Larger tires, tread design and power loss?

[resistance]

After lifting my Renny, I replaced my stock 235/70/16 street tires with 245/75/16 Cooper STT's, which have rather agressive tread design. Since then, my vehicle has dramatically lost power. I feel it mostly when I need instant acceleration and while going uphill. I hit the gas, engine roars but no, I don't get enough response.

My question is; is it the SIZE of my tires that causes power loss, or the chunky TREAD DESIGN? What should I do to regain acceleration? Get the same size (245/75/16) street tires, or go back to smaller street tires?

 

[HoosierJeeper]

The size...regear to 4:10's (should be pretty common in Europe...with all the CRD's).


Tread might play a little factor, but not that much of one...

 

[ThunderbirdJunkie]

It's the tire size.

HJ, 4.10s are not common. 3.73 gears were standard in the 2.8 CRD models.

 

[HoosierJeeper]

It's the tire size.

HJ, 4.10s are not common. 3.73 gears were standard in the 2.8 CRD models.

2.5 CRD's had the 4.10's IIRC (thought Tony was boasting that a while back)?

 

[ThunderbirdJunkie]

2.5s did but not all CRDs

 

[incommando]

Re-gearing will help, but it is not just the diameter: I'll bet you moved to an "e" rated tire that is much heavier than stock. Unsprung weight has a greater effect on performance/mileage by a factor of up to 10

 

[tjkj2002]

Every 1lbs of added rotational weight is like adding 10lbs to the vehicles total weight,it adds up fast.So if your tires weight 20lbs more then stock it's just like you added 800lbs of weight to the KJ while in motion when you actually only added 80lbs.

Plus the larger tire size play's hell with the transmission which makes it "seem" to loose alot of power also.Having the speedo recalibrated helps some but regearing is the only way to get most of the lost power back.

 

[J-Thompson]

I mostly agree
The loss is due to tire OD more than any thing
I have done a ***** pile of digging on this very subject
what I have gathered is this
70% of the loss is OD
20% is weight
10% is tread design ,crappy street tires can get crappy MPG compared to good ones
the loss to OD is called "effective gearing"
The KJ needs a certain ratio to function ,as designed
taller tires change that ratio

nuts and bolts you need 4.10's or you need to live with it
I chose to live with it
on a side note
what works on 1 model may not be the same for the next
example
KJ lost power when we put 245/75's from stock
TJ noticed almost no loss from a 29" tall ,at best, to a 32.5" tall tire
but it noticed a loss with a 31 X 11.5 Bias ply Super Swamper

 

[Ry' N Jen]

On our last 1800 Km trip this past week, our rig was loaded with 750 lbs.
of camping and off road equipment. (I weighed it at a truck scale before and
after loading!)
And with the 245-75-16" M/T R's, we found ourselves driving with overdrive
off on the steeper inclines on the freeways/highways.
Time for regearing for our KJ as well!

 

[J-Thompson]

On our last 1800 Km trip this past week, our rig was loaded with 750 lbs.
of camping and off road equipment. (I weighed it at a truck scale before and
after loading!)
And with the 245-75-16" M/T R's, we found ourselves driving with overdrive
off on the steeper inclines on the freeways/highways.
Time for regearing for our KJ as well!

I should say
We had no issues with the heavier old style MTR's
and a good load in the Jeep
4 people + stuff
This may also be due to the 45rfe

 

[resistance]

Thank you for the valuable feedback. "Regearing" is changing gear ratios, right? God, I never thought I would need regearing with 245/75 STT's

How is it done? If I need to change a gearwheel in the transmission, I'd better forget about it (insufficient technical assistance here). I could even go back to stock size tires..

 

[J-Thompson]

Thank you for the valuable feedback. "Regearing" is changing gear ratios, right? God, I never thought I would need regearing with 245/75 STT's

How is it done? If I need to change a gearwheel in the transmission, I'd better forget about it (insufficient technical assistance here). I could even go back to stock size tires..

you will need to change out the ring and pinion gear in both the front and rear diff.

 

[resistance]

you will need to change out the ring and pinion gear in both the front and rear diff.

Well, that cancels my option to "regear", because it's nearly impossible to find the parts and get it done here. I imported a lift kit, paid huge amounts during importation and also had a very hard time getting it installed.:favorites68: I don't even want to think what the cost would be this time. So I'd better buy a set of "smaller" tires.

Next week I'll be going on a vacation and I don't think my 245/75 STT's are a good choice for that long trip. And so that there will be no regearing, the question is; what would the largest size for street type tires be to get most of the power back, if not all? Can I go bigger than 235/70?

 

[tommudd]

Why wouldn't the STTs be a good choice?
I never noticed much difference in mine from 245-70 up to 265-70, the big hit was at 265-75s !!:icon_twisted:

 

[HoosierJeeper]

Well, that cancels my option to "regear", because it's nearly impossible to find the parts and get it done here. I imported a lift kit, paid huge amounts during importation and also had a very hard time getting it installed.:favorites68: I don't even want to think what the cost would be this time. So I'd better buy a set of "smaller" tires.

Next week I'll be going on a vacation and I don't think my 245/75 STT's are a good choice for that long trip. And so that there will be no regearing, the question is; what would the largest size for street type tires be to get most of the power back, if not all? Can I go bigger than 235/70?

245/70 possibly.

 

[LibertyFever]

I've read through the posts resistance and I can't tell if you have an automatic or manual transmission but I do understand you have a CRD, am I right?

I have a gas powered Liberty but I expect the CRD models function the same way. If you have an automatic transmission I would expect that you need to program the BCM for the larger tire size. I could be wrong but I understand that the BCM controls the transmission and when it shifts. That could very well be your problem. Correctly programing your tire size into the BCM also will correct your speedometer.

I have 245/75R16 AT tires on my gas powered Liberty Sport and if seems to perform well with the tire size programed into the BCM without regearing. My hwy gas mileage is about 22 MPG but the AT tires are just way too noisy for long road trips. I may soon put the quieter 235/70R16 tires back on just until the first snow flies.

 

[resistance]

No it's a gasser with automatic transmission. I think I'd better buy smaller street tires, less noise, more MPG, more accelerataion, etc. But I don't know what size to prefer, 235/70/16 or 245/70/16 as HJ says?

I have little time left for the long trip. Any help will be greatly appreciated.

 

[HoosierJeeper]

Probably 245/70/16, you'll got like 95% of the stock performance it back, but after this it'll probably feel like 100% back.

 

[resistance]

Probably 245/70/16, you'll got like 95% of the stock performance it back, but after this it'll probably feel like 100% back.

Thank you HJ. But before I buy, any other ideas? Let me repeat the question:

what would the largest size for street type tires be to get most of the power back, if not all? Can I go bigger than 235/70?

 

[HoosierJeeper]

Pretty much just the 235/70 or 245/70, not a huge performance difference between them, if any, visually the 245's will look nicer with the lift, and maybe more tires to choose from in that range.

Have any ideas on which tires you want?