speedometer gear question

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trudysduster

trudysduster

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Going down the road in the Dart my speedometer read 70 MPH but along side the van I was actually going 62 MPH according to it. So, does that mean I need a gear with more teeth or less teeth. I have not pulled the gear yet. but need to know which way I will need to go. Thanks
 
I am running 245/60/15 tires with 3:55 gear. do I need a 34 or 36 tooth gear. seems like I am in the middle. Probably a 36 would be safer.
 
Take the number of teeth you have now and multiply times 70 then divide that number by 62

to double check I would look at the odo at a mile marker and then drive to the tenth mile marker and look at your odo again. if it says you went 11.2 miles then multiply the number of teeth you have now by 1.12.
 
trapster said:
Take the number of teeth you have now and multiply times 70 then divide that number by 62

to double check I would look at the odo at a mile marker and then drive to the tenth mile marker and look at your odo again. if it says you went 11.2 miles then multiply the number of teeth you have now by 1.12.
Click to expand...

The only way this first idea will work is if you know that both speedometers are accurate. Also you need to drive one hell of a lot further than 10 miles to check for accuracy.

Just pick the correct gear from the chart! It's so much easier. 35 teeth in this case.
 
trudysduster said:
I am running 245/60/15 tires with 3:55 gear. do I need a 34 or 36 tooth gear. seems like I am in the middle. Probably a 36 would be safer.
Click to expand...

I think you're pretty close in the 36 number.

Got a GPS? Plug it in and check it against the speedo!
 
Thats true, I have a speedometer ap for my phone.

But I wouldn't count on it being that accurate either
 
trapster said:
Take the number of teeth you have now and multiply times 70 then divide that number by 62
... then multiply the number of teeth you have now by 1.12.
Click to expand...

I drove my ebay 68 from CA to Ohio. It's a long story but because of a vibration, I could only cruise at 70 MPH. 2800 mile trip by myself without a working radio so I had plenty of time to think about life, love, and speedometers. I started checking the odometer against the mile markers in 20 mile stretches and writing the numbers down. I got the exact same numbers you are; indicated 70 was actually 62. Did the math coming up with 1.12, pulled the speedo gear from the trans when I got home, counted the teeth, multiplied the number of teeth by 1.12 and bought a "new" speedo gear off ebay that was closest to it. (I probably cheated to the high side because I knew that as the tires wore they'd get smaller giving more revs/mile) Had to do it again when I changed tires later. The result is my speedo is dead nuts accurate at 70 according to my GPS.

Do this! You do not have to know your tire diameter, gear ratio, etc. That stuff doesn't matter AT ALL and this is real world accurate.
 
well, I plan on checking it out on Sunday. will see what I have in it now. Thanks guys.
 
If you measure your tire width and no your gear ratio the chart works great, it will keep you within 2 mph or closer, if its off more than that get your speedometer calibrated.
 
just pulled the gear and it is a 33 tooth gear. According to the chart I need between a 34-36 gear. I am 8 MPH fast according to an accurate speedo. How much MPH per tooth does it slow down. If I have a 33 now and need to slow it down 8MPH at 70, would I need a 34 tooth or should I go with a 35. Thanks
 
It should just be a percentage thing and it is just math, if you want it to slow the speed down 13% add 13% more teeth, or go from 33 to 37? 70*33 = 62*37

This should match the chart so I am thinking something is wrong with one of our calculations.

34 teeth should get you 70*33/34 or 68 mph
35 ............................ 70*33/35 or 66 mph
36 ----------------------- 70*33/36 or 64 mph
37 ----------------------- 70*33/37 or 62.4 mph
38 ----------------------- 70*33/38 or 60.8 mph
 
trudysduster said:
just pulled the gear and it is a 33 tooth gear. According to the chart I need between a 34-36 gear. I am 8 MPH fast according to an accurate speedo. How much MPH per tooth does it slow down. If I have a 33 now and need to slow it down 8MPH at 70, would I need a 34 tooth or should I go with a 35. Thanks
Click to expand...
35, per the chart. With 26.5" tires and 3.55 gears, 35 teeth is perfect.
 
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