Speedometer not reading correctly...

[Paul Steinberg]

This might be more information than you might want, however, it does explain how to correct your speedometer reading, if you are so inclined, and your car has one of the listed transmissions...

Transmission Speedometer Gears

Decoding Chevrolet VIN, trim tags, cowl tags, engine, engine block casting numbers, cylinder head casting numbers, intake manifold casting numbers, transmission, interior codes, and paint codes.

chevellestuff.net

 

[John ED Renstrom]

Correcting Error

TH400: Speedo driven gears have from 34 to 45 teeth. Each tooth added lowers the speedo reading about 2.5%. Each tooth removed raises the speedo reading about 2.5%. In our example, we want to lower the speedo reading from 60mph indicated to 52mph traveling. We need to add teeth to lower the reading by spinning the speedo cable slower. Remember, one tooth = 2.5% so we need to add about six teeth. (15.4%/2.5% = 6.2 teeth).

this is the only one we need to follow as the 235 tires are smaller then the original equipment on the cars. what he is not telling you is GM has 8 set of gears and two adapter sleeves for this transmission. they also produced a number of reducers and increases that went on the transmission. they are all color coded. what I have never done is pull the gear out to see what was in the car. just to lazy to crawl under there. but once you determine the gear you have you can then add by this statement 2 or 4 teeth( gears are 2 teeth different) to move the speedometer down 5 MPH

 

[Mike Stevens]

speedo reading

I have radial 235x75x15 tires on my coach. I knew there is a difference even between makers of tires on the actual size of the tire no matter what size they call them. I checked my speedo with my GPS unit. It is 3 mph slow. Not enough to worry about for me. I have never been stopped for speeding. When I worked at IH we had big metal tubs of speedo adapters. If it was within 3 mph slow or fast we did nothing. Anything else and we wrote up a change order for the repairmen.
When changing the adapters you should also check the driven gear in the tail shaft or wherever in the trans. I had one that worked until the trans was rebuilt at 130k miles. After the rebuild the speedo was not working properly. A check of the driven gear and it was found to be very worn. Evidently it no longer match well to the rest of the trans. The fun part was when IH said they did not have a gear for my trans. Had to go to a Chrysler dealer.
Mike

 

[John ED Renstrom]

curiosity got to me today and it's Paul's car. I crawled under there to look. figuring his is the same setup as most of us. they have the big sleeve in the tyranny. the 40 to 43 tooth one. his gear was the 41 tooth. so if this formula is correct going to the 44 tooth gear should drop the speedometer 5 mph to correct for the 235 tires. a word of warning you pull that sleeve out you better be ready to catch about 5 Qts of fluid. the black gear is 40 the yellow is 41 and the green is 42. I don't have the 43 or 44 one.

 

[Russ Austin]

Good info, have added to my "fav" listing on the ol computer.

 

[Paul Steinberg]

Stopped by my friendly transmission shop today, and picked up a 43 tooth gear. I was told that each tooth will change the speedometer by 6 miles per hour. Without knowing how far off the speedometer is presently, I don't know if this is going to be too much of a change or not. He also showed me that the housing that the gear goes into is only for a specific range of gears, and that you need to make sure that the gear that you install into the housing, that the housing is proper for it. Picture to follow..

 

[Pat Post]

Well, if your speedometer isn't reading it most likely is a blown head gasket.

 

[John ED Renstrom]

the first pictures shows you have a 40 to 44 tooth sleeve. if the tyranny man is claming 6 MPH a tooth you have a 41 and it's off 5mph. a 42 tooth will drop it 6 it will read 1 slow. but that ain't what the formula says.

 

[Paul Steinberg]

I came across this website today, and it has some excellent information about speedometer gears and how to pick the correct one to correct an incorrect speedometer reading.

Is Your Speedometer Inaccurate? Here Are A Few Tips To Correct It

An inaccurate speedometer can be a hassle for car owners. A correct reading might be easy to obtain. Find out more right here.

www.chevyhardcore.com

TCI Auto Speedometer Gear Calculator - TCI® Auto

Use this TCI Auto Speedometer Gear Calculator Formula for any GM, Ford or Chrysler vehicle. This is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line!

www.tciauto.com

 

[John ED Renstrom]

Will that help a little? The three mph per tooth makes guessing a lot easier. If I remember correctly, with the 235x15, most pro cars read five mph faster at 60 mph. So, to slow it down, you need to add two teeth to the drive gear. Or was it five slow? Either way, with a GPS and the three mph per tooth, it would be easy to get it closer.