear, before trying any other combination. If I substituted slightly taller tires, I could probably get away with the 44 tooth driven gear with the 3.73's.


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