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Discussion Starter #1
Kind of an odd question, something I should have been able to find someplace else but... So my speedo is calibrated to the the car as is. Technically, from my understanding of things, switching the rear gear ratio (explorer swap (good or bad is a different argument)) and even wheel size should change the calibration. How do you guys keep it correct?

Am I not really understanding it, or over thinking it?
 

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Kind of an odd question, something I should have been able to find someplace else but... So my speedo is calibrated to the the car as is. Technically, from my understanding of things, switching the rear gear ratio (explorer swap (good or bad is a different argument)) and even wheel size should change the calibration. How do you guys keep it correct?

Am I not really understanding it, or over thinking it?
You are correct... but these speedos are not that accurate so that small changes don't matter much (e.g. under 5%).

For big changes in tire size or rear gears, you must compute the difference or use an online calculator to select the proper gear for your speedometer cable. About a 10 minute job to change this...

-Rory
 

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Thanks 2ndGen. Do you think it's worth it for me to do all that (I am in the process of locking down an Explorer rear end)? Stock is the 2.8, and going to the 3.73. I'm leaning towards yes, although it would be nice to be able to honestly say "No officer I have no idea how fast I was going".
 

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"I don't know how fast I was going" --- "Here, let me write it on this ticket for you"
Even if you don't calibrate your speedo, you should know how fast you are going. Find a measured mile (highways with mile markers work good), hold a steady speed, and time yourself. Divide time into 3600 to get speed. Your cell phone probably has a stop watch feature. One of the first things I do with any new car I get.
 

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Thanks 2ndGen. Do you think it's worth it for me to do all that (I am in the process of locking down an Explorer rear end)? Stock is the 2.8, and going to the 3.73. I'm leaning towards yes, although it would be nice to be able to honestly say "No officer I have no idea how fast I was going".
Absolutely... your speedo will be off by approximately 33% ((3.73-2.8)/2.8). This assumes you don't change tire size too and that your speedo is reasonably accurate now.

-Rory
 

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Yes but the speedo will say 90 when you're actually going 60!
 

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Discussion Starter #7
Ya, I actually usually have a gps up that measures moving speed, but I like to have the dash match, for the invetable time when that fails.

I will be moving up to 17 inch wheels as well. So the plot thickens. I have a few more projects before I can even get back on the road with it... but one thought led to another led to another. I appreciate all your input.
 

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Ya, I actually usually have a gps up that measures moving speed, but I like to have the dash match, for the invetable time when that fails.

I will be moving up to 17 inch wheels as well. So the plot thickens. I have a few more projects before I can even get back on the road with it... but one thought led to another led to another. I appreciate all your input.
The size of the wheel doesn't matter. It's the overall circumference of the wheel and tire that counts. That gives you the revs per mile and that number is required for computing the required speedo gear set.
 

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Discussion Starter #9
Very true, and I understand that. I just put wheel for simplicity sake... depending on tire size... the circumference will more than likely change... enough to mess up the speed calibration? Maybe maybe not, depending on current size vs. future size... I would bet it probably won't make a difference worth calculating out relative to the gear change.
 

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Going to 3.73 you will have your speedo way off......what trans are you using?
Up to a certain point all you need to do is change the speedo cable gear....with higher ratios you will need a new speedo cable gear and additionally a new gear in the tranny....not real hard but for sure more work....
There are calculations out there for this. I seem to remember using one out of the CJ Pony parts catalog when i built my car...
Tom
 

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If this is for an auto trans equipped car, the main shaft gear is not replaceable, and you may have a problem finding a suitable speedo driven gear for the 3.73 rear end. There are inline converter boxes available that can help, but you will probably need to find a speedometer shop to have it installed and calibrated.

If you have an electronic speedo such as the Classic Instruments speedo or some Autometers, you can calibrate them at the speedometer head.

If the 8.8 rear end has a speed sensor on it, you can use an aftermarket electronic speedo and use the 8.8 rear end speed sensor, and it should read correctly, regardless of the rear end ratio. Only an increase or decrease from stock tire circumference will affect the accuracy of the speedo.
 

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Discussion Starter #12
Currently I have an FMX... eventually it will be a T5. Like I said, I have a few other projects going on it before it will be on the road again. I will probably just use the gps speed for a while, it's not like I'm going to racing through traffic either. I was just thinking in future tense and thought I'd put it out there.
 

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I haven't had a chance to re-gear my speedo, so for now I took my Garmin for a ride. It has GPS so it shows constant actual speed. Speedo shows about 15-20 mph faster then actual speed with 3.90 gears. Quick simple way to get a reading.
 

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oops....someone beat me to the GPS post.
 

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Discussion Starter #16
Awesome link. Thanks so much
 

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I know that this is kind of random but can some one show me were the speedo cable enters the car and goes into the guages on a 1965 mustang. it is a manual transmition. is it in the firewall? is it in the shop manual. picture please.
 
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