ACCURATE SPEEDOMETER?

The trip odometer should be close. You can also use the mile markers on the side of the highways assuming those are correct in your area. I was told by a local road worker in Wisconsin that they are very accurate, but that may vary by states.

As far as 6th gear is concerned, using only 5th gear will cause more engine wear, but it may or may not hurt fuel economy. The only thing to do there is to test it out. I would think that with the small increase in tire diameter, you should still be using 6th gear. You do have a very high gear in your axles, so it is probably worth testing.

Chris

Your speedo with be off a good bit. You will be going faster than it records. With 65 indicated you will be doing about 73 MPH. With 3.07 gear it has got to be a bit of a slug at times too with 31's. Below is a link where you can run some numbers yourself

TheSnoMan.com

On a foot note, I do not know were this myth started. The engine is not going to wear out any sooner in 5th and may actually last longer if 6th is so tall that engine lugs. A few extra RPM never hurt any engine and lower RPM lugging is potentailly far more damaging and tough on bearings.

----------------- TheSnoMan.com

The (ones that have posed here) older TJ speedometers became correct when 31" tire were put on.

Your mile per minute is a good base to tell it is close.

The only other way is to find a measured mile or a friend with a GPS or even ask a cop to pace you because 'you just got new tires and don't want to break the law'. Only a cop's speedometer would be reliable for this, cars can be off too according to tire size.

I find 'my' engine 'much' prefers to be running at 2300-2500 rpm on the highway which I get with my 4th gear. (have 33's and 3.31 gears) That rpm gives me good throttle control and prevents engine lugging. I also get a pretty stable 11L/100km or 23 mpg running there.

If I go to overdrive or 5th gear, the rpm drops to 1750 or so at 65 mph which makes for the need of the full gas pedal travel to adjust speed or hold a hill or wind vs a twitch of the pedal in 4th. This low rpm drops my gas mileage by at least 5-8 mpg.

You can find out easy. Just compare gas mileages when using only 5th on the highway and when using 6th mostly. I have one trip I do lots to get bush camping which is 201 km from gas station to gas station so I have an easy check point.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)

So, assuming that my speedometer is correct, running approximately

2500 rpms in 5th gear at 65 mph will NOT hurt my engine or cause increased engine wear? If I were to shift into OD (6th), what is the approximate rpms that I should be running so as to not cause lugging or increased engine wear?

THANKS

Oh, nevermind. I think I just answered my own question. : D

I am assuming that the approximate 2500 rpms would apply to the OD (6th) gear as well to prevent increased engine wear and lugging?

Which would probably be somewhere in the neighborhood of 70-75 mph in

6th at approximately 2500 rpms.

Your minimal tire size increase likely made your speedo/odo more accurate as they tend to read high from the manufacturer. That way warranties expire sooner and manufacturers save money. I'd bet you are very close now. For fuel economy, use the highest gear you can while staying in the power band (not lugging). Lugging will become immediately apparent and you do it by feel, not by someone telling you what RPM to run. If you shift into too high a gear, you will notice a power response loss and ultimately your engine will lug. Downshift.

tw

Running in the higher gear is the bad thing, not running in the lower. Lugging is bad.

'You' engine has it's own 'sweet spot' of the rpm where it seems to run the best with the best response and best mileage. That is usually where it likes to sneak to when you are not paying attention.

In 'my' engine's case this is 2300 rpm. Anything below 2000 on the highway starts the 'lugging' process and causes engine 'ping' or a rattling noise under load.

1750 or my OD (5th) at highway speeds is way too low for my setup.

Mike

Your speedo is way off.(big tires will do this) In 5th with current tires, 2500RPM would yeild about 74 MPH and 6th (.82 OD) 2500 would yeild about 91 MPH based on quoted tires sizes and axle ratios.

Again you can do the math yourself

TheSnoMan.com

Yup, that is where my OD starts to have good power, over 75 mph.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)

Your site appears to be totally useless garbage.

No one 'knows' their freaking tranny ratio or their t-case ratio, most do good to have their tire size and gear ratio.

'Comparing' tire sizes of old vs new to get a new speedometer reading is garbage. The old tires were 'not' certified to have the correct speed vs the gauge speed, so that is a GIGO situation. Garbage In Garbage Out.

If I punch in my numbers for how fast I should be going at 2300 rpm and say my T-5 tranny in 4th is 1:1 and my Dana 300 t-case in high is 1:1 and my gears are 3.31 and my tires are 33", the 'calculator' gives me a NaN

If I put in my tire size and gear ratio I get infinity for a 'new' gear ratio. Gee I thought gear ratios were set by the 'physical' gears, not by some mythical number.

Care to explain?

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)

Um, your statement is quite wrong. For instance, overdrive on my truck reduces the RPM by 30% so yeah I am pretty sure my engine will wear out sooner if I choose to never use it. I will agree with you on the lugging part, especially if it causes pinging, but no, I will not sit here and agree with you that my engine will last just as long turning 3000 RPM as it does turning 2100 RPM. That's insane.

Chris

Jeep especially seems to like to have the speedometer high for some reason.

The largest stock tire that is listed for my 88 Cherokee on the sticker is P225's, yet P235's make my speedometer accurate according to measured 'miles' and many GPS checks. The cops must think so too, because they don't stop me.

Same for most TJ's. They 'do' list a 31 as the largest stock tire and according to what has been reported here, that makes them accurate with no t-case speedo gear change needed.

I don't know how it worked, but my 86 CJ7 is accurate with 33's. It was

4mph + off with 31's (reading 65, doing 60) when I got it.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)

The GIGO is you Kike, you call yourself a Jeep Guru and do not even know what trany or gear ratios a NSG 370 6 speed has . A real Jeep freak would know this stuff so that leaves you out. One thing consistant about you is that things you do not understand (which is a lot) you attack with BS. BTW, that link has a function for even simpltons like you (if you are smart enought to use a tape measure) to compute speedo error with new tire size. You never where one for math and physics but a great one for BS. I still chuckle about your 3.31 power gears with 33's and not using OD anymore and getting in a big twist about beable able to get in 4lo a lot (because of big tires and tall gears) and 4 hi at any speed. Your comments are purely ego based you should realy not be given anywhere near the advise you do because about a lot of things you are quite clueless.

----------------- TheSnoMan.com

Maybe with a tail wind and down hill it does otherwise it has got to be a slug in OD under anything other than perfect condition but you and your ego would never admit that. See to me OD is just another gear to us all the time in a PROPERLY GEARED 4x4. But then you do not know what that is either.

----------------- TheSnoMan.com

It is bad running in a lower gear ratio when effective drive ratio is to tall because of axle ratio and tire size because your ego and pocket book does not know what is correct.

Your sweet spot depends a lot on load and effective gear ratio in vehicle and the demands placed on engine to move it at a certain RPM

I see you are too cheap to use 89 octane or better fuel to which is no surprize. None of my vehicle ever even think about pinging at any RPM because I NEVER use 87 in any of them except a old 79 J20 with 8 to 1 compression that was made for 87.

And you lake the understanding and comittment to fix it properly because your ego and pocket book holds you back.

BTW, if you want to restart this fued I will tear you a new one because you provide a lot of times to do it. The choice is yours. Your little more than a BSing troll with abig EGO problem.

----------------- TheSnoMan.com

Gee how did I put close to 200K and a few 4x4's 20 to 30 years ago with 4.10 and 4.56 gears and no OD???? RPM as long as it is not excessive (like 4000 to 5000 RPM cruises ) will not shorten life at all. 2500 RPM or cruise for a V8 is childs play. How about a Toyota that when 220K with a 3000 to 3500 RPM cruise and more and was using a quart maybe every 3500 miles then. Also true RPM drop is determined by OD ratio and to get a 30% drop you need a .7 (like on a 700R4 or

4L60 or .69 on a Ally 5 speed)

----------------- TheSnoMan.com

Well, it just so happens I HAVE a 700R4. Don't come off as the only person that knows how to figure out gear ratio calculations. Geezus, I think sometimes you argue just for the sake of arguing.

Chris