98 cobra w/ 4.10 gears rpm/speed

brian98gt

brian98gt

Founding Member
Mar 12, 2002
377
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0
new jersey
#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?
 
#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
 
#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.
 
#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...
 
#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.
 
#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...
 
#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
 
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