Just realized why my revs are so high...

[dragonriot]

So, I've been reading about all these guys running at 1800RPS at 65mph, and I just couldn't figure out why I was running at 1800RPM at 55mph... well... I figured it out today, just finally did the math. I have 265/75R16s, and so I am running with 4.56 gears... =( That means I don't get as good of mileage as the rest of you guys with Yukons, and it would cost me a lot of power and offroad ability if I changed it to 3.42 or 3.08... I'm not sure if I'd rather have the power or the mileage, but I guess 20mpg on the highway and 16mpg in the city is good enough for me.

[DmaxMaverick]

I think you did your math backwards. You should be seeing 3.73 in your final drive. NOT 4.56. Those gears weren't an option on your vehicle, and require a special aftermarket set to fit them into your diff. With 4.56 gears and 265/75/16's, you'd be lucky to get 75 MPH against the governor. You could move to 3.42 or 3.08 gears, but you'd lose significant power and towing ability, and probably not gain much at all in economy. Certainly not enough economy to pay for the gear swap.

My SWAG is your tach is off. If the alternator has ever been replaced, it probably has the wrong pulley on it.

[dragonriot]

I think you did your math backwards. You should be seeing 3.73 in your final drive. NOT 4.56. Those gears weren't an option on your vehicle, and require a special aftermarket set to fit them into your diff. With 4.56 gears and 265/75/16's, you'd be lucky to get 75 MPH against the governor. You could move to 3.42 or 3.08 gears, but you'd lose significant power and towing ability, and probably not gain much at all in economy. Certainly not enough economy to pay for the gear swap.

My SWAG is your tach is off. If the alternator has ever been replaced, it probably has the wrong pulley on it.

I did the math at www.4lo.com, 1:1 transfer case ratio, 2340 RPM, 31.9" tire diameter, 65mph and a .75:1 final transmission ratio... it came out to 4.56 gearing.

I doubt my tach is off, because I've only ever replaced my alternator with OEM alternators... I can bury the speedo in overdrive and pushing somewhere around 3000RPM at 85... I have a mechanical wastegate controller and performance chip from Heath Diesel, not that it would change anything to do with my gearing, but my redline is higher now... under full throttle, my truck shifts somewhere around 3800RPM. At 55mph, the truck feels like it isn't pushing, it's just smooth at that speed... at 65mph, it feels like it's working a bit harder, and at 75mph it is running around 2700RPM.

Each speed I put into the calculator spits out the right RPM that I'm actually running when I use a 4.56 gear ratio... at best, if I'm reading the tach wrong, or off by 50 or so RPM, the lowest I could have is 4.11... which would put my 55mph revs at just under 1800, 65mph revs at about 2100 and 75mph revs at 2400. Add that to the fact that my door sticker says it came stock with 245/75R16s... and I bought the truck with bigger tires, so yeah... it's either 4.56 or 4.11.

[DmaxMaverick]

Something else is amiss, then. Your truck didn't come from the factory with 4.56, or 4.11 gears. Have you owned this truck since new? The gears are very expensive and hard to come by. It would have had to be deliberate (as in, not an accident). You may be right, but it doesn't make a lick of sense otherwise. Either your tach is off, you aren't getting into OD, or the converter isn't locking, I think. Go to the horse's mouth and calculate the gear ratio by counting the turns at the wheel and pinion. If you raise one wheel, one turn will rotate the pinion slightly more than 2 turns with 4.11 gears, and less than 1-3/4 turns for 3.42. Or, pull the diff cover and count teeth.

[dragonriot]

Something else is amiss, then. Your truck didn't come from the factory with 4.56, or 4.11 gears. Have you owned this truck since new? The gears are very expensive and hard to come by. It would have had to be deliberate (as in, not an accident). You may be right, but it doesn't make a lick of sense otherwise. Either your tach is off, you aren't getting into OD, or the converter isn't locking, I think. Go to the horse's mouth and calculate the gear ratio by counting the turns at the wheel and pinion. If you raise one wheel, one turn will rotate the pinion slightly more than 2 turns with 4.11 gears, and less than 1-3/4 turns for 3.42. Or, pull the diff cover and count teeth.

I've not owned it since new, it was built here in Wisconsin, the owner took it to Texas, kept it there until 2006, and then it moved back her in time for me to trade my 1970 Blazer for it from the same guy I bought the Blazer from in the first place. I'm willing to put real money on the fact that I do in fact have either 4.11 or 4.56 gears, and that the guy I bought it from swapped them out for towing or something, just because he's a moron.

[6.5 Detroit Diesel]

i would be with Dmax here. those gears are not stock. your mileage threw me a little, but then i realized that your driving a yukon here, not a pickup, so that helps. if it is running 4.56's, then that must lauch hard!

3800 is a little high to be shifting, IMO, and if you are pushing only pushing 3000 at 85 mph, something is out. my truck with 4.10's is pushing well beyond 3000 at 85, and that is with a proper readout on the gauges.

[stingthieves]

OE replacement alternators are rebuilt by the lowest bidder and stuck in a pretty box. Don't be so sure about that pulley size - just because the box said Delco on the outside.

[jerry598]

Isn't there supposed to be a little aluminum tag on the differential that indicates the gear ratio? Maybe they were discontinued or maybe it just fell off. What do the RPO codes show on the glove box? Maybe they would indicate the factory specs.

[DmaxMaverick]

The factory spec. is 3.42. Or, 3.73 if it's a Z71, some towing packages, or special order. 4.11 gears were never optioned in a Yukon. Some years offered a 3.08 and 2.73, but I don't know which (very rare), and I've never seen a 2.73 equipped (factory) Diesel. The RPO will show the ratio as originally equipped. His problem is, it isn't acting like stock gearing, not even close. A 4.11 wouldn't be too uncommon for a swap, as the rear ends and gears were available (a pickup option). 4.56 gears in that rear end are very uncommon, and only available from the aftermarket. Some special use GM trucks were optioned with 4.56, and even 5.13 gears, but they are only one tons, 3500HD, or Workhorse chassis (including panel trucks), and the diffs aren't interchangeable (full floaters only). The lower of the low gears were only available to fleet order vehicles (utilities, etc.).

[DaveBr]

I have 4:10's in the 93 and it runs at 2050 RPM @ 60mph.
The 98 has 3:73's and runs about 1850 - 1900 @ 60 mph.
Something does not sound right. Either tach or speedo could be off.