speedometer 15 mph fast - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 07-11-2017, 08:51 AM Thread Starter
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speedometer 15 mph fast

The speedometer on my 67 GTO with 400 is running 15 mph fast. I checked with a GPS unit. I have 235-60-14 on the front and 245-60-14 on the back with cragars. Friends say it is the tires making it run fast. I wouldn't think it would make it read that much faster. Any ideas? Thanks in advance.
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post #2 of 4 (permalink) Old 07-11-2017, 10:16 AM
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Your surmise is correct...the tires would not make that much of a difference. To compute the tire size change vs speedometer error here is a website I use:


I think you're going to have to get into speedometer gear changes. here is a good site from TCI trans on calculating gear changes vs tire size, etc. :

Speedometer Gear Calculator and Charts - TCI® Auto

Hope this helps......and Welcome to the forum. Keep asking questions...someone here always has the right answer.
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post #3 of 4 (permalink) Old 07-11-2017, 10:19 AM
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There are different size speedo gears for different rear end ratios and a lot of these cars have parts from another one.
If the rear end has been changed to a different ratio the speedo gear also has to be changed for that ratio.
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post #4 of 4 (permalink) Old 07-12-2017, 08:27 AM
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15MPH fast at what speed?

Here's a primer on what you're getting into:

Originally Posted by 64GTOConvertible View Post
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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