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Old 07-05-2020, 05:34 PM   #1 (permalink)
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Rear end swap revisited

Hello everyone.

Long time no see. I hope everyone is staying healthy during all this Corona craziness. Stay safe.

I have asked about a rear end swap before and it has always been on my mind. It was one of the reasons I started the MPG/RPM thread a few months ago. For my Ranger I seem to get the best mileage on the highway at 1850-1900 rpm. The engine gets so smooth and nearly disappears audibly. The problem is that it's only around 48-50 mph and I wind up causing quite a bit of road rage for other drivers. I know that some would say "Ah, forget them" but I drive the same 2 lane country blacktops to work most everyday and I really need to share the road responsibly with my fellow drivers. That and I wouldn't be able to live with myself if someone got hurt because an impatient driver felt he had to pass me. Even though it would be my fault directly, it might be indirectly. When the road is empty I'm right there at 48 mph until someone catches me. Passing zones are few and far between on most of these roads, so I speed up and if they still want to pass then at least I'm at the posted speed.

I now have access to a 3.55 rear end that puts me in that optimal rpm range at 55 mph. Even at 60 mph I'd be in the rpm range I am currently at and getting between 30-32 mpg depending on the weather and route. My goals are to reach 35 mpg and still have my Ranger look like a truck.

With all that long winded stuff said, here is my question......

IIRC the 2003 Ranger gets it's speedometer signal from a Variable Speed Sensor. I think they are mounted on the rear end. If that is the case, then do I need to do anything with the computer to make sure the speedometer is still correct or can I just swap it in and the VSS takes care of the adjustments? Do I have to let the computer know I swapped gearing or will it automatically know because the signal will be different? Also, is it the same signal that controls the tach? If so, then the same question applies. If not, then how do I make sure the tach is also corrected?

Thanks and stay healthy.

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Old 07-05-2020, 08:20 PM   #2 (permalink)
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The tone ring in the differential will turn at the same speed as your tires so no difference. You are only changing the ratio that your engine sees so you do nothing and the speedometer reads correctly.

The tach is driven by the engine computer so no changes are needed there either. You will notice slower rpm at 55-60 than you have now and that is all.
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Old 07-05-2020, 10:53 PM   #3 (permalink)
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Me-and-my-metro might be right; I don't know what a tone gear is. But I think you might need to adjust the speedometer and odometer signal after your swap. There are aftermarket products that can adust the voltage from the VSS so that the ECU gets what it is programed for. But before buying anything, get a good, free GPS app onto your smartphone and compare dash indicated speed with the GPS. You'll know if the two are off. Then, compare the dash indicated distance with the GPS over as significantly longish stretch of driving. I use a 100 mile ride out to the desert.
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Old 07-05-2020, 11:54 PM   #4 (permalink)
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The tone ring is mounted to the carrier inside the differential. The speedometer is calibrated by rotations per mile based on tire size. Changing the gear ratio in the differential does not change number of times the carrier turns per mile. If you change tire sizes you need to recalibrate your speedometer with this type of setup. The sensor and the tune ring generates a low voltage AC signal based on speed that is interpreted by the pcm and converted to the speedo signal.
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Old 07-06-2020, 08:50 AM   #5 (permalink)
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Thanks for the advice both of you.

I do have a larger tire size on there now. I checked the speedo and odometer before the tire change and they were very minimally off. Just a hair over 1 mile for every 100 driven. I figured that was pretty good. The current set up is 5% oversized tires. I did the same math and checked after the change. IIRC it went up to something like 6.5 miles for every 100 driven. So I'm thinking the speedo and odometer are fairly "calibrated" from the factory. At least in my truck. Vehicles are production line products and there are always some variances. Mine must have been one of the "good" ones.

I'm going to pick up the rear end next weekend. Not sure when I'll get it into mine, but after some testing and many full tank runs, I'll post up some results.

Wish me luck.
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Old 07-06-2020, 12:15 PM   #6 (permalink)
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Just do it.

Expect mpg improvement about half the difference, 3.55 to 2.98 would be ((1-3.55/2.98/2)+1) *32=35 mpg.
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Old 07-06-2020, 01:45 PM   #7 (permalink)
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Thanks arcosine.

I guess I should have been a bit clearer. I'm not trying to get all my gains from the gear swap. 35 mpg is my total goal for now. Once I hit that I'll start investigating larger projects. I've got a few other ideas on the project table now to help. Right now the rear end is a 3.73 so a switch to 3.55 isn't big by any means. I would have loved to find a 3.45 but they are only in the older Rangers that have the cable driven speedometers. Having to stay with the VSS type, it seems the 3.55 is as low as I can go. I wish I could afford having the just the gears changed, but it's almost $1k. I can find a good rear end for $150 and swap it with a friend for a case of beer and a couple of pizzas. Now if there was an easy way to swap in a 3.27 from an Explorer or a 3.31 from a Mustang that might be a different story.
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Old 07-06-2020, 03:25 PM   #8 (permalink)
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looks like some explorers have 3.08 in 8.8 in.

https://www.fordranger.net/threads/w...-do-i-have.38/

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