Old Question New Tricks?

[Eagle]

I sort of remember that I may have posted a link to my tire/gear/road speed spreadsheet on Google Docs awhile ago. Long enough ago that (if I did it) I no longer remember the link, or the user name and password I would have used for Google Docs. If anyone remembers seeing it, or has it bookmarked, please post the link.

 

Mine offers two advantages: First, it gives you RPMs for both 4th gear and 5th gear. Second, the RPM calculations are based on actual tire manufacturers' revolutions per mile, so they are much more accurate than calculations based on theoretical tire size (which is what pretty much everyone else is doing with their on-line calculators and tables).

[lil_loco]

I sort of remember that I may have posted a link to my tire/gear/road speed spreadsheet on Google Docs awhile ago. Long enough ago that (if I did it) I no longer remember the link, or the user name and password I would have used for Google Docs. If anyone remembers seeing it, or has it bookmarked, please post the link.

 

Mine offers two advantages: First, it gives you RPMs for both 4th gear and 5th gear. Second, the RPM calculations are based on actual tire manufacturers' revolutions per mile, so they are much more accurate than calculations based on theoretical tire size (which is what pretty much everyone else is doing with their on-line calculators and tables).

That'll be nice to see!

[lil_loco]

I sort of remember that I may have posted a link to my tire/gear/road speed spreadsheet on Google Docs awhile ago. Long enough ago that (if I did it) I no longer remember the link, or the user name and password I would have used for Google Docs. If anyone remembers seeing it, or has it bookmarked, please post the link.

 

Mine offers two advantages: First, it gives you RPMs for both 4th gear and 5th gear. Second, the RPM calculations are based on actual tire manufacturers' revolutions per mile, so they are much more accurate than calculations based on theoretical tire size (which is what pretty much everyone else is doing with their on-line calculators and tables).

That'll be nice to see!

[lil_loco]

It give you some idea of how larger or smaller tires will change the way your car performs. Example, if most of your driving is on the freeway at 70 mph (for example) and your engine turns at 2250 rpm. By knowing the size tire you have now, you can calculate the change in engine speed if you go to bigger tires. Bigger tires will mean a slower turning engine at the same speed. Say perhapse 1850 rpm instead of 2250 rpm. And the typical engine produces less horsepower/torque at 1850 rpm than 2250 rpm. So your truck will not accelerate as well or go up a hill as well with the bigger tires. Hope this makes some sense.

In that theory if one was a bit lead footed and prefers to cruise high speeds but take time getting to them then a slightly larger tire would be good? Slightly ...... not massive boggers ;)

[Eagle]

In that theory if one was a bit lead footed and prefers to cruise high speeds but take time getting to them then a slightly larger tire would be good? Slightly ...... not massive boggers ;)

2001 Cherokee. On stock 225/75-15 tires the speedometer was off by about 2 MPH at 70. Speedo read faster than actual speed. It's now running on 235/75R15s and the speedo is dead nuts accurate at 70 MPH, confirmed by GPS.

 

So one tire size makes a difference of 2.86%. In terms of engine RPM, we're talking in the vicinity of 2250 RPM, so the one size difference works out to 64 RPM -- at 70 MPH.

[FrankTheDog]

I sort of remember that I may have posted a link to my tire/gear/road speed spreadsheet on Google Docs awhile ago. Long enough ago that (if I did it) I no longer remember the link, or the user name and password I would have used for Google Docs. If anyone remembers seeing it, or has it bookmarked, please post the link.Mine offers two advantages: First, it gives you RPMs for both 4th gear and 5th gear. Second, the RPM calculations are based on actual tire manufacturers' revolutions per mile, so they are much more accurate than calculations based on theoretical tire size (which is what pretty much everyone else is doing with their on-line calculators and tables).

It's pinned in the DIY section.

[HOrnbrod]

It's pinned in the DIY section.

That's no help. What's the link?

[FrankTheDog]

I'm at work and I knew where it was off the top of my head.

Not compuker savvy enough to link it.

[benjy_26]

I'm running an AX4 with 3.55's behind my Squirrel and 31" BFG AT's. Describing it as a 3 speed with overdrive is pretty accurate, though it keeps up with traffic on the freeway just fine.

[Eagle]

Thanks to Frank the Dog, I found my own post. And the link still seems to work:

 

https://spreadsheets.google.com/pub?hl=en&hl=en&key=0AvuGamS7bzt-dHJxYy1QajhuVUFUbUVDUWdiZnVFbmc&output=html

 

When the spreadsheet was copied into Google Docs, it truncated fields in which the text is wider than the column width. Where each set of data says "Revs per M", that refers to the box just to the right, and it's not RPM, it's Revolutions-per-Mile for the tire. Within a particular tire size that number will vary slightly depending on the make and model of tire. In general, I used an actual manufacturers number for a tire suitable for use on an XJ or MJ, typically a name brand all-terrain tire.

[lil_loco]

@Eagle THANKS! that must have took quite the time to make! Too bad the tire I wanted to know about was the only one not on the list..... should I assume 33-10.5 the same as 33-12.5 or 32-12.5?

[lil_loco]

I'm running an AX4 with 3.55's behind my Squirrel and 31" BFG AT's. Describing it as a 3 speed with overdrive is pretty accurate, though it keeps up with traffic on the freeway just fine.

Shy of my ability to choose shift points I guess you are running the closet set up to mine.

 

I'm going to try to trade some leftover TJ parts (oem half doors) for a set of 31" duratracs or mtz's then. That way if the 33" really hurt my ability to clear an intersection before being to the highway I'll be able to swap back.

 

Thanks everyone for your input and stories! Feel free to keep sharing for other angry squirrel owners :)

[FrankTheDog]

Use the 33/12.50/15 for comparison, it'll be close enough.

[benjy_26]

 

This is not the TBI engine, but, it'll do as an example.  The most practical reason to swap gear after a tire change is to keep the revs of the engine as close to the TQ peak as possible for the longest time possible.  This will ensure greater responsiveness and arguably will promote efficiency.  

 

The 2.5 peaks out around 3000 rpm.  A cruising rpm of about 2500-2800 would be ideal for most applications.  I suggest to gear accordingly.  

[lil_loco]

3000 rpm seems painful no? Running hotter and screaming......

 

I was always told 2000-2250 was ideal

[benjy_26]

Redline for the 2.5 is 5600rpm.  3000 isn't unheard for a 4 banger.  Keep in mind, it's gotta spin to make power.   Based on that dyno sheet and the grimm jeeper table, I plan on going with 4.56's or 4.88's in mine when I swap in a 5 speed. 

 

If you look at the dyno sheet, there ain't a whole lot going on in this engine below 2400rpm.  

[lil_loco]

True point. It's a steep peak. So much learning going on here! That's why I enjoy CC

:)

[benjy_26]

By my calculations, I'll be doing 67 at 3000 rpm with 31's if I go with 4.88's.  A bit high, but it's in the meaty part of the power band, doesn't beat the engine too much, and is still faster than the 60mph speed limit imposed on me inside my city limits. At 70mph, I'll be going less than 3200rpm.  Good enough for cruising long distances, I think.  Besides, this leaves me open to step up to 33's and not suffer TOO much.  The only thing I worry about with the 4.88's is the one tooth contact on the front D30.  May be a weak point.

 

If you plan on wheeling your MJ, the lower cogs really start showing their usefulness.  With 4.88's and an AX5, in low range, 1st gear (np231), I'll be  crawling at less than 5mph right at 3000rpm.  ALMOST perfect for dealing with difficult, rocky terrain.  If I drop a crawl box between my T case and tranny (in the works), that crawl speed goes down to under  TWO MPH!  

[benjy_26]

True point. It's a steep peak. So much learning going on here! That's why I enjoy CC

:)

Learning is what it's all about!

[Eagle]

3000 rpm seems painful no? Running hotter and screaming......

 

I was always told 2000-2250 was ideal

You were told wrong.

 

These engines are derived from the old AMC I-6 that dates to 1964, which was long before overdrives became standard. The typical AMC "back in the day" was a three-speed manual tranny and geared to approximately 24 MPH per 1000 RPM. That meant 60 MPH was 2500 RPM, 72 MPH was 3000 RPM. The engines easily lasted 200,000 miles -- my brother's 1970 Gremlin went over 300,000 and he autocrossed it regularly.

 

2250 is a good cruise RPM for the 4.0L but it's well below the torque peak for the 2.5L. Cruising at 3000 RPM won't hurt the engine at all, and it's hardly "screaming."

[lil_loco]

Well now i won't be so afraid to "make a like noise" with the skinny pedal!

[benjy_26]

A good exhaust goes a long way towards making high RPM excursions in the 2.5 enjoyable. When I first got my truck, I was granny shifting like a mofo It sounded like I was about to drop a rod! I swapped in a custom exhaust based around Magnaflow components and I get it up past 4000 now without too much drama.

[FrankTheDog]

With a real quiet exhaust you can really rev it without making to much commotion.

I have a huge muffler on my 4.0HO 5 speed MJ and I sometimes accidentally leave it in third and I don't catch it until I look at the tach and see it at 3500 or so. With 3.07's and 30/9.50/15's I rarely take it out of fourth.

[lil_loco]

I don't even think my truck has a proper catalytic right now.....

[benjy_26]

Cats are optional sometimes, depending on the age of the vehicle and local laws. I run mine cat-less and it passes with flying colors. I have a 24" Magnapack muffler feeding a fulal case Magnaflow muffler, both in 2.5" and it sounds pretty neat, I think.