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The right gear ratio

W

wyattearp

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Location
McCall, Idaho
Well I am back leaning toward putting the 6.5 in my 85 Chevy Crew Cab. I am looking at running 37" tires. I know with the Cummins 12v, 3.73's would be fine. But what about the 6.5. I am looking at getting 250 hp out it. I have run the numbers through some of the gear ratio calculators. I plan on doing towing on occasion (wood) and a cabover camper from time to time. Thoughts. I have a window of opportunity to change directions as the 3.73 gear set is do into the builder later this morning or afternoon. Thanks!
 
NV 4500. My understanding is that the 6.5 likes to run at a somewhat higher rpm than say the Cummins. With a NP 241 transfer case and the low first gear, crawling speed and/or towing a load out of woods at slow speed is an issue. But towing at highway speed might as well as the optimum rpm for best mpg's.
 
If you plan on towing with 37's, I would look at 4.56's or 4.88's. Even the CUMMINS would have been worked pretty hard with 3.73's, and would be alot happier with 4.56's.
 
I agree, with the 37" tires your should really go lower then 4.10s. The 4.56s sound good and maybe lower like thethermenator said
 
I know somebody who had a 98 DODGE with a 12 valve in it, and his stock 3.73's sure SUCKED with is 37's and automatic with top of the line BANKS ad on kit for 01. He went to 4.56's and was MUCH happier.
 
wyattearp said:
This rig will be more of a daily driver. I may even move down to 35" tires with only a 4" lift. 4.56's seem awfully short.
Click to expand...


With a 31" tire most run a 3.73 or 4.10, so 4.56's with 35's or 37's is actually right in line with a stock 31" tire and 3.73's. Plus going to a taller tire will have more weight which gives more parasitic draw and will require a slightly lower gear to get the same performance.
 
What I should have asked is what rpm does the 6.5 like to cruise at and for best mpg. Gear ratios are dependent on tire size, purpose, etc.
 
depends on fuel system, turbo set up, ect. Mine likes ~2200-2500 with a wh1c (older hx35) but an ATT or center mount VVT might be even happier up to 2600-2700.

These beasts will rev and they dont seem near as strained at those rpms.
 
X2 on what 3500GMC said.

When I started reading this thread my first thought was that I needed to post a link to the grimm jeeper ratio calculator. It is a very handy tool for figuring what engine RPMs, gearing and tire size combo will work best for your situation.

With my K2500 6.5 (in my sig), 4.10s, and 265/75/16 tires I've found that 60MPH puts me at 2000RPM. If I'm doing any amount of out of town highway driving without any stop and go stuff I can get my best MPG at between 1900 and 2000 RPM.

Don
 
With the truck in my sig, I run about 2300rpm at 65mph in 5th. I usually run at 70mph most of the time so it sits just right at 2500rpm. The ATT and the 6.5L are a match made in heaven for upper rpm range and I will admit she sure likes to rev. I did a mileage test a while back driving like I stole the damn truck, including fast accelerating up to 70mph and running it hard. I was still averaging 18.5mpg. I am doing another mileage test now factoring in regular cruising and some in town, without accelerating like I am leaving hell. Will post back with results. I am expecting 19-20mpg.
 
Working off of Grimm Jeepers gear ratio calculator, this is what rpms I come up with 4.56 gears and 37" tires.

1:1 OD

55 mph 2278 1708
65 mph 2692 2019
75 mph 3106 2329

If I went with 35" tires you can add 100 to 180 rpms. This would confirm what Thefermanator and Joey D surmised. Thoughts? Because once I go down this road and change gear ratios to 37" I can only put in the 6.5.
 
wyattearp said:
Working off of Grimm Jeepers gear ratio calculator, this is what rpms I come up with 4.56 gears and 37" tires.

1:1 OD

55 mph 2278 1708
65 mph 2692 2019
75 mph 3106 2329

If I went with 35" tires you can add 100 to 180 rpms. This would confirm what Thefermanator and Joey D surmised. Thoughts? Because once I go down this road and change gear ratios to 37" I can only put in the 6.5.
Click to expand...

EVEN teh CUMMINS will be happier with 4.56's VS 3.73's. It is a lower RPM engine, but not that much lower.
 
From guys I know on other b oards, the Cummins and specifically a VE pump 12v in a Chevy with 37's is happier at a much lower rpm, say 1800 - 1900. I know several who have 4.10 gears, running 35" tires and double overdrive.
 
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