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Larger tires Huge Mileage drop?

tkjeeper

NAXJA Forum User
This past summer I had small stock 28's on for the highway driving I do and switched to my winter tires with a small lift, 31" Bridgestone Dueller A/T's. So the way I figure it the tires are 10% larger in diameter. I would get over 700 km's on a tank of fuel with the old 28's, now with 31's I'm barely hittin 450 km's. Now I've kept track of the rpm old and new and I'm still under 2000 rpm's at 100 kph. If my rpm's arent siginificantly higher and tires are only 10% larger, why the drop of 30% fuel mileage?? Any help would be appreciated.
 
tkjeeper said:
Now I've kept track of the rpm old and new and I'm still under 2000 rpm's at 100 kph. If my rpm's arent siginificantly higher and tires are only 10% larger, why the drop of 30% fuel mileage?? Any help would be appreciated.

Did you keep the stock gearing?
 
more wind resistance more rolling diameter more unsprung mass (bigger tires weigh more) can contribute to the lower mpg's also with the larger tires did you change your spedo gear?
 
The speedometer gear may be the entire answer here, of course the tires weigh more and you should not expect to get better milage but you are rolling ~18" farther with every revolution of the tire but it takes more power to make them roll......I would expect a drop of about 10-15% in overall milage, not 30%.
 
Yes I should have mentioned, stock gearing, stock speedo gear. 30% seems quite high, no leaks of any kind by the way.
 
you're putting more miles on it than your speedo says.

also, some sites have said that best efficiency rpm range for the 4.0 is 2500-2800rpm.

try highway driving out of overdrive see if it makes a difference.
 
Ive checked with my gps and the mph is dead on with the gps! As far as the optimal rpm range being 25-2800 I had no idea! I just assumed that the lower the rpm's the better the efficiency, I will try next week driving out of OD! I have also checked with the gps the distance to work to check my odometer, it's dead on also. Keep the ideas coming, this is good stuff!
 
With my roof rack and the 4 lights on top of my 89cJeep with a 4.5 inch lift and 33's, I am lucky to manage13-14. I recently took the roof rack off and gained about 2 mpg back, and am now running my winter wheels (31's) and pulling about 16.
 
tkjeeper said:
Ive checked with my gps and the mph is dead on with the gps! As far as the optimal rpm range being 25-2800 I had no idea! I just assumed that the lower the rpm's the better the efficiency, I will try next week driving out of OD! I have also checked with the gps the distance to work to check my odometer, it's dead on also. Keep the ideas coming, this is good stuff!

is your GPS rounding up? I dont see how it could be dead nutz on, its mathematically impossible.

Are you checking the obvious like proper tire pressure, your not running rich (check for codes and have you ever replaced o2 senor).

From what you said so far its not adding up that you experience that fuel difference with those tires AND you have confirmed with GPS.

Lastly how are you calculating the mpg are you starting off with a toped off tank, then refill to top X amount of gallons then divide by milage on your trip odometer? Keep doing this for a month and see how the milage goes. One or two trips can be explained by driving habits, hills, lots of idiling, low tire pressure etc and throw off the performance of one tank. Average over a few tanks and you will get your true real world MPG.
 
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I have not checked codes and don't know if I'm running rich, tire pressure I will double check today, O2 not changed as long as I've had it 2 yrs, it's a "93 HO by the way. I know it's hard to believe but my gps just got for Christmas say right on, as close as I can see on the speedo of the jeep but the gps is on to the first decimal. The beauty of this is that I just started this problem this winter when I put the tires on, coincidently the same time I negotiated a raise at work for them to pay all my gas! But I'm still interested and curious,
 
tkjeeper said:
Yes I should have mentioned, stock gearing, stock speedo gear. 30% seems quite high, no leaks of any kind by the way.
If you changed tire size and changed nothing else, your speedo was off before you changed the tires. It's impossible for the speedo to be correct before and after the swap. The net effect is still the same, you were never getting the mileage you thought you were before. Maybe someone before you was running bigger tires and swapped speedo gears?
 
motorcityxj said:
its mathematically impossible.
No it's not considering most speedos aren't correct from the get go.
My speedo is a tenth of a mile off running 70mph with 31 BFG KO's (all terrains) 3.55 gearing Aw-4. My ex's 5.2ltr, 44re, 255/75's ZJ reads 4mph faster from mine at the same speed (and XJ's are notorious for reading faster too). I seem to average 13-16mpg on long trips with a 3' lift, 31/10.50's, a roof rack with spare tire, Bushwacker fenderflairs, and no lower air valiance on the front. I've also got the weight of rock sliders, T/C skid, hitch, and a 30lb sub box.
 
tkjeeper said:
The beauty of this is that I just started this problem this winter when I put the tires on...

And this, I believe, is more likely the cause of most of your problem. I have lost ~100 miles per tank since cold weather came. With extended warm-ups (love that remote starter!) in the morning, "winter" gas and running in 4WD I have dropped down to about 12 mpg (uncorrected for tire size, but compared to my 15 city averate in summer, also uncorrected).
 
Just an update, went out geocaching today and just got to my location literally 15 minutes away with the family of course, and got out and smoke was coming out pretty good from the right wheel house, looked under and a small fire in the caliper! Good thing I had an extinguisher! I knew there was a brake problem but I assumed it was a warped rotor because it came and went. So I believe a caliper seized up and maybe that was coming and going thus causing the poor mileage, maybe but maybe not, I'll keep track.
 
Dragging a brake will definitely cause a mileage drop.
 
Yeah, your mileage sounds right I'm getting between 375 -440 km per tankful,never known anything but, Bought my xj without engine and put engine,lift and 31's on right away. I have 3:55's I'm hoping to re-gear this spring,I'm told It will make a big difference.
 
So I just filled up tonight, I guess I will go a week with trying no OD, what is the opinion on this idea everyone, I'll try it for a week then back to OD for a week. By the way the gas gauge doesnt work so I see how many litres it takes to fill and reset the odometer every fill. Last week 72L took me 450 kms. I'll keep you posted!
 
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