anybody have 0.63 overdrive with 4.11s?

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steel1212

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#1

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When I got my t-5 rebuilt I told the guy to rebuild it with t-5z parts 2.95 first and 0.63 overdrive gear as I like my 4.11s. According to the gear calculators I should be running at 2342. According to my friend that was following me and telling me we where going 70 my rpms where around 2500-2600 (my speedo doesn't work yet). I was just wondering if anybody else had any real world opinions.

 

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bnickel

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lubbock, texas

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#2

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couple of things. his speedo could be off as well, even OEM speedos can be off right off the lot, my cherokee is about 5 MPH fast with no changes to anything including tire size. could be that you're clutch is slipping a little in OD as well. also the gear charts are just an approximation and not an absolute.

 

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10secgoal

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#3

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And an answer to a formula is only as accurate as the info input. Is possible you entered 26 for tire hieght, but it is really 26.2

 

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steel1212

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Yeah, I went and checked its 26.1 but that would make the rpms go lower not higher. I've thought about the clutch thing. I was never happy with the shop that sold me that thing since after I already had the clutch and they had given me a 50oz flywheel that there was no such thing as 28oz and 50oz! My clutch is probably not up to the 300hp I told them to make it for.

 

9

96 DOHC Cobra

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#5

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It's also possible your tach isn't as accurate as you'd hope.

 

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steel1212

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#6

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I guess thats possible as well but its not that old and it is autometer.

 

Edbert

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#7

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Use the calculators but use them with a grain of salt. At 60mph in 1:1 drive with a 25.5" tire the difference between a 4.11 and a 3.89 is about 175 RPM.

A "normal" OD gear is either 0.67 or 0.66 which makes the above compaison still differ by less than 200RPM. The difference between that 0.67 and your 0.63 is so negligible that I think you are wasting your time worrying about it.

There are so many variables that the simple mathmatics cannot account for, such as road temperatures, which can throw the numbers off by a percentage or so. A tire that stands 25.5 inches tall in your garage can grow significantly at 85+MPH. Also, when planning for the "correct" ring and pinion it is usually advised to ignore OD and focus on the final 1:1 gear (3rd in an AOD or 4th in a T5).

I made this chart to help myself when I was choosing, maybe it will help you too.

 

chepsk8

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#8

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I have 3.55's, with a .67 OD T-5, motor is lugging at 2200 RPM at 70.

 

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D.Hearne

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#9

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I don't think tire "growth" at speed affects anything. Afterall it's the distance around the tire that's a constant that affects your rpms. There ARE variations in tire sizes that do affect your calculations, there's even a variation as the tire wears. Not all tires or brand of tires are actually the same exact size when it comes to circuference, which is the measurement that you use to calculate the rpms at a certain speed. The most exact way to measure speed is to use a stop watch over a set course or measured mile then compare your speed to the rpms on the tach. Even then, unless you have a digital tach, it's not an exact science.

 

jst6

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Nov 28, 2005

#10

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steel1212 said:

Yeah, I went and checked its 26.1 but that would make the rpms go lower not higher.

Click to expand...

I checked your tire size in you signature, and the size would be 25.82" diameter. with the .63 od and the 4.11's you should see just over 2500 rpm's at 75 mph. that's better than my old ford ranger pu would do. it was at 3300 at 75 mph.

 

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steel1212

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#11

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jst6 said:

I checked your tire size in you signature, and the size would be 25.82" diameter. with the .63 od and the 4.11's you should see just over 2500 rpm's at 75 mph. that's better than my old ford ranger pu would do. it was at 3300 at 75 mph.

Click to expand...

useing this calculator gives me exactly 26"

 

jst6

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#12

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even ur calc shows 25.83"

 

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steel1212

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#13

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LOL guess I should have used the more accurate part at the bottom...oops.

 

blue66tang

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#14

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Yep that is right. I had 4.11 with .63 and thats what it was. I now have 4.30 and its still only about 2450 for 70 mph. I skipped a bunch of posts so sorry if this has already been said.

 

Edbert

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#15

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I hear the term "lugging at 60-70mph" in overdirve a lot, kind of makes me wonder, overdrive is for cruising not accelerating right? That is what I always thought. I also find it sort of interesting that all of my newer vehicles (97/2004 F150 and a 97/2005 Mustang) run right at 2,150 to 2,300RPM at 70mph. I guess that is sort of the "sweet spot" according to OEM engineers.

 

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bnickel

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#16

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every engine has it's own sweet spot. the OE's design the sweet spot to be around 2100-2400 rpm because that's where a stock engine has the most power/torque under the curve and also where it seems to burn cleanest. a mild street motor will still fall into this sweet spot, but some high HP motors do not like to run that low in the power band, hence the lugging. it could alo have something to with the gears in the car as well.

 

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D.Hearne

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#17

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bnickel said:

every engine has it's own sweet spot. the OE's design the sweet spot to be around 2100-2400 rpm because that's where a stock engine has the most power/torque under the curve and also where it seems to burn cleanest. a mild street motor will still fall into this sweet spot, but some high HP motors do not like to run that low in the power band, hence the lugging. it could alo have something to with the gears in the car as well.

Click to expand...

Also the reason some motors get better mileage with a cam or gear change.

 

blue66tang

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#18

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Yep when i had my built 289 in it (blew up, but now i am almost done with my 332). It would lug realy bad. My power didnt even begin to come on till about 3000rpm, so it was always below that. Unless i wanted to drive 100mph everywhere.

 

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steve wilson

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#19

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I'd have to agree with edbert. For highway cruising, the lower the RPM the better for fuel mileage, as long as you can maintain speed and accelerate mildly. Hell, my wife's 2L VTEC pulls fine at 2000 RPM, and it's not a low end torque engine. Are you actually running enough cam that it won't pull clean at that RPM?