VSSB and speedo what rpm at 65 mph

[arrowheadracing]

Just wondered how the vssb works. I just installed a set of 285 75 tires and the rpm and mph didnt change from what they where originally. Meaning my rpm is the same as it was before at 65 mph. Which was 2300 rpm. I figure that the tires are roughly 2 inches taller, and that would drop the rear gear ratio to something lower ( supposedly my truck was ordered with 3.73, but it seems to be 4.10's which is what came standard from what I am told ) I have to change the brakes , so I ll be pulling the axles to do it, and I ll check it then.

According to the info posted here, I would have 4.10s with that kind of rpm. My mpg is roughly 17.5-18, which coincides with the 3.73 mpg.

So what do you guys run , with what rpm and what size tires ?

Thanks

[ALSAIL]

Your effective gear ratio did in fact change if you increased the tire size. However, if you didn't correct the speedometer the readings will still be the same. The speedo senses T-case output shaft speed so the ratio between that and the engine depends only on the transmission/T-case gearing. So if you were driving at the same RPM as before, you were actually travelling faster. Once the speedo is corrected, you will see an RPM drop at the same road speed.

[slagona]

The VSSB just takes a signal and converts it for your speedomete/odometer. Changing your tire size (or anything else in the drivetrain) requires a change in the VSSB settings to get a correct reading on your speedometer.

It's fairly simple to recalibrate the VSSB, there are some great web pages and articles here on The Diesel Page which can walk you through it.

For some great links, search this forum for VSSB - take a look at the thread titled "255 85 16 tires" and follow the links...

--Scott

[DennisG01]

I corrected my VSSB for 265/75 tires. 65MPH = 1900RPM. If I remember correctly, when I had 245/75's, the RPM's = 2100. I don't remember for sure. This probably doesn't help you, though, as I have a 3.73 rear.

[90K5-62]

Our '97 Suburban K2500 6.5TD has the 3.73 rear end, stock size P245/75R16 tires and is mechanically in original stock condition. It turns 2,000RPM @ 65MPH, both as indicated by the existing, original gauges.

Good luck with your project.

--Ian

[moondoggie]

Good Day!

This is simple math.

Per my 9 Mar 04 post in Speed vs RPM with 4.10 rearend (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005936)(Click on the colored text.):

rpm @ 60 mph = (60 mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (103.14 inches/tire rev) X (3.73 driveshaft rev/tire rev) X (0.75 engine rev/driveshaft rev)

rpm @ 60 mph ≈ 1719 rpm
rpm @ 65 mph ≈ 1862 rpm

(285/75R16 tires are ~ 32.83

[Hye]

Your tach uses the alternator to determine engine speed. The alternator pulley diameter and the number of field poles inside the alternator both affect what the tach will show as engine speed. There are different size pulleys and not all alternators have the same number of poles. If you ever replaced your alternator, consider yourself lucky if your tach is still accurate. Bottom line is even if your speedo is correct you can't depend on you tach and speedo to determine your axle ratio.
The crank position sensor puts out four pulses per engine revolution, period. Unfortunately, GM didn't use this info to drive the tach after they went to electronic fuel injection on the 6.5L's. One of these days, I'm going to build a digital tach that uses that signal. All it would need to do is count pulses and display the results every 1.5 seconds to show engine speed to the nearest 10 RPM's.
But first I have to teach the ferral hogs around here to fly so I can charge a premium for hunting rights on my 100 acres and use the profits to buy a Dmax. Does a Dmax have a crank position sensor?