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Speedometer Issue

4K views 82 replies 8 participants last post by  scottydsntknow 
#1 ·
I swear I already posted this but can't find it and haven't received any response so I apologize if I posted twice. I still need help with the following issue.

I have a stock '98 GT with the stock 17" rims and stock size tires. I'm get probably 10+ MPH too slow. I'm going down the freeway where everyone is doing 65-70 and my speedo shows about 55. To check my theory I checked the odometer too. I pushed the knob in for the trip meter at the mile marker and by the next mile marker I had only gone .7 according to the odometer. Great in the way my miles are way lower than reality and keeping my value higher. Bad in the way I'm risking get a ticket for going over the posted speed limit. Any ideas? My speed shows smooth movements, no jerkyness or erratic movement. Just knowing my tach I know the tach is way too high in 5th gear compared to the low mph. It's a stock man trans too. Scratching my head...
 
#2 ·
Get a speedometer app for your phone and see if it matches. If not, you might have had the rear gear ratio in your car changed at some point by a previous owner.
 
#3 ·
Ok. Assuming I find my belief is correct. How do I determine which gear is correct?
 
#4 ·
If you want to be accurate, then count the teeth. Take the diff cover off so you can see the gears, count the teeth on the ring and pinion. Divide the ring teeth number by the pinion teeth number. Example: 41 teeth on the ring and 10 teeth on the pinion = 4.10 gear ratio...
 
#8 ·
When I had 3.73's put in my '98 Cobra the installer just changed the gear at the end of the speedo cable going into the transmission and the speedo was right on the money. I think it is just one bolt holding the clamp on it and then a lockclip holding the plastic gear on. There should be a chart showing the different colors that correspond with the different amount of teeth on the gear. They are cheap to purchase. I would check but sold the car a while back.
 
#9 ·
According to the VIN Decoder I had a 2.73 axle ratio when the car was produced
 
#23 ·
The tire spinning trick is the most sure fire way to tell short of pulling the diff cover and counting teeth.
 
#24 ·
I did the speedometer app and it's definitely off. At 30 the app showed 38. At 50 it showed 60.
 
#26 ·
On the list of things to do... just very low priority right now with my son graduating in three days and then a huge party for him this Saturday.
 
#33 ·
Ok. The answer is 5 1/2 driveshaft rotations for 2 tire rotations. Got a couple pics of the rear axle while I was under there too.
Auto part Metal

Cylinder Gas Metal
 
#39 ·
Ok. Cool. What does the "L" mean? Wouldn't this car have come originally with a 3.28?
 
#40 ·
I know that a 2.73 or 2.75 gear would be extremely tall. This is according to my limited knowledge.

I am not sure if tire dimensions have an effect in regard to the math used in determining the gear ratio without removing the rear diff cover.
 
#44 ·
That makes sense, but I have no idea why the speedo would be off, unless the rear end was returned to stock after a gear change. This scenario could leave you with a speedo gear that is still wrong.

The joy of working on older, modified vehicles.
 
#52 ·
Might just drop the cash and change the gears to 3.73 and go from there. I keep reading about using some tuner but how does that affect a 98 with cable and not electronic speedo?
 
#63 ·
You got me. I typo'd. I know, no one ever does that. The sticker says 225/55R16 which is what I had (currently sitting in my garage waiting to be sold on CL). I hope this helps clear up my typing error.
Text Font Car Vehicle
 
#62 ·
It sounds like you bought a car that had a gear swap at some point. Before you bought the car the gears were replaced with the stock ratio, but the speedometer gear was left from the previous gear swap. When you changed wheels and tires you compounded the problem.

Change the speedometer gear back to the factory gear tooth number. When you replace the tires with 245/45/17s the speedometer should be reading correctly.
 
#64 ·
Thanks. That's the plan. Hopefully selling the old rims and tires can help offset the cost of the new tires.
 
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