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?? Has sombody changed my rear end ??

Snot Rocket

Snot Rocket

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Bethany IL.
Ok so I just got my 94 2wd 1500. On the test drive I noticed the speedo was reading a little fast. Ask the PO about it he said it read 4 to 5 mph faster than what you were going. Bought the truck got out the gps and he was right. Now it had the right size of tires on it and acording to the build specs it has a 3.43 rear end.
What I'm seeing from the driver seat is 60mph is 2k on the tach. To me that is about right for 3.43 but the gps has it at 55mph.
It needed some new tires on the rear anyway so I steped up to 31-10.50-15. They are around 1 to 2 inch taller than stock. Now the speedo is only 1 or 2 mph fast. (I need the better mpg)
Can anybody confirm that 2k at 60mph is right for 3.43 gears? That would let me know if the gears have been changed or the speedo is messed up.
If that is the correct rpm for 60. Then would anybody have a idea of what the gear ratio is for 2k at 55mph?
By the way OD and the locking coverter is working fine.
 
You don't specify what size tires you started with so I assumed 245/75/15 since that seemed to be a common stock size. With 245 tires, 3.43 gears, and 2000 RPM you should be going 68 MPH. With 31/10.50/15 tires, 3.43 gears, and 2000 RPM you should be going 72 MPH.

If you have 3.73 gears you'd be going 63 MPH with the 245 tires and 66 MPH with the 31" tires at 2000 RPM

I used the gear ratio calculator at grimmjeeper(dot)com. It's a real handy calculator. I'm not sure what other ratios were used, but use this calculator and see what you come up with.

Don
 
Also keep in mind that the alternator driven tachs in these trucks are horribly inaccurate. The easiest way to tell what gears you have for certain is to pull the diff cover off and count the teeth(also most ring gears will have the teeth and ratio count stamped on them).
 
Yes the stock size is 245/75/15.
I'm going by 2000rpm on the tach with it in overdrive and the converter locked up. That is diffrent than 2k at drive shaft or the tires.
I'll check out that site and see what I can figure out.
 
Check the wiring on the transmission for shorting out as the insulation falls off near the sensor on the harness. Then pull the speed sensors looking for debris on them. Check 4x4 wiring as well.
Factor in a gas pulley on the alternator to add 200 RPM to the tach. You need to measure the pulley.

Possible the VSS was re-calibrated for different tires.
 
I did not know that about these tach's. It's a good day to learn something new.
Now that mention it the alt pully looks smaller than the one on the 6.2 truck.
 
Also keep in mind that some tachs even with the right pulley will read 10% or more off. I do have the correct pulley, and my tach is off 10% across the entire RPM range. When I had a gasser pulley on it the tach was off almost 400 at 2K RPM's. Needless to say it shot into the red ALOT.
 
LT235/75R15C is the actual stock size.

I know my 1999 1 ton and dads 98 1 ton, both of which are 4.10 gears calibrated for 29.5 inch tall (roughly) LT225/75R16D turn 2 grand at 60 in overdrive. I dont think LT235/75R15C is much smaller than LT225/75R16, so I would say the tach isinaccurate. see if it has the gas pulley.

good luck!
 
Well got the pulley changed. Got the rpm's were they should be. But according to this site.
http://www.angelfire.com/fl/procrastination/rear.html

1750 rpm, 29" tire, .73 OD, and my GPS showing 55 mph = 3.70 rear end.

1750 rpm, 29" tire, .73 OD, and speedo showing 60mph = 3.43 rear end.

I will have to get some rear end lube to be for sure.
But it is sure looking like it has been changed. For now the 31" tall tires have brought it close to being right. If I ever want to go back to the stock size tires I will have to figure out how to adjust the speedo.
 
jack up both back tires, put tranny in neutral, turn one of the tires 1 full rotation and count the number of times the driveline rotates. 3 rotations plus a little more = 3.42. almost 4 rotations = 3.73. Easy to do and quick. Helps to make a chalk mark on the driveline to keep track of things.

Gotta have both tires off the ground otherwise you get a doubling affect with the spider gears doing their thing.
 
I'm pretty sure you would have to turn both wheels at the same time to do that.
If its not a pos track you turn one wheel and the other would just turn the opposite direction with out turning the drive shaft.
 
Nope, just have to turn one wheel. Yes, the other wheel will turn the opposite direction on an open differential. The drive shaft will still turn.

The other way is jack up only one side, but you need to turn the tire 2 rotations.
 
I just ran my first full tank of fuel threw this truck. Not to happy with 16.5 mpg.
My 6.2 was 20 mpg almost every day. From what I have read that is about the norm for this truck. It just sucks that both trucks only go 450 miles on a full tank. Just this one takes 10 more gallons to fill it. Really makes me wish it still had the original rear end in it.
Time to read up on fuel saving mods I can do.
 
Snot Rocket said:
I just ran my first full tank of fuel threw this truck. Not to happy with 16.5 mpg.
My 6.2 was 20 mpg almost every day. From what I have read that is about the norm for this truck. It just sucks that both trucks only go 450 miles on a full tank. Just this one takes 10 more gallons to fill it. Really makes me wish it still had the original rear end in it.
Time to read up on fuel saving mods I can do.
Click to expand...

This is why we need, and the new trucks have, 5 and 6 speed transmissions (auto and man.). So the need to pick a differential ratio and suffer the consequences is pretty much eliminated. A 6 speed trans has, one over drive for the highway, and one over drive for get out of my way. :rof:
 
Snot Rocket said:
I just ran my first full tank of fuel threw this truck. Not to happy with 16.5 mpg.
My 6.2 was 20 mpg almost every day. From what I have read that is about the norm for this truck. It just sucks that both trucks only go 450 miles on a full tank. Just this one takes 10 more gallons to fill it. Really makes me wish it still had the original rear end in it.
Time to read up on fuel saving mods I can do.
Click to expand...

The addition of a turbo is what is costing you the most on liage from the 6.2 to the 6.5.
 
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