Got a question, when driving my Chevelle around I have noted that the Speedometer reads on average 10mph faster than actual speed, and the Odometer racks up miles 1/10 of a mile faster than actual. I am wondering what could be the problem, I recently realized that I have different size tires on the front of the car than on the back, its not a huge size difference but its different, I have 205's on the back, and 195s on the front. Would this be the cause of the discrepency in accurate readings? Or is it something more difficult to deal with like the speedo gears? TIA. -Steve

Your Rear tire height, rear end gear ratio, and the speedometer gear in your transmission effect your gauge reading. Easiest (or cheapest) to change is the plastic gear in you transmission. If you know your rear end ratio and rear tire size you can figure out how many teeth you should have on your speedometer gear in your tranny. Here's some info and a formula:

http://www.tciauto.com/Products/TechInfo/speedo_gears.asp

All of what Steve said. Front to rear differences make no difference, it's the rear that's driving the speedo. You mention 205 tires but what aspect and what wheel size? A 205x60 is going to be smaller than a 205x70. My sources show a 7.35x14 tire as original equipment. That converts to roughly a 205x70x14. If you're running, say, a 205x60x14, that's a 6.4% overall diameter reduction and your speedo will read about 4 mph too fast and, consequently, put more miles on the odometer than you travel.

http://www.miata.net/garage/tirecalc.html has a good java calculator for differences in tire/wheel sizes.

http://www.chevellestuff.com/tech/tires.htm is a conversion from old style (7.35x14) to metric and letter sizes.

http://www.chevellestuff.com/tech/speedo.htm is another page to help with transmission gears (Courtesy of Fred Aldrich)

The 1/10th mile and 10mph difference seems more than just a tire size difference. Have the rear end gear and/or transmission been swapped from original?

When I calculated my speedometer I used the above sites. To be sure of my speed difference I used my cell phone GPS on a highway, set my speed at 60 and compared it to the GPS. I went to the dealer and bought several driven gears. Come to find out the calculators listed were slightly off and I ended up using another gear than it called for. I compared my speed to my GPS again and the police roadside radar signs and it is dead on.

I am not too sure on whats been changed and what hasnt, the car is from what I have seen and checked on numbers on the POP and the housings and casting numbers, everything is original to the car aside from tires, the car has less than 50,000 miles original. I dont know what to do to fix it, as my original post suggested i thought it was tires, speaking of which, the rears are 205x75x14 the fronts are 195x75x14, What would this do? Would that affect anything? Thanks. -Steve

Front tires have nothing to do with your speedo calibration. As stated above, any change in the tire size from stock will cause your speedo to be off. The answer is to change the driven gear, which is in the transmission where your speedo cable connects.

Well now that I know what I have to do, and can go get a new gear, i was wondering what causes the gear to not work right? Do they just wear out over time like any other moving part? Bad lubrication? defective? Age? Is there something that can be done to prevent it from happening again? As I mentioned I dont know how long its actually been reading wrong, from previous memory I believe at least 10 plus years. Thanks for all your help everyone! -Steve

The gear doesn't know you put different tires on. A different size tire will result in fewer or greater rotations per transmission output shaft revolution.

Changing gear ratios has the same effect. By changing tire circumference you are in effect changing your gear ratio.

Front tires have nothing to do with your speedo calibration.
Unless you're driving a mid-to-late '60's Oldsmobile. The speedo is driven off the left front wheel on them.





The proper procedure is to gear the transmission-to-speedo cable so that the ODOMETER reads correctly. If the odometer is correct, but the speedometer is off, the speedo has to be removed for calibration.

Realistically, everyone I know sets up the gears or the reduction box to make the speedometer correct; and the odometer accuracy is ignored but hoped to be "close enough".

Thanks for all your help everyone, I will see what happens when I get a new gear, Though I dont like that the miles roll on so fast, I drive 100 miles it reads 110, that adds up fast especially on a true low mileage vehicle. Though I have finally decided what I want to do with it. I have after some research discovered that aside from the lack of some heavy duty parts on the car, I have nearly a factory Police Package. Though it is the "Thrift" package for police, it is a four door 300 Deluxe, all the base options so it wouldnt be very hard to paint her up after i get some minor body dents reshaped, and then get her all set in the right colors and equipment. I am really excited about turning her into one, would make a nice cruiser and show car that way. But I am still torn about simply restoring her to her former family cruiser beauty though... -Steve

Before you buy a gear, you will have to check to see how many teeth are on the ones you have. The speedometer cable attaches on the drivers side rear of the transmission. Where it screws onto the transmission, there is a small bolt and bracket that hold in the gear housing. If you undo the bolt, you can pull out the housing. You'll lose some tranny fluid unless you have the car sitting a little higher on the drivers side. The DRIVEN gear will be in the housing you pull out, you can see the DRIVE gear through the hole in the transmission. Count the number of teeth on both. The factory mixed and matched the number of teeth on each gear depending on your rear end and tire size. You can use the sites above or post the tooth count here to figure out what gear you need to buy.

With 10% error, I'd try a driven gear with 2 more teeth than you have now - to start. FWIW

I had a like problem with my car. I have 3.31's with a 26 Tire. I had to play around with my speedo gear and also install the 4.11 adapter to get it right. I used a GPS and confirmed my readings. I just had my cluster restored and was told that my speedo was off almost 10mph and Tach was off by 500 Rpm. It makes sense why I couldn't use TCI's chart. The key would be to use a GPS to get your speedo close.

Im not sure how this works, it just has not made sense to me just yet, But,,,,,
I have a Turbo 400 in my 67 Chevelle, and my Speedometer is running about 18-20 mph fast.
I just checked and I have a white gear with 36 teeth, which color would I need to correct my speedo?
Turdo 400
26" tire
Now 36 teeth (white)
18-20 Fast

Each tooth on the driven gear equals about 2.5 MPH. You need to add teeth to slow the speedometer down so you need to add about 7-8 teeth so if you can get a 43 - 44 tooth gear you should be close but if you can't you will need to change the drive gear and driven gear. Should be a dark grey gear BUT you will have to change the housing

Thanks,
I will check the dealership to see if they stock the gears, if not I will order online.