98 cobra w/ 4.10 gears rpm/speed

brian98gt

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Mar 12, 2002

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new jersey

Jul 12, 2006

#1

• Jul 12, 2006

• #1

i have a 98 cobra which i purchased last month and i thought was completely stock so i went to have 4.10 gears put in and to my surprise, there were 4.10's in already. my question is what speed at 2,000 rpms in 5th gear is correct. cuerrently my speedo reads 59 mph at 2,000 rpm. since i didn't have the gears put in i'm not sure if the previous owner corrected the speedometer with the correct speedo gear. and just out of curiousity, where does the speedo gear go? is this something i can have checked easily?

 

Gallows

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Mar 30, 2005

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Summerside, PE, Canada

Jul 12, 2006

#2

• Jul 12, 2006

• #2

I have a 97 and actually had my speedo gear out today. On the rear of my transmission on the drivers side you will see a cable/wire fastened with a metal clip and 11mil bolt. Take the bolt out and wiggle the plug out. The gear should be white for 4:10's with the stock 245-45-17 tires?? I'm getting 4:10's installed next week and have 275-40-17's and need to know what gear I need.

Cheers
Stephen

 

brian98gt

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Mar 12, 2002

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new jersey

Jul 12, 2006

#3

• Jul 12, 2006

• #3

thanks, i appreciate the info

 

missouri_guy197

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Aug 31, 2002

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St. Louis, MO

Jul 13, 2006

#4

• Jul 13, 2006

• #4

Good speedo/RPM calculator...

http://www.svtoa-atlanta.org/cgi-bin/trans.cgi

BTW, if your speedo isn't corrected all the speedo/RPM calculators in the world won't let you know what gear you have in there...Get a speedcal if you don't have it corrected...


It does look like it's corrected already though...

 

brian98gt

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Mar 12, 2002

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new jersey

Jul 13, 2006

#5

• Jul 13, 2006

• #5

thanks, that site was very helpfull. seems like i'm off around 4 mph in 5th gear. as rpms go up, will the difference be greater? at the track my speedo read around 110 and my speed on the time slip was 99.

 

missouri_guy197

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Aug 31, 2002

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St. Louis, MO

Jul 13, 2006

#6

• Jul 13, 2006

• #6

At the track your speedo will read higher when you cross the line than what your slip reads because they are taking the average speed of the last 66ft of the track (called the trap) and not your actual speed at the finish line.

Also, something to consider is that the stock tach's are usually off quite a bit...I know mine is off about 200rpm...I've set my shift light to 3000rpm and revved up to 3k on the tach and it doesn't come on until I get to around 3200rpm...

You can also use this gear calculator to find what RPM you should be doing at any given speed, or vice versa.

http://www.corral.net/tech/gearcalc.html

Also, if you can find someone with a GPS, that's probably the best way to know if your speedo is on or not...You may even have a Speedcal already and it's not set up properly too. To find this out just pull up on the shifter bezel and if there's a black box tucked up under the dash where the aux 12v outlet sits that's a speed cal that is correcting your speedo...

 

SVT32VDOHC

waiting for the next hack atta

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Nov 22, 2001

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Motor City

Jul 14, 2006

#7

• Jul 14, 2006

• #7

4.10s 80MPH should be a 3,000RPM's. After my speedo gear stripped out, I put the same 21 tooth white speedo gear in, and now it reads 90MPH at 3,000RPM's. So.............I think the gear on the tail of the trans must be messed up, but I haven't looked, I really don't care enough.

 

missouri_guy197

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Aug 31, 2002

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St. Louis, MO

Jul 14, 2006

#8

• Jul 14, 2006

• #8

Actually 80mph is more around 2800rpm with 4.10's...83mph should be 3000rpm...

The white gear is a close approximation for 4.10's, which is why a speedcal is a much better thing to use to calibrate for gears...Plus if you ever want to change gears you don't need to get another gear...I've had 4.10's, 4.56's and now 4.30's and I've only had to flip a couple dip switches...

 

SVT32VDOHC

waiting for the next hack atta

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Nov 22, 2001

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Motor City

Jul 14, 2006

#9

• Jul 14, 2006

• #9

How do you use the Speed Cal? Does it need to be wired in, or is it plug and play into stock mustang plugs and wiring??

 

brian98gt

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Mar 12, 2002

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new jersey

Jul 14, 2006

#10

• Jul 14, 2006

• #10

whoever put the gears in must have used a speedo gear cause i'm only off by like 3 mph which i can live with. thanks for the help everyone.

 

missouri_guy197

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Aug 31, 2002

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St. Louis, MO

Jul 14, 2006

#11

• Jul 14, 2006

• #11

SVT32VDOHC said:

How do you use the Speed Cal? Does it need to be wired in, or is it plug and play into stock mustang plugs and wiring??

Click to expand...

Speed cals are pretty much plug and play...you have to pull a few wires out of the stock VSS harness and plug it into the Speedcal harness but nothing crazy...Takes about 20-30 minutes to install once you get the car up on ramps...not much longer than installing the speedo gear...

Here's the instructions:

http://www.dallasmustang.com/page/speedcal.inst.php

 

brian98gt

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Mar 12, 2002

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new jersey

Jul 22, 2006

#12

• Jul 22, 2006

• #12

just curious cause its probably illegal,could you use a different size speedo gear or calibrate a speedcal to make your speed lower than actual therefore lowering your actual miles. just a thought

 

Built98GT

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Oct 17, 2005

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OKC

Jul 24, 2006

#13

• Jul 24, 2006

• #13

Speed-cal is the way to go