Possible Gear Change?

[dustin9187]

I've had my cobra for a little over a year now and ever since I got it I thought that the speedometer and odometer were off a little. I felt like I was going slower than what the speedometer was saying, and therefore traveling less miles than what the odometer said. So I drove the cobra to work one day last week and clocked the miles, and then did the same with my daily driver. 24.2 miles with the cobra, 21.4 miles in my other car! So at least I know I'm not crazy in thinking this. But the thing is, I was told from the previous owner that the car was all but stock, so I didn't really think that someone had done a gear change. Is there any other explaination other than that the gears were changed and not the gear for the speedometer/odometer? And if this is the case, how can I fix it?

[jstanley55]

What size tire do you run?

[dustin9187]

I was told they are the stock tires. So P245/45ZR17, right?

[Z282SLO]

Assuming your Cobra actually has the stock size tires (forget what you were told - check the tires themselves), your car likely has a different gear ratio (probably 3.55) or a significantly mis-calibrated speedometer/odometer.

Different tires and gear ratios are common mods that will cause factory speedos to read incorrectly. Sometimes, you could just have a faulty speedo that is way off of spec, although that is rare.

There is a lot of info re: speedo calibration on the web. Here's one useful article -

http://www.hotrodhotline.com/calibrating-speedometer

Good luck.

[93Cobra#2771]

Sounds suspiciously like a 3:55 gear may be in your car, and hasn't been corrected for.

It is a simple matter of swapping out your speedo gear at the trans end of your speedo cable for the correct gear. Don't ask me which gear, as it's been years since I've done it.

[vinchez78]

I thought you didn't have to change the speedo with 3:55 gears??

[buddha]

What RPM to you run at say 70mph? You could compare that to a stocker with 3.08 gears.............

[escogt]

I thought you didn't have to change the speedo with 3:55 gears??

You don't have to change the speedo, you have to change to the correct 'speedo gear' that goes in the tranny.

[sounnder]

My speedo was off by 10 mph when I got it. With the help of the guys on the site I learned my car has 3:55's. Ordered the correct speedo gear and took care of the problem.

[Z282SLO]

What RPM to you run at say 70mph? You could compare that to a stocker with 3.08 gears.............

That would be helpful. Maybe a member with a stock Cobra could post that info.


Although not as user-friendly, here's the maximum rpm in each gear for a stock Cobra (from the 2/93 Road & Track) -

1st - 40 mph @ 5900 rpm
2nd - 68 mph @ 5900 rpm
3rd - 102 mph @ 5900 rpm
4th - 135 mph @ 5900 rpm
5th - estimated 150 mph @ 4440 rpm (later shown to be a bit optimistic)

Anyone want to confirm the 4th gear info? (I'd love to but my Cobra's far from stock!)

[93Cobra#2771]

What RPM to you run at say 70mph? You could compare that to a stocker with 3.08 gears.............

That's assuming he has a stock trans. Can't do that with my T5Z, as it has a .63 overdrive. I looked at his posted numbers on the mileage run with his DD and his Cobra. The error came out to roughly a 3:49 ratio rear end, hence my guess at a 3:55.

I supposed he could check 4th gear (1:1 ratio) at say 55mph and everyone else could see what they turn at 55mph in 4th and figure the amount of error from there.

Could also do the driveshaft rotation test, just need to make a nice chalk mark on the driveshaft to reference and start counting turns.

And finally, could drop the differential cover and count teeth on the ring gear

[buddha]

Or any 5.0 with a 3.08 gear.

[vinchez78]

I thought you didn't have to change the speedo with 3:55 gears??

You don't have to change the speedo, you have to change to the correct 'speedo gear' that goes in the tranny.

Thanks P, I may need ur help on that whenever I get the gears

[escogt]

I thought you didn't have to change the speedo with 3:55 gears??

You don't have to change the speedo, you have to change to the correct 'speedo gear' that goes in the tranny.

Thanks P, I may need ur help on that whenever I get the gears

You got it, brother!!

[WillyB93]

I drove my car today and totally forget to pay attention to the RPM verse MPH. I will try to drive it tomorrow and get details on it.