Weird speedometer issue

Mike Honcho · Oct 2, 2019

So I'm having something strange going on with the speedometer in my 94 gt. The speedo has been off since I bought the car, it was a 2.73 car with a 5 speed and previous owner swapped in a 3.27 rear axle with 285/35/18s. No big deal. Used the gear calculator on LMR, ordered a 20 tooth driven gear, installed, and the speedo was still off. Here's where it gets weird, the speedometer is off more at higher speeds. Around town the speedo is fast by about 5mph. But with the speedo reading 90mph, gps read 67mph. The higher the speed, the further the speedo is off. Thought it must be a bad speed sensor, replaced it. Drove the car, now 90mph is reading 75mph so it's some better, but nowhere near accurate. Starting to think there must be and issue in the insturment cluster or eec, any ideas? Maybe I just need to move up to a 21 tooth driven gear?

07GtS197 · Oct 3, 2019

First I would verify what gears it really has since its so far off. It has to have a different ratio. My 87 grand marquis has 4.10s, originally 2.73s, and when it says 70 Im really going about 48. It sounds like youre in the same boat, I think you have steeper gears than you think.

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Mike Honcho · Oct 3, 2019

07GtS197 said:
First I would verify what gears it really has since its so far off. It has to have a different ratio. My 87 grand marquis has 4.10s, originally 2.73s, and when it says 70 Im really going about 48. It sounds like youre in the same boat, I think you have steeper gears than you think.

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That is possible. Axle tag is still on the rear end but I will verify. That actually would probably make more sense. I kept thinking that the speed should be off an equal amount regardless of speed, but obviously I'm shifting through the gears and each gear has a different ratio so it couldn't stay the same.

SnakeBit! · Oct 3, 2019

if the rear gears have been changed, then it will be off more, the faster you go. jack the car up and see how many times the driveshaft rotates for one revolution of the tire.

lutter94 · Oct 3, 2019

Of course it will be inaccurate based on a percentage not a certain mph. It's not weird, its how ratios work. If you go from 2.73 to a 3.27 you will gain 20% in engine RPM's for the same given speed. Or loose 16.5% of your speed (reading 90 mph would truly be 75 mph). Yes I know 20 doesn't equal 16.5. 2.73/3.27 = 83.5%. 3.73/2.73 = 1.20%

Here's some math for ya:
With your 285/35R18 tires, so 25.9" tall. And a T5 trans, your final drive ratio is 0.68 IIRC.

If you have 2.73 rear end gears, a true 70MPH will be right at 1690 RPM. See below for all of them. Not saying your tach is 100% perfect, but you should be able to tell between the 200-300 RPM steps.

2.73 @ 70 MPH = 1690 RPM
3.27 @ 70 MPH = 2025 RPM
3.55 @ 70 MPH = 2200 RPM
3.73 @ 70 MPH = 2310 RPM
4.10 @ 70 MPH = 2540 RPM

Sounds like you are currently setup for 2.73 gears, but actually have 3.27. If you're doing 90 reading 75.....math is working out perfectly at the moment.

What drive gear do you have? You had it all apart correct? Did you count teeth?

Mike Honcho · Oct 3, 2019

lutter94 said:
Of course it will be inaccurate based on a percentage not a certain mph. It's not weird, its how ratios work. If you go from 2.73 to a 3.73 you will gain 37% in engine RPM's for the same given speed. Or loose 26% of your speed (reading 90 mph would truly be 65 mph). Yes I know 26 doesn't equal 36. 2.73/3.73 = 73.2%. 3.73/2.73 = 1.37%

Here's some math for ya:
With your 285/35R18 tires, so 25.9" tall. And a T5 trans, your final drive ratio is 0.68 IIRC.

If you have 2.73 rear end gears, a true 70MPH will be right at 1690 RPM. See below for all of them. Not saying your tach is 100% perfect, but you should be able to tell between the 200-300 RPM steps.

2.73 @ 70 MPH = 1690 RPM
3.27 @ 70 MPH = 2025 RPM
3.55 @ 70 MPH = 2200 RPM
3.73 @ 70 MPH = 2310 RPM
4.10 @ 70 MPH = 2540 RPM

What tooth gear did you have before 20? How far off was the speedo then? Are you saying it didn't change at all?

Yeah, I know it's not weird. I just wasn't thinking about it correctly, just had a brain fart I guess. It had a 18 tooth in it originally. It did change, just nowhere close to correct. Lower gears is probably the issue the more I think about it. I'll verify today. You have it backwards, if I'm going 75mph(gps) the car reads 90mph.

Mike Honcho · Oct 3, 2019

Well the bad news is I feel like a dipshit for making this post. The good news is the car has 3.73s instead of 3.27s like the axle tag says. At this point I'm just glad I got it figured out. New speedo gear is on it's way.

07GtS197 · Oct 3, 2019

I figured as much but its an easy and cheap fix so dont sweat it.

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lutter94 · Oct 3, 2019

I editied my post a hand full of times lol, didn't give your op a perfect read thru. Looks like you quoted that before my last edit lmao. Reading = speedo, truly = gps.

Mike Honcho · Oct 3, 2019

lutter94 said:
I editied my post a hand full of times lol, didn't give your op a perfect read thru. Looks like you quoted that before my last edit lmao. Reading = speedo, truly = gps.

No worries. Got it all figured out, should've been thinking the first time around. I think I'm just getting frustrated undoing all the mess from the previous owner.