ACCURATE SPEEDOMETER?

If your pedal is near the metal, your engine is lugging at two grand and using more gasoline than at a cruising speed of three grand, also fourth is direct, and doesn't use any gears nor bearing like in overdrive. So it saves the transmission, too. God Bless America, Bill O|||||||O mailto: snipped-for-privacy@aol.com

Well, I do have an appt at the dealership tomorrow around 5 pm (eastern time). When I find out whether or not my speedo is right on or off a few mph, I will certainly post the results to let you all know. In the meantime, I really don't have a lugging issue or a power loss with the 3.07s and 31s. I can start up a moderate sized hill doing 65 (according to the speedometer) in 5th gear, and not have to stomp the gas pedal to continue going up the hill at a nice pace. I am wondering if the spec sheet on my jeep is wrong, and I actually have 3.73s. I will look at the rear diff case tonight and see what the stamp says.

Instead of going off the deep end again and biting, hook, line and sinker, why don't you just answer the questions about your site?

How can you get an accurate speedometer reading by comparing two unknowns? That is a GIGO situation to me.

For one example, comparing the old and new tires. Makes no sense to me. That will only tell you how much more or less one tire is off from the 'other', not how fast you are going.

For example again, when I bought my CJ7 with 31's it was only going 60 mph when the speedo said 65 mph according to a GPS. I then put 33's on and it became correct according to GPS. That is the only way to know short of a measured mile.

How come my CJ7's numbers don't work in the rpm vs mph box?

258, T-5 tranny in 4th should be 1:1, Dana 300 t-case is 1:1, 3.31:1 D44 rear.

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)

Gas mileage tells all for highway running.

You can block it and jack up one wheel then count the driveshaft turns when you spin the wheel twice to get your ratio. A chalk mark on both the wheel and one ear of the driveshaft helps for counting.

Your driveshaft will spin just over three times, or close to 4 times depending on which ones you have when you turn the wheel twice.

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)

Mike, I didn't look at that site, but the calculation is quite simple in case you don't have it.

MPH = RPM*TireDIa/(GearRatio*TcaseRatio*TransRatio*336)

Chris

Jeff, I'm relieved that at least someone chimed in with the relevant answer on the speed - virtually ANY speedometer will be dead on if you use the odometer to measure your mile since both the speed and distance shown is derived from how fast a shaft turning, not from actual distance covered. The same error in speed is translated to distance. You need a "measured mile" - I like a 4-5 mile stretch since it averages out the speed better and minimizes timing errors.

As for eng> Your watch is a bit clunky, but the method is correct.

Yup, exactly. I'm thinking she'll be closer with that tire change.

tw

Are you making a special trip to the dealer to check your speedo??

tw

Borrow a handheld gps and verify your speed.

Not really. I have had this appt scheduled for a few days now. I am not really good at some things when it comes to this stuff. However, I did try the measured mile trick on the way home tonight, and used the formula that someone posted here earlier (3600/time it takes to go

1 mile) = actual speed. When my speedometer reads 60, it took me 54 seconds to go 1 mile. Hence I am assuming my actual speed is approximately 66.5 mph (3600/54=66.6).

As for the 3.73s, no such luck. : ( I am stuck with the 3.07s. Although as I mentioned earlier, it's really not that bad. At least not as bad as I had expected. There is a noticeable difference from the stock sized tires, but I really don't notice it until I get into

5th gear. Then again, before this 6 speed, I was used to driving a 5 speed anyway. And finally, there's the great debate of whether or not to use 6th gear and if so, at what speed/rpm.

One thing.....what is engine pinging and what does it sound like? I have a 'funny' noise that my jeep makes (seems to be coming from the passenger side) while going uphill. To me, it sounds like keys jingling very softly and intermittently. It never happened (that I remember) prior to the installation of the bigger tires. The engine is not skipping, sputtering, or seem to be lugging at all when this noise occurs. It usually starts right at the beginning of a moderate sized hill and continues intermittently until I get to the top of the hill. And it NEVER happens when the engine is cold (first 30-45 minutes of driving hills or not) or while going downhill. I don't know if this makes a difference, but I have been using 87 octane fuel since I bought it new in November of 2006. Just to see if it made a difference, I filled her up with 89 octane today. Gonna run her a couple of days and see if I still notice this noise.

The "key jingling" sound you hear is probably pinging. That is actually a good way to describe it. I used to always tell people it sounds like light tapping on a piece of metal with a small hammer.

Chris

Any idea why this would happen when I don't seem to be lugging the engine? Could it be strictly related to the increased tire size? I know how to 'feel' an engine to shift without lugging (been driving a stick for the past 13 years) or to shift into a lower gear while going uphill if engine seems to be lugging. Other than the 87 fuel I was using, I have no clue. Is this harmful if it continues, and is there a way to avoid it from happening? I drive approximately 35 minutes each way to work, and it only happens 2-3 times each way for about 10 seconds each time (not a straight 10 seconds, but intermittently 2-3 times in that 10 seconds).

When your tire diameter increased, it effectively increases the load on the engine. Kind of like adding weight to the vehicle. Yes it is harmful to the engine if you let it keep happening. What you are actually hearing is the fuel igniting in the cylinder before it is supposed to. It is caused from excessive cylinder pressure. Over time it can cause broken pistons and piston rings, blown head gaskets, and in extreme cases, a cracked cylinder or head. Higher octane fuel will stop the pinging, as will keeping the transmission in a lower gear.

What you can do to test the cost effectiveness of the high octane fuel is to run one tank of 87 octane while using the lower gear and check your fuel mileage. then do the same with 89 octane and see if you can run like you did before the tire change. If the fuel mileage offsets the cost of the higher octane fuel, then that is the best way to go. Some vehicles will see a good increase in mileage if the fuel allows it to be run in the higher gear. The only way you can know for sure is to test your particular vehicle.

Chris

Hey Snow, look out! There's another dead horse. Better get your baseball bat.

-- Old Crow "Yol Bolson!" '82 FLTC-P "Miss Pearl" '95 YJ Rio Grande BS#133, SENS, TOMKAT, MAMBM

Thanks Chris. That was helpful. :) I will just have to remember to fill my tank with 89 octane instead of

1. Is there much of a difference in effectiveness between the 89 and

93 as far as pinging is concerned?

I agree. To me the light ping means lugging and bad mileage. Running in a lower gear ups 'my' mileage radically when this is going on.

I also have a manually tuned carb engine with a 'hot' Accel powered spark and hot timing curve so mine runs best on 91 or 92 octane.

In 'my' case, it makes 100 miles per tank difference in gas mileage so it is well worth the extra for the high test.

In our old Cherokee 4.0's case, 91 octane gave slightly lower mileage if anything at all. It liked regular.

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)

Well, the higher the octane, the more resistant to ping your Jeep will be, but the question remains whether or not you need 93 octane to do the job. If the 89 octane eliminates it, then you're good to go. If not, you can try a 50/50 mix of 89 and 93 or try all 93. The goal in the end is to get the most distance traveled with the least amount of money, and no engine damage. Right now where I live, 87 and 89 are the same price for whatever reason, so I just use the 89 octane.

Chris

You know, this goal description is the polar opposite of my entire life in relation to the automobile. :)

tw