Rear End Gear vs. Tire Size to match the speedometer - Ford Mustang Forum
 
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post #1 of 6 (permalink) Old 10-14-2011 Thread Starter
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Rear End Gear vs. Tire Size to match the speedometer

I have another question in an attempt to try and save a couple hundred bucks on an attachment, or re-calibration of the speedometer?!

I'm planning on putting 3.73 gears in my '96 GT this winter in preparation for Solo II season next year. I also use this car for my daily commuter and in my commute I go right through every speed trap in the area, so an accurate speedometer is a must. With that said, one of my rims is cracked so I'll have to replace the rims before next season as well.

Is there a calculator, formula, or some other way to figure out what wheel/tire sizes to purchase in order to offset the lowers speeds of the 3.73 gears and still have the speedometer (relatively) accurate? 18's vs. 17's, 19's or whatever!?

I know the obvious answer, "The gears will lower your speed, so you'll be safe," but I'd rather it be accurate than being exponentially off by 5 or 10 miles per hour at given speeds. Since I have to spend $1,000.00 in wheels and tires, as well as new gears and install kit, I'd rather save the money from a recalibration, add on attachment, etc.

Thanks,
~ Dethan

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post #2 of 6 (permalink) Old 10-14-2011
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Well there are couple things in this thread worth being covered, most of it could be done by searching threads. Couple of things that could help you is your trying to get your car ready and are spending that kind of money it might be worth buying a tuner. This will handle your speed calibration issues etc. There are several ways from buying recalibration tunes and what not but this mod will benefit you as well along with fix the reading of your speed and any other future errors. Just a suggestion. And 4 new tires with new wheels and gears well cost you well over 1000. Just the gears alone will run you to about half that with install costs not even covering the tires/wheel prices.

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post #3 of 6 (permalink) Old 10-15-2011 Thread Starter
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Quote:
Originally Posted by 02stang46 View Post
Well there are couple things in this thread worth being covered, most of it could be done by searching threads.
I guess I scanned specifically for this particular issue, and not looking for pieces here and there. Sorry about that.

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Originally Posted by 02stang46 View Post
it might be worth buying a tuner. This will handle your speed calibration issues etc. There are several ways from buying recalibration tunes and what not but this mod will benefit you as well along with fix the reading of your speed and any other future errors.
When I purchased the car it came with an SCT SF3 Tuner, but I don't see anything in it that would adjust the speedometer? Are you saying, when I do buy a tune that they can adjust the speedometer calibration through the tune or computer modification? I do plan on purchasing a Bama (or similar) Tune once I have everything purchased and installed, but that won't be before spring. If they can reprogram the speedometer with a computer change, that is awesome and question answered. I was under the impression it was mechanical?

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Originally Posted by 02stang46 View Post
4 new tires with new wheels and gears well cost you well over 1000. Just the gears alone will run you to about half that with install costs not even covering the tires/wheel prices.
I'm going a bit on the cheap, as I'm not competing nationally and it's just a hobby/fun thing for the wife and I to do. I "just" replaced the tires on it with some Falken ZIEX ZE-502 for $500.00. Unfortunately, after doing so, I found a crack in one of the Wheels and will have to replace it before next year. So, I was thinking if I could get new wheels and transfer the tires to them and I'd be good for a year.

As for the rear end gears, I'd replace those myself with a replacement kit, shims, gaskets and so forth. So the cost there is about $200.00. With Wheels and Gears, and Gear Kit it would be around $1000.00 total if I can keep the same sized wheels. If not (having to increase size to compensate for speed changes and offset for fender width), then I'd have to buy new rear tires (same brand/model different size) with new RIMs.
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post #4 of 6 (permalink) Old 10-15-2011
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No problem, but yeah the tuner you mentioned will calibrate your speedo upon uploading the tune. I did however try to look to see if it still did with the Stategy tunes and not Custom tunes but I had no such luck finding anything. I am 90% sure this will fix your issue as just plugging your handheld into the OBDII port you can ajust settings and revamp your ECU. So your best bet without purchasing custom tunes (much better!) is to simply plug in your SF3 tuner erase the original tune and reinstall the strategy tune and your computer should be correct and recalibrated thus eliminating your problem. If this fails there are other ways by simple needle placement as well where you do not have to spend a dime, but lets expend other measures before that one.
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post #5 of 6 (permalink) Old 10-15-2011
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I thought for '96 you needed a new speedo gear for the trans? If so, it's pretty cheap and shouldn't be a difficult install.


2001 GT Convertible
/1967 Coupe
All the usual bolt-on's /200 I6, C4

And a few Explorers too... 2008 4.0 XLT 4x2, 1996 XLT 5.0 4x2, 2003 Sport Trac XLT 4.0 4x4
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post #6 of 6 (permalink) Old 10-15-2011
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I thought for '96 you needed a new speedo gear for the trans? If so, it's pretty cheap and shouldn't be a difficult install.
+1

94-98 GT using T5s and T45s use mechanical pick ups and need a new gear to correct the speedo. I think the T45 uses a 8 tooth shaft though so the best you could correct for is 3.27s with a 21 tooth gear.
http://www.fordracingparts.com/download/charts/129.pdf

You will either need a handheld tuner or one of these:
Ford Racing Mustang Speedometer Recalibration Tool (94-10) at AmericanMuscle.com - Free Shipping!
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