[skullhunt]

I've read a ton of posts about running larger tires and most of the comments pertain to fitment. What about power issues? How much will a departure from stock tire size add to loss of power/inefficient gear ratio? I've got a '95 4WD with a newly installed crate motor and a rebuilt trans. It's got a 3" lift and I'm running 30" tires. The van has notable power loss on hills, head winds, hauling trailers, etc. Compression tests are good. I replaced the stupid CPI fuel system AND computer. It's not throwing any codes. I have an extra set of wheels and I'm waiting for a cheap set of stock sized tires to come up so that I can rule out (or in) tire size for the loss of power.
I have been driving a 2003 AWD with a 2" lift and stock tires for years and while it's no race car, I've pulled plenty of trailers up hills and it's got more power. I know somewhere on this site I read an old post from a guy that was switching to 4:10 gears to run his 33" tires. I also have read about folks switching to 3:73. Both of my vans have 3:42 (though I suppose the difference between the 15" and 16" wheels changes the equation). I guess my main question is - how much power loss should I expect from running a 30" tire? Is is worth doing the experiment with the stock tires? And, does a 3.73 ratio allow for comfortable hwy speeds?
Thanks!

 

[Ruh-Roh]

This table might help:

Gear Ratio Chart | Revolution Gear & Axle

www.revolutiongear.com

 

[Rod's Trucks]

My 1998 AWD with 3.73's and 30" tires has no trouble with highway speeds. I've run long distances at 80+ with normal hills etc. When I get to the mountains, I do notice some more pedal is required, and it sometimes drops out of high gear( OD) to maintain the speed limit. I seem to be averaging about 15mpg, but those checks are few and far between, since I only put a couple of thousand miles on it each year.
Rod J

 

[Mmusicman]

It's got a 3" lift and I'm running 30" tires. The van has notable power loss on hills, head winds, hauling trailers, etc.

Yes.. taller tires ABSOLUTELY kills your cruising power! My 2000 AWD is an absolute DOG with stock 3.42 gears and 31" tires. Even on straight flat roads any head-wind KILLS me.. mileage drops dramatically, and my foot is always in the throttle. The lift, the wind drag, taller tires, and running A/C is often too much. It's sometimes impossible to maintain highway speeds without downshifting.. cruise-control is out of the question.

I dropped from 31.5" rugged AT tires to 30" LT highway touring tires and my power increased a little, and fuel economy increased substantially. It does better overall.. but still lacking.

That small tire downsize was a big improvement for me, but I'm still way out of the efficient power band. A 3.73 gear would be the right correction for my tires (would put me back to equivalent of 3.42). But if I want the added bonus the 3.73 with stock tires offers.. I will need to go to 4.10 gear.

If I change gears, I will go to the 4.10's. The only thing holding me back is that I have to do the front as well, and it looks like it might be quite a big job to do. I've already started seriously looking into it. I may even live with mismatched gears for a while until I get to the front. I rarely use my 4x4 and when I do it's in soft dirt anyway where mismatched gears would still work.

I have been driving a 2003 AWD with a 2" lift and stock tires for years and while it's no race car, I've pulled plenty of trailers up hills and it's got more power...
I guess my main question is - how much power loss should I expect from running a 30" tire?
Is is worth doing the experiment with the stock tires?
And, does a 3.73 ratio allow for comfortable hwy speeds?

I think you already answered your own question.
You already know what a 3.73 with stock tires feels like.. huge difference!
We have a stock 2004 work van with 3.73 and stock tires... it burns rubber and accelerates like a V8!

But will the smaller tires look goofy?
No doubt they will improve your performance.
The correct solution is a "gear correction".. or maybe just downsize a little more.
I've seen 29" tire setups that look fantastic!
I'm at my ideal tire size.. for me it's going to be a gear correction.

The rpm difference at highway speeds with more gear is only about 200-300rpm more.. which is nothing. Take your 2003 out on the highway... does it feel like it's lacking or has an issue?

Again, the 3.73 will only "correct" for your taller tires size, putting you back to the equivalent of the 3.42. Like me, if you want even a little more.. you'll want to go with the 4.10's.

 

[Mmusicman]

* * AMAZING "TIRE>GEAR>RPM>SPEED" CALCULATOR! * *

This has got to be hand's down the absolute BEST calculator anywhere on the web!

Completely interactive!
Change one number and they all change interactively immediately.
Compare A>B too!
Also fills in the unknown numbers for you with transmission, transfer case, tire size, etc

STOCK ASTRO TRANS: (both have same overdrive)

• 85-92 - 700R4 trans (0.70 overdrive)
• 93-05 - 4L60E trans (0.70 overdrive)

TIRES (speed differences negligible):

• 85-02 - 215/75-15 (27.7")
• 03-05 - 215/70-16 (27.8")

TRANSFER CASE:

• Stock AWD not listed (1:1 ratio)
• NP231/NP233 (2.72:1 ratio)

(this is only needed if you need low-range calculations)

---------------------------------------------------

QUICK REFERENCE: (here's a few gear/tire=rpm statistics @70mph)

• 3.42 - 27.7" = 2033
• 3.73 - 27.7" = 2217

• 3.42 - 30.0" = 1877 (below power band)
• 3.73 - 30.0" = 2047
• 4.10 - 30.0" = 2250

Note: each tire/gear change is good for about 10%

Here's the INCREDIBLE calculator:

http://www.grimmjeeper.com/gears.html

 

[skullhunt]

Update: I borrowed a set of 215/75/15's from a buddy and went on a test drive. Van felt a bit more froggy in town, but as soon as I hit a sustained climb, it dogged out and performed much like it does with the 30" tires - gradual loss of power and transmission searching. So... I wonder if some of the issue is just a heavy van that is under-powered for it's weight (it has a fiberglass high top, camping build out, steel platform roof rack with ladder, rocket box, custom front bumper...) - I definitely did not try to save ounces on the modifications. At any rate, thanks to Mmusicmans "tire gear rpm speed" calculator, it's easy to see that with the stock 27.7 tires and the 3.42 gears I should be making 2033 rpms and only 1877 with the 30's so I was hoping to feel a bit more of a difference. I plan to install a tach so that I can have some actual data, but real numbers aside, the van just feels like it's not quite geared right, even with the wimpy stock sized tires on.
Also - at some point I pulled the VSSB or DRAC, or whatever it's called on this van, and sent it off to get re-manned and to get the speedo calibrated for 31" tires (I ran 31x10.5's for awhile, abandoning them mostly due to fitment, but also due to the same power issues in this thread). Speedo is a bit off now that I'm down to 30" but it's simple math and doesn't bother me. My question is, if I decide to swap gears to 3.73, how will the VSSB module act? Do other folks calibrate it when they do their gear swaps?
Thanks!

 

[Mmusicman]

So... I wonder if some of the issue is just a heavy van that is under-powered for it's weight (it has a fiberglass high top, camping build out, steel platform roof rack with ladder, rocket box, custom front bumper...)

Yes.. I feel the Astro is underpowered for it's weight. At 4000lbs, it's close to (if not more) than the weight of typical cars and trucks. Then add all the extras. Most typical full size vans come with V8's, yet weight difference compared to Astro is similar. It's not that a V8 has to have "high horsepower".. but that it has adequate midrange torque (which is what accelerates you or moves you up a hill)

The stock V6 4.3 is not a weak engine (and does quite well when fresh with proper gearing)
But the V8 does the extra-duty without the added strain, which actually often results in better fuel economy under loaded conditions. The 4.3 is 262 cu/in, the 5.7 is 350 cu/in (almost 100 cubic inches difference and close to 35% more)

The tire/rpm difference (as you can see from the calculator) from 31" tires to 27" tires is about 10%. This isn't substantial. A little more pep, and a tiny bit more pull. This is the same (corrective) difference between 3.42 and 3.73. This is why I intend to go to 4.10 gears (with taller tires), for yet another additional 10% (for a total of 20% increase over stock).. although it will only be a 10% improvement over the correction.

For me, the added wind load of a head wind PLUS the 10% loss of torque to rear tires PLUS running the A/C was more than my 4.3 could often overcome. Just dropping 1" tire size was enough to overcome this deficiency, although it's still close to borderline. AND this is on straight level roads! lol My mileage however did improve dramatically.

A V8 is typically good for roughly about 25% more torque (over the V6), and that doesn't even include gearing increases. My 92 has a high HP hotrod motor, but my 2000 will get a near stock 350 with plenty of normal mid-range torque. Add that to the gear correction, and hill climbs will be a breeze without even trying... struggling will be a thing of the past. My co-worker gets 22mpg with his Savana van with V8, while I get about 15 average.

 

[AstroWill]

With a new engine you shouldn't really be having any power issues esp with stock sized tires.

When you replaced the engine did you use the old sensors?

If you didn't replace your knock sensor(s), go ahead and spend the ~$20ea to replace(do it properly and torque to specs) and do another test drive.

Edit: 14lbft

 

[Ruh-Roh]

For what it's worth, my original tires were 245-60R15 Goodyear Eagle GTs. The diameter is just 26.6". I have a 3.42 diff, so the van is on the fuel economy side of the daily driver range. If I go to the Cooper 295-50R15s on the rear, their diameter is about the same. When I replace the diff I've been thinking about going to a 3:73, which would give me some margin If I decide on a taller tire.

[Off topic: I need to get on the interstate and check to see if the speedometer/odometer is correct for the current set-up. That is if I can find any mile markers (The state has been taking them down). ]

 

[Mmusicman]

They make simple GPS speed calculators for your phone.. they work quite well

 

[Ruh-Roh]

They make simple GPS speed calculators for your phone.. they work quite well

Thanks man. I really appreciate the information you share on here.

 

[Pengyou]

For me, the added wind load of a head wind PLUS the 10% loss of torque My co-worker gets 22mpg with his Savana van with V8, while I get about 15 average.

What year is the Savana? The newer ones come with 5-6 speed auto transmissions and much more efficient fuel injection systems than our vortecs.

Thanks also to the OP for the question. I have been looking to increase tire size to get more ground clearance...have not thought much about the power issues mentioned.

 

[Pengyou]

If you put taller tires on your van, can you increase the power your engine puts out enough to offset the additional power needs? Put in a new intake manifold? new exhaust system? etc. I know that more torque is what is needed.