speedo gear question

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yeah, I changed to the white 23-tooth gear and mine is still off by 10%...it rolls over 1.1 miles for every actual mile traveled

the easiest way to check accuracy is to reset your trip odo at a mile marker post and check it at the next one :nice:
Actually, you should check it over a much longer run if you want to get an accurate measure of how far off it is. 15-20 miles will give you a better reading. It's really tough to check the speedo - easiest way is with a long unobstructed Interstate run where you can set the cruise on a specific speed, and then set a stop watch with the mile markers. Time how long it takes to cover say 15-20 miles and note the speed you set your cruise at. Then compare that to the actual miles covered (by mile marker) over what time you measured. It's best to set your cruise a good bit under the speed limit - as that gives you the best chance at being able to maintain that speed for a long period of time. If you're going fast enough to have to pass others, more than likely you'll have to vary your speed - which throws the test out. This will let you know if the speedo and odo are off by the same amount or different amounts. On my speedo, the odo and the speedo a separately calibrated - it's possible for them to be off by different amounts.

If you're still off with the 23 it's because you need to change the drive gear on the tranny shaft - a fairly major operation. Alternatively, the speedo companies sell black boxes that you plug your speedo into, and then it plugs into your tranny -- and they'll make any ratio adjustment you tell them to inside the box. That'll let you put a gear that will last on the speedo cable (like a 19 tooth) and then just make the adjustment in their magic box.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.
jaidedeye said:
well i get roughly 200 miles to a tank of gas.....if my speedo is off by 8mph what kind of gas miliage am i actually getting?

8mph off at what speed? Your speedo is not consistently off by 8mph (unless it's broken), it's off by a percentage of your actual speed.

8mph off at 50mph is about a 16% error. 8mph off at 100mph is a 8% error.

Take a trip down the interstate. Reset your trip odometer as you pass a mile marker. Drive a good distance like 10 or 20 miles (even numbers are the easiest). When you pass the last mile marker, note the exact miles on your trip odometer.

Divide the higher number (most likely your odometer miles) into the smaller number (the miles driven), and that'll give you the error. That number can be used to figure out gas mileage, your indicated speed for various true speeds, etc.
Nice explanation TRW - I mentioned "percentage" error in my first reply, but they didn't seem to either pick up on that, or didn't understand it.

Jaided - example ---- So, if you're 8 mph off at 60 (it reads 68 but you're going 60) - Then for every 68 miles it says you go, you only went 60 -- assuming your odometer and speedometer are off the same %. As mentioned above, you must check each independantly if you want to know that for sure. Let's assume the odometer is off that amount. That means your odo is reading 13% too many miles - so your fuel economy is off (you think it's better than it actually is) by 13%. Instead of covering 200 miles, you're only covering 176.5 miles. Use that number to recalculate. 20 miles to the gallon just turned into 17.5 mpg.

jpc - you have to do this stuff if you want your speedo to be accurate. Go to the link I posted in my first reply, and you'll see how you compensate for different gears in different models. The speedo simply measures how fast the driveshaft is turning. When you change the rear gear in the car (2.73 to 3.73 for example), the driveshaft now must turn 3.73/2.73=1.37 times faster than it did before to move the car at the same road speed. That means instead of turning over 2000 rpm at a certain speed, you'll need to turn 2730 rpm with the bigger gear. If you don't change the speedo cable gear, to allow for that difference, your speedo will be off by that percentage.
jaidedeye said:
damn this sounds like a pain in the ass....

Not really. There are plenty of ways to get your speedo close. A simple swap of the speedometer driven gear can usually get you close if you haven't done a really radical rear end change (4.10s and higher usually require additional changes).

Even when it leaves the factory, you speedo doesn't have to be too accurate -- within 5% is normal, I believe. Even police cars with "Certified Calibration" speedometers are only within 2 to 3%. Any changes in gearing or tire size after that can reduce the accuracy even more.

You can either make the changes to bring the accuracy close again, or you can live with it being off. Just do some calculating to know what speed you have to indicate to be going selected speeds -- like 30, 45, 55, 65, and 70 mph -- so you don't get a ticket by having to guess. Know that your odometer will be showing more miles than you've actually driven, but this would only be a problem if you want to sell it later.