Speedo Correction for Larger Tires

[BajaSportsmobile]

Get a Dekota Digital SGI-5 for about $80.00. I use them all the time and they work great. Use a GPS to see your actual speed and the adjust your speedo exactly.

 

[blupaddler-SMB]

Not to make a broad statement, but I don't notice as much of an issue with the 33's (285) on my van. Is it because they aren't that big of a tire?
Or is my van just too old and dumb (1995) to know the difference? Do all vans have issues with speedo correction, or just after a certain year?

 

[JetUnit]

I went to the Chevrolet dealer with my Chevrolet Express. I'de change my tires for 33" tires and they only charge me 50$ to make the modification in the computor. You better look with them before spending money for nothing!!!

 

[Wanderer-SMB]

Get a Dekota Digital SGI-5 for about $80.00. I use them all the time and they work great. Use a GPS to see your actual speed and the adjust your speedo exactly.

Hey Ramsey,

Can you use the SGI-5 module on a 2009 E350? I've been to a couple of Ford dealers and they don't have the capability to adjust the speedometers. Apparently, they need a special programmer and not all dealers have it.

Is it easier for me to buy an SGI-5 and install it on my van? Is it permanent?

Thanks,
Dave

 

[derJack]

Digital Speedos are easy to configure electronically. And if one has much bigger tires, the shift point should be also.

But what about the good old ones :e7: ? Mine is from 1993 and has an IDI + E4OD.
No electronic speedo.

Any ideas? I`ve 4.10 + 35" now. These are 13,5% longer than stock 245-75/16.

 

[jage]

But what about the good old ones :e7: ? Mine is from 1993 and has an IDI + E4OD.
No electronic speedo.

Any ideas?

Just an example from google, not recommending: http://www.transmissioncenter.org/Ford_ ... _Gears.htm

Basically the different number of teeth create different RPM for the gear which translates to your speedometer directly via cable. Generally not possible to be 100% accurate (even factory speedos) but once you locate the gear it should be an easy fix.

From the above page *their calculator is... lacking:

Gear Teeth = ( Drive Teeth x Rear Ratio x (Tire Rev/mile) / 1000 )

35s have 593 revs/mile according to http://www.mickeythompsontires.com/tech.php?bulletin=g3

therefore:

Gear Teeth = ( 8 x 4.10 x 593) / 1000
Gear Teeth = 19.45

Since your 35s are not truly 35" but smaller, revs/mile goes up and gear teeth goes up, so round up to 20 teeth speedo gear needed.

And I'm just going off those two pages, you'll want to double check my math, the input gear being 8 teeth and everything else... :t1:

 

[MadScience]

To add to an old thread: I'm trying to sort my speedometer on my '99 Tijuana conversion.

Some general questions that I have not managed to find answers to: What exactly is the speed signal from the VSS on the transmission used for? Will it effect transmission shift points (this seems desirable)? Will it keep the torque converter from locking up if it's not in the correct range (that would be poor)?

-- If the answer is just shift points and speedo/odometer, then I might be done and I'll either add a signal converter, or swap gears on the transfer case sensor.

Now for some confusion on my particular van:

The VSS on the transmission was not connected, but the connector was extended and is plugged into what looks like a mechanical VSS on the transfer case (np208 I think, tag is missing). Moving the connector back onto the transmission resulted in a roughly 4x increase in the speed indicated by the speedometer. This seems in line with 2000 pulses per mile from the mechanical sensor and 8000 from the 4r100.

So, I went hunting for a converter (hoping that I might be able to re-program it). However I can't find anything. Not where the cable was made longer, not at the harness interconnect on the top/front of the transmission, not at the connector that goes into the PCM, and not at the speedometer itself.

It's obviously possible that this was spliced somewhere mid-cable run, but I'd really think someone would avoid opening up one of the large bundles. Is there some other common place for folks to splice such a thing in?

Could the PCM have been re-programmed to accept a 2k signal?

Finally: My transmission (and engine) are Ford remanufactured units, installed, as far as I can tell, by a Ford dealer, prior to the conversion. So I don't know whether they fully conform to '99 specs, but would assume that they did not swap too many other parts.

 

[MadScience]

OK, I really want low range to shift with the new transmission, so I need to move the sensor reading to be in front of the transfer case. I'm certain something has been re-programmed on my van, but I've not been able to figure out what or how, so I'm just going to translate the input accordingly.

Since I just want this to work now, I ordered one of the commercial signal translators (don't tell my ee-professors, they'll figure out how to invalidate my degree). Still, there's value in something that's been proven to work in an automotive environment.

I'll see how it goes. Specifically I'm curious if any rounding errors screw with torque converter lock up.

 

[Gtbensley]

I was able to program my speedometer from the cluster on my 1995 e350. It's somewhere located deep in a service manual on how to do it. But basically you make a jumper wire and jump a connection under the dash that puts the cluster into a mode that can be adjusted.
This can only be done something like four or five times ONLY before it's will not adjust anymore.

You input a value so you need to have a conversion chart and do some math. I did mine a few years ago so I'll have to refresh myself on how....currently on the road up in nova Scotia.

It absolutely does change the shift points. I screwed up my first value and it was going into OD at 25mph.

I now am pretty close, at indicated 65mph I'm going 64mph on a GPS.

 

[GAR-SMB]

I wonder if a tire shop like Les Schwab can reprogram the speedo after ya purchase the tires.

Gar

 

[louis.robins]

I always use the GPS when I travel.

 

[1der]

I use GPS for speed validation and also to confirm odometer readings.

Typically at fill up I zero out the trip meter on the Speedo and zero out the GPS distance/trip meter. After 100 or so miles (or more) I compare the two. This gives me the error correction % for calculating true distance traveled for accurate mpg calcs.