Question about axle ratios...

[2broke4this]

So I've spent quite a bit of time searching through the forum for an answer to the question that's been bugging me. I have a 92 Chevy G20 with a 350 and a 4 speed with overdrive. (I know I need to keep it out of overdrive when I'm towing d ). It has an oil cooler, and a big transmission cooler and a relatively recently rebuilt transmission. Here's the question... I've got quite a bit of rear end noise, so I'll have to address that... but I'm wondering if I should change the gear ratio while I'm at it. The van has a 3.42 rear end... and I'm thinking about going to 3.73 gears... Would this be a good idea? I will be towing a 1987 19 foot Bayliner Cuddy Cabin most of the time. I don't intend to do too much with the van besides haul stuff and tow the boat around. Right now, the van is already getting miserable gas mileage (12-14 mpg unloaded). I'm wondering if changing the gears may help that, because now, if there is even the slightest incline (Indiana hills) the van downshifts, whether or not I'm towing. I'm hoping to move to a 3.73 rear end with limited slip, so I wouldn't have wheelspin pulling the boat out of the water. Sorry for the long post, I just wanted to provide as much info as I could. I've read a lot of the posts on here and it seems like there is a HUGE wealth of knowledge here. Feel free to ask for any information that might be helpful, I'll try to come up with it. Thanks in advance!

-Carl

 

[Silvertip]

Re: Question about axle ratios...

If you are changing the gears by all means go with the 3.73. With a van you are essentially pushing a barn door through the air and the deeper gears will free up the engine a little, cause less shifting and provide better towing ability. Now, when you change the gears, make sure you change the speedometer drive gear or your speedometer and odometer will not read correctly. You may also find that fuel economy may acutally be better with the deeper gears, again because the engine is not working as hard. As for downshifting, you are not actually experiencing a downshift. You are feeling the torque converter clutch releasing and locking. The converter clutch provides a 1:1 lockup between the engine and input shaft on the tranny. In other words, it eliminates the slippage in the torque converter. I specifically order all my tow vehicles with 3.73 gears (or as close to that as I can get). Depending on the load you are towing, you may be able tow in overdrive and contrary to the "NEVER" do that crowd, as long as the tranny is not hunting up and down, that's not a problem. Obviously heavy loads required towing in direct.

 

[bjcsc]

Re: Question about axle ratios...

I agree, although do not count on a drastic change. I changed the ratio in my CJ-7’s axles from 2.73 to 3.11 (small change like the one you’re considering) and I noticed better performance, esp. in 5th gear as I pretty much couldn’t even use it before the change with my 31” tires. Still, if I had it to do over again I’d be looking for 3.54 or 3.73. You can use this formula to see what the change would be at trans 1:1 (when you’re locked in drive).

RPM= mph x gear ratio x 336
..................Tire diameter

If your noise ends up a simple fix, you can get to the same point with smaller diameter tires. If not, consider buying the axles with the ratios you want from a yard. R&P changes are tricky, expensive to have someone else do (~$800 per diff. around here), and you could run into other things like carrier sizes being different for <3.73 and 3.73 and up (that’s how Jeep axles are). Research it well…

 

[2broke4this]

Re: Question about axle ratios...

Thanks for the information guys! At this point, I'm just trying to gather as much information as I can. I'm leaning toward trying to find a used axle that already has the 3.73 in it... hopefully with limited slip. I've got friends who would be willing to help with that project, but having one rebuilt is definitely a lot more money. My tires have less than 2000 miles on them, so I'd prefer to hang on to them for a while....Do you think I should go even farther? Would 4.11 be too much? Would that kill my mileage? I had read/heard that going to the 3.73 may actually help out with the MPG, so I'm happy to hear that I wasn't misinformed. Right now, on just a very slight incline, the torque converter releases, but if there is more than that, it actually does downshift a gear and has to work really hard to maintain speed. I don't have a tachometer (yet) so I have no good way to keep track of RPMs, but that should be coming relatively soon because I hate not knowing what is going on under the hood. It's much better now that we've fixed the timing (it was incredibly retarded), but I'm still trying to find the best way to get the most use out of the van. I don't have a lot of money into this project so far, which is part of why I'd like to keep going with it. I really appreciate all your help and opinions! Thanks guys!

-Carl

 

[Silvertip]

Re: Question about axle ratios...

No need for all the calculation. 3.42 to 3.73 is a 9% change. If you don't change the speedo drive gear, your speedometer will show faster than you are actually going (more revs, for the same wheel speed) and gas mileage will increase 9% but that's only because the speedo is fast. Change the gear and everything is back to normal. The gear is simple to change. If your speedo is an electric drive, then a tire shop or the dealer will need to recalibrate it.

 

[bjcsc]

Re: Question about axle ratios...

The calculation wasn't given to figure out the error in your speedometer :/ , although you would eventually have to address that, it was given so you can "try out" different ratios before you commit to one. You must know the RPM range your van pulls best at, right? You know the point at which it downshifts, right? You can use the formula to get the RPM at specific speeds with specific ratios. If you play with it, you'll find the ratio that gives you the RPM you want at those speeds. That's the one you go with. 4.11 might be it, or the 3.73 might do it. 4.11 is not too tall since you have overdrive, but it will reduce your top end some (formula will tell you how much)...

 

[Silvertip]

Re: Question about axle ratios...

I understand that. Actually, RPM alone is not a good measure of where best towing is. Engine manifold vacuum is a better way to tell if you have it right. There certainly is a relationship between RPM and vacuum but picking rpm and then hoping manifold vacuum is right is not the way to approach the problem. Manifold vacuum affects what fuel economy will result from the change. While vacuum may be high (desireable) at cruise, engine rpm may be way high which screws up economy as well. Axle ratio is about compromise. 4.10 gears will tow very well but engine rpm will be quite high in any gear and fuel economy will suffer. Many decades of towing have proven to me that 3.73 is a very good choice for the recreational user. Save the 4.11 for the construction/work trucks, unless you have a really heavy load to tote with high wind resistance. The reason some vehicles were so touchy with torque converter locking and unlocking was so frequent was because manifold vacuum was the signal to lock/unlock the converter. If vacuum stayed high, the converter stayed locked.

 

[2broke4this]

Re: Question about axle ratios...

Hmmm, so I should take the time today to change the EGR valve that's been sitting in the van for almost a week now? If that is sticking (and I'm almost positive it is because when I tapped on it everything smoothed out) that would kill my vacuum, wouldn't it? Silvertip, bjcsc, thanks for all your help... I'll let you know how it all works out... It sounds like the 3.73 will be the best option for my purposes... now I just need to either get ahold of a rear end at a yard or bite the bullet and spend the money for a professional rebuild. The rebuild would probably cost more than I have into the van.... wow, that was a lot of ellipses. Thanks!

-Carl

 

[Silvertip]

Re: Question about axle ratios...

Just one more thought. If the rear end has a slight whine, that's no reason to change anything. Some differentials simply whine from new on and live normal lives. If it gets progressively louder and the whine begins to turn into a growl, it's time to make the change. A tablespoon of moly disulfide powder may help quiet the noise. It's the same stuff used in moly disulfide grease but in powder form. In the good old days used car dealers used hardwood sawdust. d

 

[2broke4this]

Re: Question about axle ratios...

Thanks for all the help... the whine has definitely turned into a growl, and is really starting to grate on me when I drive the van... good thing it's not my daily driver! I guess it's time to get some estimates to rebuild and change out the gears....

 

[bruceb58]

Re: Question about axle ratios...

Towing a 19 foot boat you could use either the same ratio you have or the 3.73.

Your truck is old enough that the change to a 3.73 would not require a memory change for your ECM. The transmission you have was redesigned in 1993 to have a electronic shift so you should be ok since you have a 92.

 

[2broke4this]

Re: Question about axle ratios...

Time for an update.... This is why I'm not a mechanic... it turns out that the growling wasn't from the differential at all. It was coming from the wheel bearings on the right side, which got bad enough that the right axle was damaged and had to be replaced. The noise is gone, and it was a LOT cheaper than having the differential rebuilt. I spent under 300 bucks and now have a happy van. We'll see how she does this summer! Thanks again for all your help... The gears may still be changed some time in the future, but I imagine I'll buy a new tow vehicle before I spend the money to do that to a 15 year old van.

 

[Mischief Managed]

Re: Question about axle ratios...

I'm a little late responding to this one but perhaps it will help someone. You can accomplish the same thing as an ring and pinion change just by putting different tires on the rig. Smaller diameter tires do the same thing as increasing the numerical drive ratio. If you are running 255 75 16 tires, going to a 255 60 16 will have the same affect as going from a 3.42 to a 3.78 gear. You'll also get better lateral stability due to the lower sidewall height.

I did this type of mod on my Durango with excellent results. I used sport truck tires as well and therefore achieved better braking and handling too.

 

[Silvertip]

Re: Question about axle ratios...

Make a 10 mile run on the freeway while counting mile posts. Your speedometer is now 10% fast as well (unless its been recalibrated after the tire change).

 

[gmctodd]

Re: Question about axle ratios...

So lcould I put larger tires on my truck and have the opposit effect? I have an 05 2500HD with the 6.0L and 4.11's with stock tires now ( LT245 75 R16). Would something like a LT 285 75 R16 help out at all. It just seems like I'm running the guts out of it on highway trips now. I know it wouldn't be as drastic as going from a 75 series to a 60 series like mentioned above, but the jump would have some effect wouldn't it

Todd
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05 GMC 2500HD
06 GMC Envoy
07 Crownline 200LS - 5.0 MPI
02 Keystone Sprinter 299RLS

 

[Silvertip]

Re: Question about axle ratios...

Changing tire diameter has the same effect as changing axle ratio. Larger diameter tires rotate fewer times each mile so engine RPM drops. Smaller diameter tires rotate more times each mile so RPM goes up. However, you purchased a truck with 4.11 gears for a reason (I hope) and that must have been heavy hauling or towing. If not, you bought the wrong the wrong truck. Remember, lower revs reduces towing ability but may help fuel economy when empty. Increasing engine rpm helps towing but generally hurts fuel economy. Whatever percentage you change the tire diameter means the speedometer and odometer will be off by the same amount so fuel economy increase or decrease will be artificial. The change you reference amounts to about 8% which reduces the 4.11 to a 3.78. Your speedometer and odometer would also register 8% slow (do you read speeding tickets?) d GM speedometers are electrically driven so the dealer or tire store can generally recalibrate the speedometer for the difference in tire size. Older, cable driven speedometers need to have the speedometer drive gear changed. That gear is located in the tail housing of the tranny and is generally a small plastic gear.

 

[gmctodd]

Re: Question about axle ratios...

Yes I do some towing. A 30ft. travel trailer and a boat. But even with the trailer fully loaded at around 10k lbs. I have plenty of power. The 4.10 gears are the only option available with the 6.0L. The truck is acceptable as is, but when it's time for some new rubber I will be going to a larger tire anyway. ( for a little better clearance at looks). Now this just makes it a better idea all around.

Thanks
Todd
-----------------------------------------------------
05 GMC 2500HD
06 GMC Envoy
07 Crownline 200LS - 5.0 MPI
02 Keystone Sprinter 299RLS

 

[Silvertip]

Re: Question about axle ratios...

Keep in mind that the 8-9% reduction in engine rpm translates to an 8-9% loss in towing performance.