FL Columbia Speedo Off 3-5MPH

[sgreer78]

As title states. I've only had this truck for about a month and I knew something was fishy when my cruise was set at 65 and Werners were passing me left and right. I went by a few of those radar things that tell you your speed and at 65 it said 61-62. So I did some math and on the way back with cruise set at 1400RPM which should be around 65 it said 65-66 and showed about 70. The guys at FL are telling me that there is a 3% error cushion and that this is not out of that %.

Now I haven't been driving for 30 years or anything, but in my nearly 9 years driving, and driving some pretty ****ty trucks, I've never had one this far off.

All I want to know is, is this normal? Before I bought this truck I've driving Volvo's exclusively for 5 years. Maybe I need to re-acquaint myself with shotty-made trucks?

[tracer]

it's funny ... i've been busy lately trying to solve the same problem. The RPM at 60 mph must be Revs/mi (tires) X Axle Ratio X Top Gear Ratio (usually 0.73). My tires are low pro and spin at 512 revs/mile, the axle ratio according to the paperwork and the plaque on the front diff is "3.73" and my tranny is a 13 speed with a 0.73 ratio. So, at 60 mph my RPM should be 1394, while in reality it shows around 1350. The speedometer has been lying too, and at the end of the day I'd be 5-7 miles short (actually ran more than what the speedo showed).

So, today I finally stopped by my truck dealer ((International) and asked to verify the numbers in my ECM (truck computer). The guy couldn't find any discrepencies BUT: when he crawled under the truck, he found on the REAR DIFF a hand-written sign that said "3:53"!

Evidently someone (probably the previous owner) had replaced the diffs from the origianl 3.73 to the current 3.53. The mechanic changed my ECM to 3.53 ratio .. and now I'll see how more or less accurate it'll become.

Neither the dealer who sold me the truck nor the International Call Center had any record of diffs having been changed.

This might be your case too. If you change diffis, the RPM in that iron-clad formula at 60 MPH will change too and your speedometer WILL lie - just like mine has been doing for the past 16 months (hopefully, no longer).

[sgreer78]

Well mine are 275/80-22.5 caps that measured out to 506.33 RPM X 2.93 X .85.

I just got a new cluster off eBay, I'm going to see if it needs to be re-calibrated itself because it's the same as the other-WRONG!

But if anything, my oil pressure, primary and secondary, and fuel gauges all work now.

[allan5oh]

I've always found speedos to be accurate, unless something is set wrong. Tachs are almost always 50+ rpms off. You have to go with a digital readout(qualcomm or other) for a good rpm readout.

[Kranky]

That truck may have originally been equipped with 11R22.5 rubber (499 revs. per mile) and then someone switched it to the 275/80R22.5 rubber (506 revs. per mile) without changing the setting in the ECM to match the smaller tire size.

Easy fix, just take it to an authorized shop for whatever brand of engine you have, and they can plug their laptop into the ECM and re set the parameters.

[heavyhaulerss]

well i'll let someone else decide if this could pose a difference but when I went from my 22.5 hywy tread to kelly kda lug there was a big difference in tire height even though they were the same tire size. if any of you have seen the kellys knee deep rubber compared to others... you know what I mean. wouldnt that throw off the speedo a lil ?

[sgreer78]

The tag on the inside of the door says 275/80-22.5 and all the parameters in the ecu are supposed to be set correctly ie: top hole .85, RPM 506.33, and rear ratio 2.93.

[Musicman]

Back when I was a company driver, one of the first things I would do when I got into a truck was check how far off the speedo was. They are almost never completely accurate, and I would say that 2 or 3 miles per hour at 65 mph would be about average. By the way, you cannot trust those “Your Speed Is” signs, as they are only accurate if they are calibrated, which they usually are not because that takes money. Using a stop watch and mile markers and doing the math is okay, but not guaranteed, because I’ve noticed that some states don’t seem to care too much about how accurate they are. The two best methods for checking accuracy I’ve found is finding a state that has “Speedometer Check Stations” such as ID on I-84 and OR on I-5, or pulling over on the side of the road at a mile marker, resetting your odometer and then driving for 100 mile markers. Compare the 100 miles by mile marker versus the 100 odometer miles and that should be very accurate in most cases.
As far as speed being calculated by tire revs per mile, trans ratio and rear gear ratio… that is not 100% accurate either. I just went through this with my new truck and found that I had to set the tire revs to about 1% more than manufacturer stated revs to get my speedo close to accurate, even now mine is about 1% fast, which by any measure is pretty damn good. Also, if your tires are not brand new, you have to account for tread wear into your computation. Ultimately you’d want to have your revs set to about 5 revs more than actual, so that when the tires wear beyond 50% you will be a little fast on the speedo and it will all average. I noticed on the Cummins site that there is a “compensate for tread wear” option in the ECM, but I haven’t played with that yet.

[INKTOXICATED]

our fleet goes 70 mph all the trucks ive driven the tach says 72 mph. but its really 70 these are 06 columbias

[BoyNextDoor]

Originally Posted by Kranky

That truck may have originally been equipped with 11R22.5 rubber (499 revs. per mile) and then someone switched it to the 275/80R22.5 rubber (506 revs. per mile) without changing the setting in the ECM to match the smaller tire size.

Easy fix, just take it to an authorized shop for whatever brand of engine you have, and they can plug their laptop into the ECM and re set the parameters.

interestingly enough my century 120 has 11r22.5's all around, and does the same exact thing. shows 65, goes 61.5 (according to gps data off of copilot). my sticker on the door also says 275/80/r22/5... the mystery continues... lol

PS the tack shows 1350 rpm @ 61.5 mph

[Dejanh]

When Detroit did my head couple of months ago they pulled my numbers off of the ECM and they matched exactly with my speedometer. This is 06 Columbia.