Help me pick my gears

[99TanHoe]

Mike

Thanks for the info on your gears. From the chart it looks like your stock tires would have been about 29". With 3.73 gears that would have put you about 2800rpm in 3rd gear. With the 31.6" and 4.10 gears that you switched to, you would have been at 2850rpm. Looks like a fairly even swap. Did you notice any difference in performance or mpg after making the swap. Just trying to understand how much the wieght of the larger tire comes into play.

---------- Post added at 08:18 PM ---------- Previous post was at 08:12 PM ----------

Stephen

Yea I know, 315/70R17's are 34.36" tall and 12.4" wide and 325/70R17's are 34.91" tall and 12.8" wide acccording to all the coversion formula's available on the internet. I was just talking in terms of 315 and 325 to cut down on typing.

---------- Post added at 08:27 PM ---------- Previous post was at 08:18 PM ----------

IMO, Don't fear the gear. 35's and 4.88's.

Not really fearing anything, just don't want to spend $2200 on a regearing and then be disappointed at the results. What I'm looking for is the best balance between mpg and performance.

Years ago when nobody was worried about gas prices, you used to see trucks come from the factory with low gearing that would pull stumps. The only problem was they got worse than terrible gas mileage and they wouldn't run more than about 75mph top end on the interstate. See where I coming from.

 

[Monzter Mike]

I got an amazing increase in mileage, but I did my intake gaskets, spider injector, distributor, oil cooler lines, replaced leaking air conditioning fittings, transmission service & gears all at the same time. I had an air leak between cylinders 6 & 8 with a broken distributor at the distributor cap hold down, so my cap was flapping up & down. After it was all back together I realized when I had the scanner on it that I had a E3 spark plug misfiring that I put in 2 -3 months ago. So I pulled out those crappy waste of money E3 spark plugs & installed some AC Delco Iridium plugs. I went from 10.5 - 11 mpg to 17 - 18 mpg on the highway. One of my buddies calls all that I did on my Tahoe a "restoration". LOL

With all the problems my Tahoe had I couldn't tell you if the gears made any difference in my mpg or not. But it's nice knowing that I'm cruising at the rpms GM intended my Tahoe to be at on the highway, and that my speedometer is about as close as it gets now.

 

[99TanHoe]

Mike
I know what you mean about doing several things at once. Although mine wasn't as drastic as yours, on my black 95' 2door I got a complete transmission rebuild including Transgo shift kit, Corvette servo's and a few other trans mods. I also advanced the timing, some retard at the shop I took it to last retarded the timing too far. The transmission guy acutally clued me in to the timing advance and I started using NON-Ethanol gas all at the same time. Over a short period of time I went from 11.5 mpg to 14.5. Your guess is as good as mine as to which one made the biggest difference.

 

[_Dylan_]

Its not just an increase in tire size, you are doubling or tripling the rotating weight of the tires. 4.10 gears even though they match the original gear ratio will not push the truck like you want it too. 4.88 has a better gear ratio for the torque needed to spin 325s at 75mph. When the engine works harder to turn the tires you are going to get bad gas mileage.

Here is a good calculator:
http://www.grimmjeeper.com/gears.html

 

[noJeepshere]

Its not just an increase in tire size, you are doubling or tripling the rotating weight of the tires. 4.10 gears even though they match the original gear ratio will not push the truck like you want it too. 4.88 has a better gear ratio for the torque needed to spin 325s at 75mph. When the engine works harder to turn the tires you are going to get bad gas mileage.

Here is a good calculator:
http://www.grimmjeeper.com/gears.html

I was just about to say this, but Dylan beat me to it. Numerically, 4.10's will put you back at "stock" revs, but you're now spinning alot more weight. So your engine will be back in its rev range, but more power is lost to spinning weight. So you'd want to go lower to overcome that. I would go at least 4.56, preferably 4.88.

For the sake of example, from the seat of the pants feeling, a Suburban with 3.73 gears feels faster (and is faster) than a similar Sub with 3.42 gears. My parents at one time had one of each, and I got to compare them. Fuel economy was actually better in the 3.73 equipped 'burb than the 3.42 because the engine didn't have to work so hard to get around. In fact, the 3.42 was pretty sluggish and had to get in the throttle deeper to go anywhere. On road trips, the 3.42 'burb got about 1/2 better mpg than the other, but the drawbacks overcame that.

So, if you are going to keep the 315 tire, the 4.56's will suffice, but 4.88's will be better. If you're going to bump up to the 325, 4.88's for sure. Basically, put 4.88's in. There.

 

[99TanHoe]

Thanks Dylan and Riley

That is exactly the kind of information and explanation I have been looking for. The 4.10/4.11 are definately ruled out now. I may even go with the 4.88. I'll have to make a decision before next week, thats when I plan to have the offroad shop do the regearing.

Dylan

I must be doing something wrong on the calculator you linked to. After I press calculate I'm only getting the transfer case final drive ratio's. Everything below that is returning a NaN. I would kind of like to see the "Road RPM at a Given Speed" data also.

 

[Fishintahoe]

Which gears are better on the highway with 35's? 456 or 488

 

[99TanHoe]

Which gears are better on the highway with 35's? 456 or 488

Joe

Hopefully someone that has had experience running those gears will answer. I've beat the gear tables available on the internet to death.

As for what the guys here have said, It think BowtieFreak is running a 383 motor, 4.56 gears and 315/70R17 tires and he commented that at times he wishes he had gone with 4.88 gears.

My motor is still stock, so I don't have near the HP or torque that he has, so with that in mind the 4.88 gears would be even more beneficial for me.

According to the charts, the 4.88 gears would put me at the transition point between the stock RPM range and the Power RPM range at 65 mph, running the 315/70r17 tires and just a little ways back into but still on the high end of the stock RPM range with 325/70r17's.

Your asking about highway usage, I can see your need for that in Texas. In my case the majority of my driving is extremely varied country roads, two lane hwy, city and interstate hwy. I see all of those conditions just back and forth to work. Thats why I'm considering the 4.88 gears because of all the stop and go and moderate speed driving.

When we need to drive a long distance we just take the Ford F150 Ecoboost and get 21 - 22mpg depending on road conditions.

I was originally looking for best MPG for the Tahoe but now I'm leaning toward performance. The guy at the off-road shop recommended 4.56 for 35" tires and 4.88 as a minium if I were going to consider running 37's. I'm thinking more along the lines of possibly running 36's at some point so thats another reason for the 4.88's. I'm going to talk to him one more time and probably just go with what he recommends, since they have built up a good name for themselves when it comes to gear work.

 

[_Dylan_]

Dylan

I must be doing something wrong on the calculator you linked to. After I press calculate I'm only getting the transfer case final drive ratio's. Everything below that is returning a NaN. I would kind of like to see the "Road RPM at a Given Speed" data also.

Works for me? Make sure every slot is filled out. 4l60e trans, nv243 transfer case, rear axle gear ratio, tire size and metric or standard...then hit calculate

Which gears are better on the highway with 35's? 456 or 488

You might be thinking to yourself, hey the RPM is going to be much higher than stock! Won't that burn more gas? I know when I drive at the RPM I get bad fuel mileage... Yes that is true, 2400 RPM vs. 2100 RPM the higher RPM is going to get worse MPG. Why? because you are going faster, fighting more wind resistance and working the engine a little harder at 85 mph than 75 mph. BUT, switching to a 35" tire and a 4.88 gear you are still travelling 75 mph, but now at 2400 rpm. You have more power to spin the tires and fight the slight decrease in aerodynamics from the lift and tires. You aren't fighting all the extra wind resistance you are at 85 mph. An easier running engine will get better gas mileage than an engine popping in and out of lockup and overdrive to maintain speed.

Either gear will be fine, but IMO I'd pick the 4.88 for more power. A tahoe isn't a mileage machine. You will see a better increase in fuel mileage by changing driving habits than picking a 4.88 or 4.56.

Joe

Hopefully someone that has had experience running those gears will answer. I've beat the gear tables available on the internet to death.

As for what the guys here have said, It think BowtieFreak is running a 383 motor, 4.56 gears and 315/70R17 tires and he commented that at times he wishes he had gone with 4.88 gears.

My motor is still stock, so I don't have near the HP or torque that he has, so with that in mind the 4.88 gears would be even more beneficial for me.

According to the charts, the 4.88 gears would put me at the transition point between the stock RPM range and the Power RPM range at 65 mph, running the 315/70r17 tires and just a little ways back into but still on the high end of the stock RPM range with 325/70r17's.

Your asking about highway usage, I can see your need for that in Texas. In my case the majority of my driving is extremely varied country roads, two lane hwy, city and interstate hwy. I see all of those conditions just back and forth to work. Thats why I'm considering the 4.88 gears because of all the stop and go and moderate speed driving.

When we need to drive a long distance we just take the Ford F150 Ecoboost and get 21 - 22mpg depending on road conditions.

I was originally looking for best MPG for the Tahoe but now I'm leaning toward performance. The guy at the off-road shop recommended 4.56 for 35" tires and 4.88 as a minium if I were going to consider running 37's. I'm thinking more along the lines of possibly running 36's at some point so thats another reason for the 4.88's. I'm going to talk to him one more time and probably just go with what he recommends, since they have built up a good name for themselves when it comes to gear work.

Those gear/tire charts are crap. Refer back to the calculator I showed you earlier.

 

[99TanHoe]

Thanks again Dylan for another good explanation,

I got the calculator to work. I just wasn't using the selector next to the metric tire size that I was entering. The selector defaults to the tire size in inches.

I now understand that 4.88's will be fine to use with 35" tires. This next question is just out of curiosity. At some point if you lower gear ratio enough and decrease tire size enough you will get negative results (Loss of top end speed and poor fuel mileage) won't you? If so, is there some known rpm for a given tire size that this will occur?