Odometer

[JF1957]

Recently I bought a gps an notice that the odometer of my 99 burb 2500 is faulse. when the odometer indicate 49 mph the gps indicate 55. I try the experience an my 2005 montana and they both indicate the same speed. I don't bought my truck new so could it be possible that the ratio diff. have been change and if so, what kind of ratio have I ? I know that the truck came alive whit a 4.10.

Thanks

[Griffin86]

Your tires could be different from factory.

[JF1957]

Recently I bought a gps an notice that the odometer of my 99 burb 2500 is faulse. when the odometer indicate 49 mph the gps indicate 55. I try the experience an my 2005 montana and they both indicate the same speed. I don't bought my truck new so could it be possible that the ratio diff. have been change and if so, what kind of ratio have I ? I know that the truck came alive whit a 4.10.

Thanks

Tires are the same size as original LT245-75-16

[Griffin86]

Hmm maybe someone swapped out the 3.73s

[jrrod2004]

Recently I bought a gps an notice that the odometer of my 99 burb 2500 is faulse. when the odometer indicate 49 mph the gps indicate 55. I try the experience an my 2005 montana and they both indicate the same speed. I don't bought my truck new so could it be possible that the ratio diff. have been change and if so, what kind of ratio have I ? I know that the truck came alive whit a 4.10.

Thanks

 

What is your VIN?

[JF1957]

That's what I think

[JF1957]

vin number is 1GNGC26RXXJ343286

[silverad03]

If you have the same transmission, transfer case, and tires as factory original then I'd guess you have a 3.73 rearend.

 

My math may be conceptually off here, but a 3.73 gear will make one revolution about 10% faster than a 4.10 ((4.10/3.73)-1) at the same rate of speed. Therefore, if your speedometer were calibrated for a 4.10 rearend and the only variable that changed in your driveline was your rearend ratio, then a 3.73 rearend would make your actual speed about 10% faster than was indicated on your speedometer. In your case, 10% of 49 =4.9 and 4.9+49=54. That's pretty close considering speedo and GPS measurement error.

 

Or, maybe your gauge cluster was swapped?

[JF1957]

I think your wright silverado, but can the rear end gear may have been change whit out changing the casing ? notice here that is a C2500 2X4. So I thing that is an important thing to know because I haul a trailer and GM permitte to haul on the overdrive whit a 2500. I suppose that's because of the tranny and the 4.10 diff. Am I wright ?

[jrrod2004]

This is what was from the factory:

 

XHH TIRE FRONT LT245/75R16/E BW R/PE ST TL ALS 120Q

YHH TIRE REAR LT245/75R16/E BW R/PE ST TL ALS 120Q

ZHH TIRE SPARE LT245/75R16/E BW R/PE ST TL ALS 120Q

QIZ TIRE ALL LT245/75R16/E BW R/PE ST TL ALS 120Q

MT1 TRANSMISSION AUTO 4 SPD, HMD, 4L80-E

U19 SPEEDOMETER INST, KILO & MILES, KILO ODOMETER

GT5 AXLE REAR 4.10 RATIO

G80 AXLE POSITRACTION LIMITED SLIP

 

[unit731]

So in non-technical terms is the GPS incorrect or the truck?

[JF1957]

The truck

[yukondan]

One other possibility

 

Tuned PCM (or in his case Mis-Tuned) ?

[saxdogg]

So in non-technical terms is the GPS incorrect or the truck?

 

Probably a mix of the two...+/- 2 mph for each is reasonable, which could equal your discrepancy.

[silverad03]

I think your wright silverado, but can the rear end gear may have been change whit out changing the casing ? notice here that is a C2500 2X4. So I thing that is an important thing to know because I haul a trailer and GM permitte to haul on the overdrive whit a 2500. I suppose that's because of the tranny and the 4.10 diff. Am I wright ?

I don't think it would be a problem going from a 4.10 to a 3.73 without changing the casing, since the 3.73 gear is smaller. Going the other way, though, may require a bigger casing. I'm purely speculating because I don't have firsthand experience with doing a swap, though.

 

As far as being able to tow in OD, I don't think the gearing in your rear differential is going to have any bearing on whether they recommend towing in OD or not. GM created the tow/haul mode which ups your line pressures and such and I haven't heard of GM truck owner's manuals specifically recommending you not tow in OD; I may have to check mine when I get off work.