RPM vs tire size

[TRCM]

I did some searching, but what I need to know from those who run it, is what SHOULD my rpm be with 31's and 3.55 gears (88 vintage AW4) going down the road @ 60 mph.

I'm trying to figure out a potential trans issue, and I can't remember what the rpm were. Looking at the various tire size vs rpm charts, even accounting for OD, the rpm they show are higher than what I remember (this truck is only used seasonally, so it's been almost a year since I drove it a much).

I'm almost starting to think my OD isn't working any more, among other things. This might explain my drastic loss of mpg.

 

[zachandandy]

According to the Grimm jeeper hear ratio calculator, 3rd gear 2309 rpm od 1731 rpm at 60.

 

[TRCM]

Any actual speeds vs rpm ? instead of chart numbers ?

 

[zachandandy]

The numbers should be dead on unless your tranny is slipping. Those aren't generic chart numbers, they are calculated using actual components. Unless you regeared your tranny, the only variable would be actual tire size, as I put in the size listed, which could be of by a fair amount depending on manufacturer and or wear.

 

[clean4drxj]

The numbers should be dead on unless your tranny is slipping. Those aren't generic chart numbers, they are calculated using actual components. Unless you regeared your tranny, the only variable would be actual tire size, as I put in the size listed, which could be of by a fair amount depending on manufacturer and or wear.

Regear your tranny? hmm I haven't heard of many people do this, most people regear their axles..

 

[VAhasnoWAVES]

what i do, to get the best estimate i can... is go to the novak adapters website and get the specific gear ratios of the trans i want to run. then plug all the information in manually. this (if it applies) its nice for a manual, as you can also look across the speed range that you typically drive and see what gears will turn what RPMs at what speeds.

 

[zachandandy]

Regear the tranny was a joke, he told us which tranny, the gear ratio, and the tire size. The Grimm jeeper gear ratio calculator has a list of trannies and automatically inputs the gear ratio. Unless he somehow regeared his trans, the # I gave are accurate.

 

[mjs302]

Regear the tranny was a joke, he told us which tranny, the gear ratio, and the tire size. The Grimm jeeper gear ratio calculator has a list of trannies and automatically inputs the gear ratio. Unless he somehow regeared his trans, the # I gave are accurate.

It is possible to regear some transmissions, (both automatic and stick). I have installed deeper first and second gears sets in Ford C-4 automatics and many manual transmissions can be regeared (T-5, T-56, NP435 and various Borg Warner and Muncie 4 speeds come to mind).

 

[zachandandy]

I love when you make a joke, then everyone wants to get technical. He has an aw4, which I am unaware of any available deeper gear sets. Also, deeper 1st and 2nd gear sets shouldn't come in to play. I guess he could have adapted a gear vendors overdrive into his xj, but it's unlikely.

 

[cody4359]

sorry i dont have an rpm for you, but a few years ago cruising down the fwy the rpm's would go up about 300, i'd accelerate and feel the torque converter lock up/drop the rpms then id go back to cruising and the rpm's would jump up again.

long story short, replaced tps- didnt fix it. had the tranny rebuilt at a shop, mostly fixed it but still occasionaly slipped. said owell it has a warranty and kept driving it. put a bored out throttle body on it, never slipped again. if memory serves me right my conclusion was it had a plugged/dirty vaccum port on the stock throttle body that went to the tranny. it's been about 2 years ago, never looked into too hard but some reason a clean throttle body fixed my issue.

 

[G.P_XJ]

Here's a great website for gearing

http://www.novak-adapt.com/knowledge/gearing.htm

 

[Johnnie Walker]

I did some searching, but what I need to know from those who run it, is what SHOULD my rpm be with 31's and 3.55 gears (88 vintage AW4) going down the road @ 60 mph.

I'm trying to figure out a potential trans issue, and I can't remember what the rpm were. Looking at the various tire size vs rpm charts, even accounting for OD, the rpm they show are higher than what I remember (this truck is only used seasonally, so it's been almost a year since I drove it a much).

I'm almost starting to think my OD isn't working any more, among other things. This might explain my drastic loss of mpg.

Most likely your TCC is not locking up like it should. Most common cause is the brake switch. Mine was not locking up (newer really knew til I got full gauges vs stock dummy lights), so I just installed a TCC lockup switch. At 72mph I run just above 2k rpm with the TCC locked up in OD.
I have 89 Renix XJ stock 3.55s (for now) and 31" Bridgestone dueler AT 693s. I can check what RPM I am at at 60 mph tomo if you like.

 

[TRCM]

The numbers should be dead on unless your tranny is slipping. Those aren't generic chart numbers, they are calculated using actual components. Unless you regeared your tranny, the only variable would be actual tire size, as I put in the size listed, which could be of by a fair amount depending on manufacturer and or wear.

That's why I was asking mainly, but I am also considerign going to 4.56 gears. Also have to take into account other factors like actual rolling radius vs tire size. I was looking for a fairly accurate range. I also was unsure of the difference between my 2 jeeps, as the OD is a diff ratio.


The Grimm Jeeper site does have a very good calculator tho, hadn't seen that one before. Doing a side by side lets me see how 4.56 gears would affect things, and it look like the engine would still be in the sweet spot rpm wise going down the highway. Even stepping up to 33's wouldn't be that bad.

I can check what RPM I am at at 60 mph tomo if you like.

If you remember, please do. That would give me an idea of how far off (or not) the calculator is. I've yet to see one that is 100% accurate, they are usually off by a few hundred rpm in the real world.

 

[mjs302]

I love when you make a joke, then everyone wants to get technical. He has an aw4, which I am unaware of any available deeper gear sets. Also, deeper 1st and 2nd gear sets shouldn't come in to play. I guess he could have adapted a gear vendors overdrive into his xj, but it's unlikely.

Sorry if I offended you, but I was just trying to say that it is possible to change a transmission's gearing. Some people that read this forum might not know that.

 

[zachandandy]

Not offended, just pointing out that tranny gear swaps are irrelevant to the op's question. The od ratio 87-90 is .705 to 1 for 91 and up it's .75to 1. Whichever ratio the Grimm calculator uses, can be changed manually.

 

[TRCM]

Not offended, just pointing out that tranny gear swaps are irrelevant to the op's question. The od ratio 87-90 is .705 to 1 for 91 and up it's .75to 1. Whichever ratio the Grimm calculator uses, can be changed manually.

Exactly. The calculator seems to be 50-100 rpm off for my 96, so now I just need to see what RPM I am getting and see how it compares on the 88.