Unfortunatly i needed a speeding ticket for it to all make sense.
There was never any engine problems, it was the Speedometer the entire time. Its 10mph over when while driving 50mph. The faster I went with more highway time just meant more miles not being read on the tripometer. So I got a surpise when I was pulled over for 69mph when I thought I was just cruising at 57mph, unfortunatly New Jersey tells you to F off if you claim its a broken speedometer and I am going to have to pay the ticket off, shame really, I have been a good boy in my Mustang.
So using my GPS as my trip and speedometer, I am getting closer to 19mpg, not bad at all considering I accelerate like an ass. And my speedometer is off because it was never recalibrated with the new wheels like I assume a normal person would take car of. I have 17' Saleens instead of the stock 15' wheels.
Are their any speedometer calbrating kits for the drivetrain so I don't have to use my GPS anymore. Plus its a bummer knowing they is probably a lot more miles on this car than what it says on the odometer.
Fun fact btw, I thought I did 95mph in the Mustang, well now consider the speedometer was broken, I probably did around 115mph.
The fact that you are running 17s rather than the stock 15s may not be your problem. I did a comparison between my 245/40/17s and the Mustang stock size, 225/60/15s using this website.
The 17s are actually shorter than the stock 15s so turn more RPMs (revolutions per mile). This would make the speedo read fast rather than slow. Try using the website with your size 17s.
Has the rear axle ratio been changed in your car? If so, the speedo drive gear may not be correct.
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CW
Craigfield, Tennessee Member Music City Mustang Club '93 LX Mustang '56 Ford Thunderbird convert '06 Harley Street Glide '01 Harley Softail Custom
The fact that you are running 17s rather than the stock 15s may not be your problem. I did a comparison between my 245/40/17s and the Mustang stock size, 225/60/15s using this website.
The 17s are actually shorter than the stock 15s so turn more RPMs (revolutions per mile). This would make the speedo read fast rather than slow. Try using the website with your size 17s.
Has the rear axle ratio been changed in your car? If so, the speedo drive gear may not be correct.
This is what I was thinking. When you go up in rim size you go down in the tire sidewall size (50 series, 40 series and so on) So I was thinking you may have after market gears. Check into that. you may need to get a new speedo drive.
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1990 LX, MAC CAI, C&L 76mm MAF, DIY ported upper, MAC 70mm T/B, BBK Equal length shorties, BBK O/R H-pipe, March U/D Pullies, Flowmaster American Thunder with Dumps, FRPP 4.10 gears, Electric Fan, 130 Amp Alt, FRPP Plug Wires, MSD Cap, MSD Coil, 14* Timing, 180* stat, BBK springs, Chrome pony's.
If your speedometer is off, I dont think it is reflected on your odometer.
Why would that be so? They both get their input from the same source, the speedo cable hooked the the output end of the trans.
Assuming the speedometer and odometer (and tripmeter) were all in sync when the factory made 'em, they would all be off by the same percentage from whatever factor caused it to be off. Such as shorter/taller tires/wheels, R&P change, etc. You can determine speedo error by using the mile posts on the expressway. As you pass a mile post glance at your odometer. Ten miles later glance again. Figure the percentage error by using the difference between the readings and the difference from 10. Example, if your odometer reads 9 miles went by, your speedo is 10 percent slow. If it reads 11, it's 10 percent fast. I've verified this with my Garmin GPS.
Same percentage of error, speedo, odo and trip. All off the same.
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CW
Craigfield, Tennessee Member Music City Mustang Club '93 LX Mustang '56 Ford Thunderbird convert '06 Harley Street Glide '01 Harley Softail Custom
Yes theyre definetly both off if ones off. You need to change the gear in you tranny. Pull the one you have, (one bolt on driver side rear of tranny) count the teeth and then subtract two. This will be the gear you need.
The odometer has to be off on this car because the tripometer is off. So if its not the wheels, does that really mean I am running shorter gears, like 3.73 or something like that?
This car is a bit mystery, I love it and everything, but I have been finding out new things every week with it.
Btw, should I buy this 1991 Mustang GT 5.0
Started at $3400, now its $1600. There is a ton of things I could nab from this including the ram air hood, extra set of wheels, leather seats, and some other stuff.
Its has to be a repaint btw, maroon firewall is a dead give away.
I might throw him an e-mail go see it in person. Count the things wrong with it and dog him to $1000.
The odometer has to be off on this car because the tripometer is off. So if its not the wheels, does that really mean I am running shorter gears, like 3.73 or something like that?
This car is a bit mystery, I love it and everything, but I have been finding out new things every week with it.
Yep. Mine is 10 off on the speedo too and has been for over ten years (or so I'm told by the former owner, so I'm guessing my mileage is a lot less too). And I'm running the 3:73s. And I Have the AOD, so I need a speedo gear. I've been just dealing with it, but it's gonna get me in trouble, I can feel it. It seems the faster I go, the more it's off, unless everyone around me are leadfoots too..hehe. Because I'm showing 85-90mph (2500+) on the interstate going home, but I do 45 in a 35 ( my 10 over) like everyone else on city roads...I need to get it fixed.
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Thanks,
Kevin
1991 LX 5.0 Convertible AOD -
Twilight Blue Metallic- 129K miles, Flowtech Headers, Flowmasters, 3:73s, Vinyl racing stripes.
I figured out my MPG problem on my 1990 Mustang. 
