285 tires speedometer/odometer errors

[jmtandem]

Anybody have a quick and easy answer for the speedometer error by running 285 tires in deference to 245's A handheld GPS seems to give it about a 10 percent error. Does that sound right?

 

[justme]

I figured mine out to be 9. 3% off with my tires. No problem I use the 10% rule. 50MPH indicated equals 55 MPH, 55 MPH indicated equals 60 MPH, 60 MPH indicated equals 65 MPH, etc.



Easy to overcome.



Michael

 

[WBerry]

check these sites. www.miata.net/garage/tirecalc www.pw1.netcom.com/~sgalaba/tiresize

 

[WBerry]

tthe 2nd one is not quite right. willtry it again. www.pw1.netcom.com/~sgalaba/tiresize.htm#ustires

 

[Jeff H]

:-{} Acording to the mile markers mine is exactly one tenth of a mile short.

 

[MikeR]

. 10 percent!!!!??????



My 285 bfg a/t's mounted on the factory alloys only show to be . 03 off. I have checked it by mile markers,speed traps, and using the calculator for the fuel mileage both before and after tire swaps.



-Mike

 

[LSMITH]

Mine is off my a tad over 9% as well, but part of it I attribute to error in the factory speedo. It keeps the wife from pointing out that I am going way too fast so I don't really want to fix it.



Mike, did you have the 265's? The calculated error for mine fron the 245's should only be about 6. 5%, so 3% would be very close to what you should see if your speedo was accurate to start with.



I think the handheld GPS is the most accurate test short of having the speedo calibrated if you can get a couple mile straight level stretch of road.

 

[jjdiesel]

speedo correction

Just did mine this past week. Finally wore out my factory 245's and put some 285's on 16" X 8" Eagle alloys. Speedo showed 9. 3 in 10 miles. Installed a Abbott ERA, now it's perfect! Easy to install and not too bad on the price.

www.abbott-tach.com or ... . 1-800 643-5973

 

[PRout]

Had my speedo calibrated at AAA twice.



When the 285's were new, the speedo was off about 9%



At about 45-50k on the tires it was off 7%.

 

[jmtandem]

Thanks for the info everybody. That is what this forum is all about.

 

[Posm]

I have an easy and cheap way to determine how much spedo is off. Using a stop watch or a digital watch with a stop watch time the amount of seconds it takes you to travel between mile marker posts and divide by 3600. If you want to increase accuracy set you cruise on a flat area and get an average from several miles. IE 60 seconds between posts = 60 mph



The reason the error changes is because of tire wear. my 235/85/16 Michilen M/S tires have lost about 1/2% of error since new. I gained about 1 inch of height over the stock 245/75/16 Michelin A/S tires, but most of it was in tread, not in sidewall hieght. The M/S tires come with much deaper tread than the A/S tires do. I would say some of the 34 -36 inch tires with very deep tread my loose 2 % of their height from new to worn out.

 

[Blakers]

My truck indicates about 5 mph faster than what it is really going with 265's. I figure it should be about right on with 285's.



Blake

 

[RON'S RAM]

Mine is off about 10% too. Does anyone know of a new drive gear for the speedometer that will correct this?

 

[Steve St.Laurent]

10% error is what I've got with my 285's too. I now know for sure because I got pulled over TWICE within 16 hours this week and the speeds both cops gave me was exactly 10% over what I was indicating. BTW, they let me go with just a verbal warning on both tickets (one was 68 in a 55 and the other was 56 in a 45). Being polite and considerate of what cops have to deal with works wonders! In both cases I pulled WAY off the side of the road, turned the truck off (asked once they got to window if I could restart the truck to cool the turbo), rolled down the windows, turned the interior light on, and put both my hands on the top of the steering wheel until they asked for my info. The 10% rule is easy, just add the first digit to your speed for actual - 35+3=38, 55+5=60, 65+6=71, etc. I've had oversized tires on all my trucks for the last 12 years, haven't corrected a speedo yet.

 

[RON'S RAM]

Steve, was that picture of you taken, pulling away after you got your ticket? I would like my speedo more accurate mainly for keeping track of fuel mileage, without using any formulas to adjust for tire size.

 

[jjdiesel]

speedo gears

RonsRam, for your '97 I'm quite sure there are three separate speed gears offered... . they install in the transmission, very easy 2 minute job. If not perfect, one of them should at least get you very close. Under 10 bucks each if I remember right.

 

[Alaskaskiff]

Parts manual lists 11 different gears for the speedomoter. Anything from 28 to 38 teeth on the gear are available.

 

[Pthomas]

interesting subject guys

Heres what ive learned,

bigger tire speedo goes slower. .

the FASTER you go the MORE it is OFF/SLOWER than actual mph. '35=39, 65=74... example

My fix is the rear axle ring gear sensor!

it produces ac voltage and more of IT the faster you go!

It also has a AIR GAP from the relucter ring on the ring gear. the closer you move the sensor to the ring the more ac voltage it produces and the speedo will go faster ! only one problem is getting It closer and not hitting it.

Ive fixed alot of abs faults with the same theory

some times on 4wabs A sensor air gap is moved enough to cause a speed sensor code. SO DONT DO THIS IF YOU HAVE 4WABS UNLESS YOU MAKE SURE THEY ALL READ THE SAME ON A STRAIGHT ROAD. after.

 

[fest3er]

Re: interesting subject guys

... It also has a AIR GAP from the relucter ring on the ring gear. the closer you move the sensor to the ring the more ac voltage it produces and the speedo will go faster...

The air gap really doesn't have anything to do with speed indication other than if the gap is *too* large, the sensor won't work at low speeds.



The sensor produces about 400mV of AC signal at close to stopped. It rises to around 2V at around 35 MPH. The sensor produces a sine wave that varies from 0 Hz when stopped to perhaps 220 Hz at 100 MPH. (These figures are approximate; I have the calculations somewhere, but they aren't handy. )



Typically, the device that receives the signal (the Abbott box, the CAB) will run the signal through a small amplifier and convert the AC sine wave signal to a square wave. The CAB (or Abbott box) then changes the frequency slightly to account for different size tires.



From this square wave,. the PCM determines the distance travelled by computing the elapsed time between the rising (or falling) edge of the square wave as it crosses zero volts. The PCM has a standard width that corresponds to a specified vehicle speed.



Thus, suppose that a 2 Hz square wave corresponded to 1 MPH. Since it is a linear relation, 20 Hz would be about 10 MPH, and 200 Hz would be about 100 MPH.



If you then install tires that are 10% larger, the frequency produced by the sender (trans or rear diff) would be 10% lower than it should be. The Abbott box would increase the frequency by 10%, thus yielding the correct speedometer reading.

Now if you want to be *strictly* accurate, the sensor signal is actually used to determine *distance* travelled. The PCM uses that figure to drive the odometer. It also divides that figure by the elapsed time so it can tell the speedometer what velocity to display.

Now you know more than you *ever* wanted to know about how the odometer, speedometer, speed sensor and CAB work together.



Caveat: this applies strictly to 98+ trucks. Earlier trucks may have had the PCM fed directly by the gear on the trans, but the PCM operation was probably quite similar if not the same.



We engineers are *such* bores.



Fest3er

 

[RON'S RAM]

Thanks for the info. Has anyone done their homework before me and found out what size gear should be used with 285 tires?



Andy, I noticed that you are just down the road from me. It's a long road though. I sent you a short e-mail.