Here are my calculations, with 33" tires 3.73 gears and automatic tranny. Please feel free to help if I'm incorrect.

1900 rpms @ 70 mph with overdrive. ( tranny shifts out of overdrive constantly).

1900 + 23.6% = 2348 rpm with 4.88 gear swap. Which I find personally to be acceptable. However, everyone else whom I've spoken to actually turns higher rpms with a larger tire running this ratio. As I've stated before I don't know alot about gears but the larger the tire diameter the lower the rpms correct? So I'm led to "assume" (and we both know what that does) that having a smaller tire than previous posters my rpms will be higher. I'm sorry if it seems like I'm dragging this through the mud but I only want to spend $1500 once. Thanks for your help.

 

Okay, I must have a touch of the tard or something. I just got my 4.88's put in last week and took a 4 hour road trip to Dallas. At 70-75 I was running about 2900 RPM's in 6th. It feels a little high for me but the response is like night and day compared to the 3.21's it came with from the factory. I will tell you this with certainty, and no you don't need a calculator for this one, my gas mileage is not very good. I am lucky to get 13-15 MPG with the new 4.88's. Not complaining because I can actually pass the eco-cars now.

 

Was Your speedometer reading 70-75 mph as you reported you were going at above?

 

See I'm with you on this one TEEJ. Bluecard have you recalibrated your speedometer?


Also how bad will the tach be off with new gears? My hypertech gives the option of changing gear ratiio so I'm assuming it must be affected.

 

I use my GPS to know what my speed I'm going. It runs about 7MPH over what my speedo says which is pretty much what every body was telling me on this board.

 

I'm not disputing your rpms or speed. What I'm wondering now is if the tach could be wrong since my hypertech programmer gives the option on changing gear ratios. Or does this just tell the computer to adjust mpg and odometer readings? questions, questions, questions. It never ends.

 

The tach will not be affected. It reads engine rpm's and doesn't care if you have 44" Swampers or the stock X "donuts" .

 

I think its just some general confusion regarding "effects" on the DEVICE that's doing the MEASUREMENT, - vs - the actual change in value.

Essentially, is the change affecting the SPEEDOMETER, or affecting the SPEED?



For example...it seems that the JK uses a Hall Effect speed sensor that reads off of the drive shaft revolutions, to calculate the ground speed, and tone ring sensors on the wheels to calculate wheel spin/ABS/Traction Control, etc...issues.

That means that the SPEEDOMETER is calibrated to interpret a given number of SHAFT RPM as corresponding to a particular SPEED...assuming it knows what gear you are in, your tire size, and your diff ratio.

If you change the diff ratio...say with lower gearing....you raise your ENGINE RPM (Tach reading) for a given real speed...as the engine has to spin faster to make the Jeep go the same speed.

So - the new diff ratio changed the speed the engine ran at, to compensate for the shorter distance each engine revolution sent you...BUT, changing the diff ratio didn't affect the Tach itself....the Tach was just reporting the facts regarding the new corresponding ENGINE RPM.



Same with the speedometer....if the shaft it reads the speed off of, is actually going faster....it thinks the truck is going faster....if the shaft is going faster because the new gears make it HAVE to go faster, to go the same speed...the new gears didn't affect the SPEEDOMETER, they affected the READING of the SPEEDOMETER.

Does this make sense?


If it does make sense, do the same for a larger tire...as the larger tires will make that shaft spin SLOWER, as every tire revolution covers MORE DISTANCE than before.

AND - as you go further for every tire revolution....the engine doesn't have to spin as fast as it used to to go the same speed.

So the tire made the tach SHOW that the engine RPM is lower than before...because it IS slower than before.

AND

Because the tire covers more distance with every tire revolution than before....the shaft that rotates to give the speedometer a reading is ALSO going slower...as it doesn't HAVE TO rotate as fast with a larger tire, to go the same speed.

__________________________


So the measurement devices are not "Affected" by new gears or larger tires...they just read off what ever input they were supposed to...but, are off by the percentage you changed the inputs by.

If you got larger tires....MORE distance is being covered than they thought.

If you get lower gears, LESS distance is being covered than they thought.

The REASON we GET lower gears when we get larger tires, is to try to achieve the rpm range that works for either off roading or gas mileage, or whatever our objective is.

All the posted charts are WAAAAAY out of date...as they assume a 1:1 final drive ratio (Which also makes it a simpler chart...)...whereas, there ARE NO rigs that actually HAVE a 1:1 final drive ratio...ALL rigs now a days have an overdrive top gear ratio that is what they are designed to cruise at hwy speeds with....to get better mpg.



A better way to pick gears than the charts, is to simply use the percentage your tire is larger, and see the proportional rpm drop from using the larger tire...

...and then use the ~ SAME percentage change, in deeper gearing, to RAISE the RPM by the same amount you dropped it with the tires.


IE: If the tires DROPPED THE RPM by 10%, get new gears that RAISE THE RPM by 10%....that wy, you are cruising at the same rpm at a given speed, etc.




IF you want MORE power, and LESS MPG...go deeper, and have HIGHER RPM, etc.



___________________

Hope that helps.

 

TEEJ, that was one of the best, well thought out posts I've had the pleasure of reading on this forum. I believe the mods should sticky it so that knowledge challenged jeepers like myself can get a better understanding of their heaps. In your one post I've doubled the amount of useful info I've learned on this forum. If you're ever in my neck of the woods, beer and crawfish on me!

 

Nope, I'm not wanting to spend the $350-$400 just to recalibrate the speedo. With the assistance of some of my buddies assigned to the traffic division we found that my JK is running exactly 5 MPH faster than the speedo reads (via laser radar)...