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Old 01-23-2009, 09:26 AM   #1
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Speedo Correction for Larger Tires

I got the following info from Heber at SMB to reprogram the van computer to make the speedometer read accurately for 315/70-17 tires.

The numbers we use to recalibrate the PCM are as follows using the PDS diagnostic tool. Select module programming, then programmable parameters tire size/axle ratio.
The column that will be selected for tire size will be marked OTHER. This will open next window where the tire size will be input.
Input tire size 315/70-17 (the rpm value that will automatically be calculated by the computer will be changed). Input new RPM value 699. Select axle ratio 3.55.
If the technician needs more info he can give us a call and we would be happy to help.


This will only work for this size of tire, but I'm sure SMB has more code combos for just about any tire you could put on an SMB. I don't know if you these changes can be made by a code reader type device (like a Scangauge). I had the local Ford dealer do it for $93 (min charge for 1 hr labor, but I'm sure it took the tech 10 minutes).

Before going this route I considered buying an electronic device (either a tuner or the specialty speedometer module), but none of the manufacturers could definitively say if it would work on a van. The van has a different PCM programming than the F-250/350/Excursion and many of the aftermarket products just were not designed for the vans.
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Old 01-23-2009, 10:19 AM   #2
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Re: Speedo Correction for Larger Tires

My speedo was way off until I used the SCT programmer. Now it is accurate within a mile an hour. (GPS checked)
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Old 01-23-2009, 11:53 AM   #3
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Re: Speedo Correction for Larger Tires

Quote:
Originally Posted by j.whitbread
My speedo was way off until I used the SCT programmer. Now it is accurate within a mile an hour. (GPS checked)
Maybe I should look into the SCT programmer. Which model of the SCT do you have for your E-series gas V10?

Quote:
Originally Posted by Switchfoot
Before going this route I considered buying an electronic device (either a tuner or the specialty speedometer module), but none of the manufacturers could definitively say if it would work on a van. The van has a different PCM programming than the F-250/350/Excursion and many of the aftermarket products just were not designed for the vans.
Yeah, tell me about it! I've wasted too much money with these things -- that turn out to not quite work on the E-series vans. The Superchips and DiabloSport Predator have both failed me.

I don't think Scangauge will do it.
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Old 01-23-2009, 11:58 AM   #4
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Re: Speedo Correction for Larger Tires

Being an engineer-type, I'd love to have a programmer device that has an "advanced" mode like the Scangauge, where you just send the PCM computer raw number codes and get raw numbers back -- so people can invent & trade ideas & hacks online. (I love my Scangauge.)
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Old 01-23-2009, 12:25 PM   #5
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Re: Speedo Correction for Larger Tires

So are we tricking the computer by entering 3.55's when we really have 4.11's?
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Old 01-23-2009, 12:34 PM   #6
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Re: Speedo Correction for Larger Tires

Not sure how it actually works Adventureduo. I think it has to do with the fact that the van PCM won't recognize the larger tires and between changing the revolutions per mile (RPM) value and the axle ratio - they arrive at something that works. According to BFG, the 315/70-17 RPM number is 601. So there are playing with both values. Our van has the 4.10 axle ratio too.

Anaheim is just up the street from us. I'll have to drop by sometime.

Bob
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Old 01-23-2009, 12:40 PM   #7
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Re: Speedo Correction for Larger Tires

It just dawned on me. I'll bet the PCM won't accept that low a tire RPM number (big tires turn slower) so they can't just use 601 and 4.10 axle ratio so they have to use a bigger RPM number for a smaller, faster spinning tire and slow it down with the taller axle ratio.
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Old 01-23-2009, 08:49 PM   #8
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Re: Speedo Correction for Larger Tires

I just drive with laptop gps. A little more trouble, but what the hay..

Jeff
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Old 01-29-2009, 05:38 PM   #9
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Re: Speedo Correction for Larger Tires

SCT Programmer????? Can I borrow it? I've been to 2 dealers with explicit reprogramming instructions along the same lines as already noted. $250 later I'm still off about 10% and the dealers think I'm nuts. If it's as simple as it seems why can't the %*&^%$# dealer do it?
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Old 11-05-2009, 06:10 PM   #10
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Re: Speedo Correction for Larger Tires

I used an SCT tunner from Mike at 5 star. His reputation can be verified on this Ford site: http://www.ford-trucks.com/forums/forum49/
My speedo is now dead on and the van runs and shifts way better than I thought it should from just a programmer. Probably the best thing I have done to a vehicle was to add his program. Just give him the weight, gears that are in the diferentials, tire size and what you expect it to do and he will customise a tune for your application with no additional cost.http://www.5startuning.com/v10/index.html
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