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-   -   Gear ratio calculators vs. real life (http://www.jeepforum.com/forum/f8/gear-ratio-calculators-vs-real-life-1580617/)

fourbtgait 09-12-2013 08:49 PM

Hello all,

Thinking of regearing from 3.54 to 4.10 with 31" tires and a T5. Average speed on the interstate is 60-65 (50% of travel), no rock crawling, travel typical forest service, timber country roads in the northern rockies, not a lot of mud, steep, washed out gravel/rock roads, again 50%). At this point in my life I do not see a reason to go to 33" tires.

Ive been researching posts on here for weeks plus trying various calculators, so it seems it comes down to real life.

The 3.54's do ok though a little boggy accelorating at 60, a liitle fast paced in 1st gear, 2wd in the hills, wish they were 3.73 but I know that slight change isn't worth the cost, hence 4.10. Even then the 4.10 only is a 16% change but according to the Grimjeeper calculator and others, the 4.10 with the T5 would give me 2293 rpm at 60 vs 1980 rpm at 60. A better rpm to travel at.

Are the calculators fairly accurate?
Is the change worth it, cost vs gain in traction and such?
Thoughts please.

Russell

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Spieg8 09-12-2013 09:19 PM

I've found that calculator to be pretty accurate. Not sure I'd make the change for 16%, but it's your money. What would the RPM be @ 60mph in 4th gear? Just because you have OD doesn't mean you have to use it (maybe save it for those few occasions when your doing 75 on the interstate, and use 4th gear the rest of the time?). In addition to the ring and pinion, you'll also need new carriers.

fourbtgait 09-12-2013 09:29 PM

2300+ with 3.54, 2600+ with 4.10.

Yes, I've priced it with new carrier for the D30, the 20 is fine as is though was looking at a Tru Trac.

Hmmm, maybe twin stick the D300 and use 2wd low in the hills instead.

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BubbaK 09-12-2013 09:41 PM

Before you make a decision... There's a really reliable guy on the forum (keith460) with a T-5, 4.10's, and 32x11.50R15s and he told me he wished he would've gone with a 3.73. Reason being that 1st thru 4th gears are all really short and don't give him a lot of range. It might be worth looking him up and picking his brain a lil bit. Good luck buddy. Keep us updated.

'83 CJ-7_258 I-6_T-4_D300_AMC20/D30_Bone Stock

fourbtgait 09-12-2013 09:46 PM

Quote:

Originally Posted by BubbaK
Before you make a decision... There's a really reliable guy on the forum (keith460) with a T-5, 4.10's, and 32x11.50R15s and he told me he wished he would've gone with a 3.73. Reason being that 1st thru 4th gears are all really short and don't give him a lot of range. It might be worth looking him up and picking his brain a lil bit. Good luck buddy. Keep us updated.

'83 CJ-7_258 I-6_T-4_D300_AMC20/D30_Bone Stock

Yes, I've read some of Keiths posts. If I remember, he Installed 4.10, then the T5 for the overdrive, finally I thought he did go to 3.73, to even it out.
Hence my curiosity of calculators vs real life. The calculators make it "appear" perfect, but...

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CJ7KAHUNA 09-12-2013 10:06 PM

I think 3.73's would be about perfect. IMO 4.10 would be more than what you are looking for. I have a 258 w/ T5 and ran 33's with 4.56 - started off in second gear and basically had a 4 speed with a granny low type set up. I think 4.10's would have been perfect for the 33's and would have given my my 5th gear back as an OD. If 4.10's are a perfect balance for dual purpose daily driver that is capable of mild to heavy off roading then they'd be a bit much for 31's for the same application.

I now run 35's and the 4.56's are perfect for me with this set up. Great for a dual purpose set up. Today - At 60mph on the GPS running the T5 in 5th I was turning just hair over 2000 rpm. Plenty of gear down low with excellent road manners.

CoryA 09-12-2013 11:03 PM

Don't forget when using rpm calculators to use the actual height of your tire not the advertised height

Matt1981CJ7 09-13-2013 06:29 AM

Good point, Cory.

There's a large difference between advertised height and actual height on several brands of tires.

BFG's aren't even close. My 32"/11.50s measured just under 31" tall and 10.5" wide brand new. False advertising.

Matt

fourbtgait 09-13-2013 07:32 AM

Quote:

Originally Posted by CoryA
Don't forget when using rpm calculators to use the actual height of your tire not the advertised height

I did.
Tires I run are actualy 30.75 brand new.
As tires wear, diameter gets smaller, rpm increases slightly.
31" is 2293 at 60.
30.5" is 2331 at 60.

Per the calculaters.

Did you ever put 3.73 in Matt? I remember reading ypur post about setting up to before an engine change.

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gmakra 09-13-2013 07:35 AM

Yes the real tire height versus the advertised height can be different.

However what I would like to point out is that the gear calculators do point out the sweet spot for overall performance which is in the 2500-2800 RPM range which is the best for overall economy and performance and this has been engineered and proven over and over. Moving out of that sweet spot with a high a gear ratio you will loose torque and lug the engine and also use more gas. To high and the engine will be screaming and also burn more gas and more than likely you will be rowing you shifter to drive.

Some guys think that the transmission will make up the difference in lower or higher gear ratios. This is really not the case since all 3 and 4 speed transmissions will have a final output of 1.0 five speeds will have and overdrive of .70 or so.

fourbtgait 09-13-2013 08:01 AM

Quote:

Originally Posted by gmakra
Yes the real tire height versus the advertised height can be different.

However what I would like to point out is that the gear calculators do point out the sweet spot for overall performance which is in the 2500-2800 RPM range which is the best for overall economy and performance and this has been engineered and proven over and over. Moving out of that sweet spot with a high a gear ratio you will loose torque and lug the engine and also use more gas. To high and the engine will be screaming and also burn more gas and more than likely you will be rowing you shifter to drive.

Some guys think that the transmission will make up the difference in lower or higher gear ratios. This is really not the case since all 3 and 4 speed transmissions will have a final output of 1.0 five speeds will have and overdrive of .70 or so.

Yes, that puts another spin on it when one looks at the other style charts compated to the gear ratio calculators.
The typical gear/tire chart shows my 3.54 with 31's at 60 in 4th gear is at 2501 rpm. .86 overdrive drops it to 2150.
4.10 same set up shows 2896, overdrive produces 2490.
Those same charts say the 31 tires with 4.10, 1-1 final drive is perfect combination for rpm range.

It would be ridiculous to go to 3.73 from 3.54. Hence my off hand remark before of doing a twin stick instead to run in 2w lo, say in 2nd or 3rd gear on the mountain roads. But that still doesnt produce optimum performance overall.

Russell

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Matt1981CJ7 09-13-2013 08:04 AM

Quote:

Originally Posted by fourbtgait (Post 15966257)

Did you ever put 3.73 in Matt? I remember reading ypur post about setting up to before an engine change.

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four,

No, I'm still dragging my feet on re-gearing. The added power of the 360 almost makes my 2:73s tolerable.

I use all 4 gears of my T-19 (5:1 first) in normal driving. The synchronized gears really help. 4th gear lacks a little in acceleration, but it's kinda nice cruising 65 mph at under 2000 RPM with my foot barely touching the throttle. If I need to pass a slow poke, I can throw it in third and take off without winding it too tight.

I wish I had your 3:54s. I think that would be the perfect gears for my setup.

Matt

fourbtgait 09-13-2013 08:09 AM

Quote:

Originally Posted by Matt1981CJ7

four,

No, I'm still dragging my feet on re-gearing. The added power of the 360 almost makes my 2:73s tolerable.

I use all 4 gears of my T-19 (5:1 first) in normal driving. The synchronized gears really help. 4th gear lacks a little in acceleration, but it's kinda nice cruising 65 mph at under 2000 RPM with my foot barely touching the throttle. If I need to pass a slow poke, I can throw it in third and take off without winding it too tight.

I wish I had your 3:54s. I think that would be the perfect gears for my setup.

Matt

Agree, with a V8 or an automatic transmission I think the 3.54 is good.
With a manual I think a touch lower would be best.

Russell

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brownbagg 09-13-2013 08:11 AM

Only thing you do is cross multply

Height of tire overt ratiio

And then cross multiply with new high of tire

You can also do rpm over tire and cross multiply with new tire height give you new rpm with gear change

Renegade82 09-13-2013 11:20 AM

Just as a point of reference. With worn 33's, which are probably closer to 32"; a 258, T-4, and 3:73 gears, mine is begging for a 5th gear at top end highway speeds. I can't give you any rpm's, or even accurate mph, but cars be passin'!


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