Possible Wheel/Tire Setup with Speedometer Cal Solution

[DJones]

After many months of thinking about it, the time has come for me to buy some new tires. I had been afraid of speedometer calibration with oversized tires, but I think I have a possible solution to that too. Based on excellent reviews and long tread life (>100,000 miles) I have decided to go with BFG All-Terrain T/A KO, but havn't decided on a size. I have three 16" aluminum spares purchased and can go buy one more, if I decide on a 16" tire.

Possible sizes:
245/70/16 (stock size, comparable to 245/65/17), 29.5"
245/75/16 30.5"
255/70/16 30.2"
265/70/16 30.7"
265/75/16 31.7"
265/75/16 31.8"
275/70/16 31.2"
245/70/17 30.6"
245/75/17 31.6"
265/65/17 30.7"
265/70/17 31.8" (two varieties available)
275/65/17 31.2"
275/70/17 32.2"
Any thoughts or opinions?
A trip to the Badlands Off-road park may be soon!

And the fun part: speedometer calibration. Since PCMforless cannot calibrate my year's PCM for a different size, I think I have found a solution. My solution uses a frequency to voltage converter attached to the vehicle speed sensor which generates a voltage based on the frequency input. Then, a transistor amplifier with adjustable gain is connected between it and a voltage to frequency converter, which sends the correct pulse frequency back to the PCM. Hopefully there is a hole through the body under the center console to run wiring for the potentiometer, which adjusts calibration. It works like this: Standard voltage swing 0-10 volts, oversized tire swing 0-9 volts, amplifier adds 11% which creates the original swing of 0-10 volts. This would need to be implemented on the front wheels as well, to be sure that A4WD and ABS continue to operate normally too. What is the maximum frequency of the VSS at 120 mph?

[The Roadie]

Here's a circuit I found in a quick search:



The output from the sensor is tiny and AC, so the input filter cleans it up for the first op amp to be used as a comparator. The output from that op amp is filtered by the 560K and 10uF, and the rest of the circuit you don't need just triggers on when the vehicle gets above 45 MPH. Your V to F converter would have to output a tiny signal to mimic the original coil output, but I'm sure you would scope that first.

Personally, and I'm an electronic designer, I'd just rather repaint my speedometer gauge face than go through all this hassle with circuits. One easy job rather than three tougher ones.

[bgwolfpack]

Or just clip the wires to #36 of illustration. No more worries.

[Trail X]

If you're that worried about it, just buy a scangauge.

[DJones]

But wouldn't it read the same data from the VSS?

[Trail X]

It does, but it displays a calibrated speed value based on a percentage factor that you input.

[DJones]

I DO want to buy a scangauge, but I also think that building a speed scalar would be fun. My circuit for my turn signals have been on the breadboard for several weeks now, but no time to test it in the application or construct the actual circuit board.

Edit: I found a module that will do this, but doesn't require any adjustment for ABS unless the gearing is 4.56 or larger. I could try it without, and see what happens.

[HARDTRAILZ]

You realize that even 32s look small w both lifts you have...

[DJones]

I didn't take that into consideration. Looks like I will look through build threads to see what others have done.

[Diacom]

Based on excellent reviews and long tread life (>100,000 miles)

I will agree with the reviews, but greater than 100,000 miles on tread life? Not so sure about that.

[navigator]

you could also just get a gps if you don't have one already. Most of them display speed.

[DJones]

The GPS unit for my radio is $60, and I will buy it to use APRS. Then other hams can know where I am. Won't solve my slight case of CDO though.

[chevycrew]

Sounds like something like this...

http://www.superlift.com/accessories/truspeed.asp

[bartonmd]

Sounds like something like this...

http://www.superlift.com/accessories/truspeed.asp

Yep, "speedohealer" is another one that's used on motorcycles with electronic speedometers. I'm sure it would work here, as well... I don't know what that one costs, but the Speedohealer is $115...

http://www.sporttour.com/shopexd.asp?id=2846

Or the "Speedotuner" is $78... http://www.hardracing.com/speedotuner.htm

Mike

ETA: or as they taught us in school... "If there's a circuit you're thinking about designing and making, you can pretty much bet that somebody, somewhere, makes it all in 1 chip for 1/2 the cost of making it yourself, that is more reliable and accurate than what you would build on a proto-board"

[DJones]

I bought some tires. BFG A/T KO 275/70R17 and they fit! There is only a small amount of rubbing on the fender liners while turning in reverse
Aired down to 15psi gives the most traction, while 32psi on-road is too soft. I'm going to try 40psi for a while.
Cruise control didn't seem to like being set to 60 on the highway.

[bgwolfpack]

David,


Sweet size tire btw.

[Mudwheelin]

Looks great!

[Zero]

Sounds like something like this...

http://www.superlift.com/accessories/truspeed.asp

Yep, "speedohealer" is another one that's used on motorcycles with electronic speedometers. I'm sure it would work here, as well... I don't know what that one costs, but the Speedohealer is $115...

http://www.sporttour.com/shopexd.asp?id=2846

Or the "Speedotuner" is $78... http://www.hardracing.com/speedotuner.htm

Mike

ETA: or as they taught us in school... "If there's a circuit you're thinking about designing and making, you can pretty much bet that somebody, somewhere, makes it all in 1 chip for 1/2 the cost of making it yourself, that is more reliable and accurate than what you would build on a proto-board"

So when will you have one of these units for us sourced or built????/huh mr Barton....huhhhhhh....when!!!

[DJones]

I hope to get parts soon. I can't imagine it costing more than $20 for everything. Plus I only need two chips unless I can find one that does everything. The parts that I really want have a lead time of 13 weeks.

[v7guy]

i'm pretty interested in this as well

i'm in need of new tires and it seems like the perfect time to throw on a lift and step up to a larger sized tire, it'd be nice to have the speedo read correctly