Tires & odometer mileage

Pretty sure I know the answer but would like it confirmed. If I take stock tires off and put 35's on and do not change the speedo gear

A. What happens to registered speed, am I going faster or slower than indicated?

B. What about mileage accumulation on the Jeep, does it show more or less than is actually being put on the drive train?

 

Larger tire will have less revs per mile = Your speedometer will read lower than actual speed

 

It will show less miles on the odometer and your speedo will show less than your actual speed.

 

Your tranny will change gears at different slower speeds too which will make your Jeep very sluggish.
Why not just buy a reprogrammer and set it up for the 35s.

 

Depending on the set up, older trannys change gears based on vacuum and rpms. Newer trannys are based on rpms and other electrical inputs. They all feel sluggish because the power to output force (torque) ratio at the wheels has been diminished, hence the desire and/or need to regear. Everybody above is correct. Your speedometer will be displaying slower than you are actually driving, and you will log fewer miles than you have actually driven.

 

It's an '06. Standard Transmission. The guy selling claimed it had been ran for some time with the 35's on it without changing the gear. That has been corrected now. He was telling me it would have less miles than indicated on the Speed O because of that.

Running that through my thick skull it didn't compute so I thought I'd come here and ask. My thoughts were it would show I was moving slower than I actually was and that it would also show less milage than was actually being accumulated.

Thanks for verifying

 

So depending on which model you are talking about they come with either 29 -32 in tires. Lets say it was 31s The circumference of a circle is 2TTr 31s circumference is 2x3.14x15.5= 97.65in and the 35s circumference is 2x3.14x17.5= 109.9in To find the percentage difference divide the bigger tire into the smaller tire. 97.65/109.9= .888535 or 89% What this means is for every 100miles driven with 35s you can add another 11miles onto the odometer. 1000 miles add 110miles to the odometer. 100000 miles add 11000 miles to the odometer. Really not that much difference on a 10 year old vehicle. IF youre getting a GOOD deal, I wouldn't let that sway your decision.

 

Offgrid, you are ONGRID when it comes to numbers!! Well said!!

 

.... Driving down the highway I was following hubby and going 60mph, ten under the speed limit. He ended up calling me soon after and I was like, babe, I can drive 70mph, its not a big deal... He was like, I'm going 78!!!


So I'm thinking when the previous owner put on the 35s they didn't adjust the speedometer... Is that the only thing that needs checked and adjusted? What does that entail?


95% of my manual driving experience is from growing up dirt bike riding, but I feel like the JK shifts well and everything, but could adjusting the tire size require regearing? What sort of things should I be noticing if it is needed?


Any feedback and suggestions is very welcome. Thanks!

 

A change in tire size if large enough should require a regearing. A small change in tire size and you can get away without changing gears. A large difference in tire size and stock gears may be ok on flat roads but you may notice that you have to down shift a lot of get up hills or that your Jeep is sluggish against a headwind as it doesn't have the torque that it once did.

If you aren't noticing any sluggishness I would check to see what gear ratio you have, it may have been changed already.
You can jack up the rear end and rotate the tire one time while counting how many times the driveshaft turns. A little over 4 rotations of the driveshaft would be 4.10/4.11 gear ratio, three and 3/4 turns would be 3.73 ratio, a little over four and a half turns would be 4.55.

 

Thanks! I'll have to test out the driveshaft turns. It is kind of sluggish on hills and I have poor torque in first for our steep grade driveway... Kinda worried that it'll need the regear. I'm trying to get into a local club to hopefully get some help without breaking the bank...

 

If I'm not mistaken you said you have 373 gears. That will,handle the 35s no problem. If you are happy with how it drives now, just by a programmer like a super chips or hypertec programmer, plug it in and adjust the computer for tire size. As prerunner said regearing helps, but in your case I don't believe it absolutely necessary. It is something you can do when you have the extra money. Regearing will give you a little more "Umph" from a stop and when passing somebody.

 

Ok. Thank you. (yes, 3.73 with the 35"s) I bet hubby would like to avoid the cost of regear atm, so hopefully reprogramming will help a bit, if only to display accurate speed. I think ideally I'd like to regear to 4.11 or 4.56, but that'll have to be down the road.

Flashcal or take it to a dealer?

 

3.73 gears work on 35s but it's sluggish. The manual trans helps a bit vs an automatic. I would suggest 4.56 when you do eventually regear.

I have 4.10 on 37s for about a year now. It's not ideal but works.

 

Yeah. 4.56 sounds like what I'd want for it.

 

Yes, exactly what OffGrid said. Get the programmer so your computer can know what your tire size is currently. That will match your Speedo up to real speed or very close. Test the real speed up against your GPS. You can also tell your computer if you plan to Tow something, Change tire height, different elevations, many options and Super Chips is menu driven and very easy to use designed to specifically work with Jeeps. It in the menu choice of vehicles..

This is exactly what I did with my 2014 JK V-6 with 35" Tires. I pull my boat all the time. Cruz 95 on the way flats with nothing behind. Tall tires will allow for that but takes away out of the hole a little just like OFFGRID and many pothers have said.

 

One more thing @ rapracing My wife has the 2003 Rubicon, the Superchips programmer fit and I adjusted her tire height 33" and speedo to read properly now. So it will reprogram yours just fine.

 

Just get a programmer..its ez. AEV procal or Superchips FlashCal F5

 

FLASHCAL - SUPERCHIPS, I vote Super Chips

 

Thanks guys! I found a highly reputable jeep modder and mechanic who is going to take a look at it and assess the ratio and then hopefully just reprogram for me.

I really appreciate all the info. Its very helpful!