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Gears....One more time

K

KJayBlaze

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I know it's been asked a lot, but here goes another gearing question. I have a 1991 Blazer with the 350/700R4/241/3.08 combo. I am going to try and run 36's maybe 37/38's if I can get a little more lift.

I'll be honest; I don't offroad and this is a highway daily driver. I know mpg and lift kits don't go together, but what rear end ratio should I go with for proper rpms/mpg for the highway. I have used the calculators, but I won't too be sure.

In stock trim, I think the Blazer came with either 29" or 31" tires. Using the calculators, at 65 mph, I should be turning 1624 and 1519 rpms respectively.

When I bought the Blazer it had 33" tires, but since I don't have a tach, I wasn't sure on rpms. But at 65 mph, I should have been turning 1427 rpms. I usually would get 16 mpg on the highway, which ain't too bad.

My engine just got rebuilt and should have about 375hp. What rpm range should I shoot for on the highway? What is the best "mpg" rpm range. I don't want it lug (which it would probably do if I kept the 3.08's with 36's even with the more powerful engine), but I also don't want a screamer if I go with a different rear end ratio. Also, because the 700r4 has a 3.06:1 first gear, will my first gear became almost useless with too much gearing? What I mean is, will it shift out of first really quickly if I go with too much gearing in the rear end?

Thanks for the advice
 
When it comes to gearing for mileage, I believe there are two things that are important.

One is to get the engine where it is most efficient. If you have access to an engine dyno you can look at the BSFC (Brake Specific Fuel Consumption?) numbers as a measure of engine efficiency at different RPMs. Programs like DeskTop Dyno will give you these values also, assuming you can give it accurate data about your engine.

The second thing I've read that seems to make sense is to try to set up the gearing so that your cruising speed on the highway turns the engine near it's torque peak. This should allow you to use the least amount of throttle but generate the best amount of torque (Cruising is a "steady state" torque thing, not an HP thing).

There you go, a few random thoughts as I sip my morning coffee and try to wake up.



:usaflag:
 
i tried to keep my 700r4 alive by cruising at around 2k rpms. id did great for a while. that was with 4.56 and 37s, once i started wheelin it it died shortly afterward.
 
I'm running 350/700R4/241/4.56's with a 35" tire and i really wished i would have went a little deeper. I'm pushing 400hp/500tq and still want deeper gearing so if i were you i would go with 5.13's IF you're running a 14ff if not i wouldn't go that low as the pinion gear gets really small on a 10 bolt with anything much more than 4.10's.

Also, cruising RPM's should be close to the peak torque of the engine for optimal efficiency.
 
Ok, I am not sure on my torque curve and where the peak is at, but it totally makes sense to me. So we have a 2 similar combo (35's and 37's w/ 4.56's) 4x4high, you wish you went a little deeper (even if it was strictly highway, no offroading?) Maybe 4.88's? How about you obijuan, if your truck was strictly highway, would you have changed your gearing at all? I have the 10 bolt axles so I can use 4.88's, but do you think the pinion would be too small? It's all highway so I think I will be fine. How is your first gear with that much gear though, shifts out of first pretty quick? Pretty strong acceleration too?
Also, do you think my 10 bolts are strong enough to turn 36-38's (strictly on-road), should be I think. When I re-gear the axles, anything else I should look at replacing or strengthen up for the larger tires? Thanks again.
 
for highway use and 37s i would have gone 4.88s. with 35s i liked it a bit better, more pep off the line and still cruised well.
i just didnt want to regear for every single tire change you know.
 
I just switched to 4.56's with 37 BFG's and like them. I am running a 383 cu. in. with some balls and the truck really gets up and goes and cruises about 2000 rpm's at 65 mph. I'm on 10 bolts cause it's a street cruiser and my hunting and camping truck.
As far as components I bought G2 genuine gear ring and pinions cause they are lifetime warranty, if they break they will give you new ones, from 4 wheel parts. They also sell rear axels, G2 from genuine gear around $110. each that are also lifetime warrantied. Mine are pitted where the bearings ride on, so eventually I will replace those.
I don't off road the hell out of it, so I don't need 1 tons.
Tarey
 
Well it seems like I can't go wrong with either 4.56's or 4.88's. I am still undecided though, not to mention, sure on tire diameter. I have some 36" Iroks that measure out to 36.8". I want to run the 36" Baja MTZ's, but my brother wants to get rid of his 38" Baja ATZ's. GNERGY, do you ever wish you went with 4.88's? I think if I run the 38's, the 4.88's are the best choice, but if I keep my tires or run the 36's, I may consider 4.56's. I would get better acceleration with the 4.88's right?
 
If you run 4.88's and a 38" (if it measures a true 38") you would be turning 1963RPM's @ 65 while in OD, usually cruise RPM's should be closer to 2200-2500 for best effiecency.
 
350/700r4/5.13's and 37's, no complaints here... but I don't have a tach or tranny temp gauge so who knows... could be dead soon.:doah::haha:
 
Scott recomended not to go with 4.88's with 10 bolts cause the pinion gear is too small and you have a better chance of breaking the teeth off of it.
Tarey
 
Thanks for all the good info guys.

Because I am not going to be offroading, my chances of breaking the teeth on the pinion should be decreased? Or by turning larger tires (36"-38"), there still is a possibility?
 
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