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This is kind of a silly question, but I'll ask anyway. I've got a 66 289 2V with automatic transmission. My rear end (which I assume was stock) recently went out. It's 8". I had to travel out of town on business, so a Mustang friend found a used 8" rear end in good condition, took it to my mechanic, who had it installed when I got back home. They told me the new rear end was a 3.5 I think - I'm going to verify that tomorrow.

I'm not exactly sure what the old rear end ratio was, but I'm pretty sure this one is higher. Car seems to perform better, but I just noticed yesterday that at highway speed, my speedometer is showing 8-10 mph faster than I'm actually going. So if it says 70 mph, I'm really only going 61-62 mph. I used a gps with speed to confirm it. This seems to indicate the rear end is a higher ratio than before.

So here's the silly question: Since I've had the new rear end, I've noticed my gas mileage has improved significantly. I would expect with the 3.5 ratio which I think means I'm turning more RPM's, my mileage should be lower. Can anyone explain that or do I have it all backwards.

Thanks!
 

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You need to change the speedometer gear for one with more teeth to correct your reading. Without knowing your tire size and current speedo gear I would guess a 21 tooth will get you really close. The mileage increase is from the lower gears(higher number) allowing your car to accelerate with less effort.
 

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Discussion Starter #4
You need to change the speedometer gear for one with more teeth to correct your reading. Without knowing your tire size and current speedo gear I would guess a 21 tooth will get you really close. The mileage increase is from the lower gears(higher number) allowing your car to accelerate with less effort.
Yeah, I had been Googling the speedometer issue and saw I needed to change the speedometer gear. And I should have included I'm running 15" tires.

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Also, the size of your tire will help. My tires are 225X60X15. the rolling diameters 25.6. TCI, if I recall, correctly, has a speedo "gear" calculator on their site. But then, there are many in "space.net". Did your "mechanic" not offer the ratio or even check? There are ways to check after the fact.
 
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Just like your speedo went up, your odometer is logging more miles than reality.
More "fake" miles = more Fake mpg.
 

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65 Fastback 289 4 spd, 65 convertible 5.0L 5 spd. 3.73 8.8
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This is kind of a silly question, but I'll ask anyway. I've got a 66 289 2V with automatic transmission. My rear end (which I assume was stock) recently went out. It's 8". I had to travel out of town on business, so a Mustang friend found a used 8" rear end in good condition, took it to my mechanic, who had it installed when I got back home. They told me the new rear end was a 3.5 I think - I'm going to verify that tomorrow.

I'm not exactly sure what the old rear end ratio was, but I'm pretty sure this one is higher. Car seems to perform better, but I just noticed yesterday that at highway speed, my speedometer is showing 8-10 mph faster than I'm actually going. So if it says 70 mph, I'm really only going 61-62 mph. I used a gps with speed to confirm it. This seems to indicate the rear end is a higher ratio than before.

So here's the silly question: Since I've had the new rear end, I've noticed my gas mileage has improved significantly. I would expect with the 3.5 ratio which I think means I'm turning more RPM's, my mileage should be lower. Can anyone explain that or do I have it all backwards.

Thanks!
With your odometer off you are showing 14% more miles then you are actually driving. But your mileage could of also gone up.
 
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