Inv4drZm
Senior Member
What about 265/75 or 265/80 ? 285s are small on my truck, but I have a 1.5" BL as well.
lift + tires + wheels = terrible mpgs
- Thread starter iin10ded
- Start date
What about 265/75 or 265/80 ? 285s are small on my truck, but I have a 1.5" BL as well.
kcdehart
New member
This doesn't make sense...compared to the rest of the exhaust system (mainly the cats), mufflers aren't restricting at all. I'm 90% sure the mileage decrease people see is because they put their foot in it more to hear their exhaust. If anything, the 'power increase' that you get (which is negligible from a muffler) is going to make the vehicle more efficient at what it does because there's less restriction (thus better gas mileage). Though the powerband may be shifted up in the RPM range just a tad which will affect mileage at different speeds.
Inv4drZm
Senior Member
Are you getting your measurements from the mpg display or are you calculating it?
Civiceg94
New member
Same here. I put on a gibson exhaust and lost about 1 mpg hand calculated. And its not because i want to hear the exhaust. Because this damn exhaust drones alot. From 15 went down to 13 to 13.5 sometimes mpg.
Lot of comments on here about "unsprung weight", which are really referring to the rotational weight of the wheels and tires. There are two aspects here.
- Unsprung weight refers to any weight that is not riding on the suspension. Wheels, tires, brake calipers, rear axle and diff, part of the front suspension. All of this primarily affects ride, not mpg.
- Rotational weight (wheels, tires, brake discs) has a multiplier effect on acceleration and mileage because you not only have to get the weight moving down the road, but you have to spin it up rotationally. It turns out 1 lb of wheel/tire weight has the same effect as about 1.8 lbs of dead weight in your passenger/cargo area. So, if you add 20 lbs/wheel to your tires, that's equivalent to 4x20x1.8 = 144 lbs of cargo weight, or roughly equivalent to another passenger riding with you. I doubt seriously that will cause mpg to drop dramatically.
In my view, the major causes of mpg losses associated with lifts/tires/armor etc. are:
- Poor metrics - failure to account for odometer errors due to larger tires.
- Underinflated E rated tires. Sidewall losses are much higher for E rated tires at low pressures than for p-rated tires. You need at least 45psi in an E rated tire to keep losses equivalent to 32psi in a p-rated tire.
- Greater vehicle height and consequent greater air resistance at highway speeds.
- less efficient gearing because of larger tires (unless you regear to match)
- Extra weight of wheels/tires. bumpers, winches, armor, etc.
- Greater windage loss of aggressive tire tread when running AT or MT tires.
- Unsprung weight refers to any weight that is not riding on the suspension. Wheels, tires, brake calipers, rear axle and diff, part of the front suspension. All of this primarily affects ride, not mpg.
- Rotational weight (wheels, tires, brake discs) has a multiplier effect on acceleration and mileage because you not only have to get the weight moving down the road, but you have to spin it up rotationally. It turns out 1 lb of wheel/tire weight has the same effect as about 1.8 lbs of dead weight in your passenger/cargo area. So, if you add 20 lbs/wheel to your tires, that's equivalent to 4x20x1.8 = 144 lbs of cargo weight, or roughly equivalent to another passenger riding with you. I doubt seriously that will cause mpg to drop dramatically.
In my view, the major causes of mpg losses associated with lifts/tires/armor etc. are:
- Poor metrics - failure to account for odometer errors due to larger tires.
- Underinflated E rated tires. Sidewall losses are much higher for E rated tires at low pressures than for p-rated tires. You need at least 45psi in an E rated tire to keep losses equivalent to 32psi in a p-rated tire.
- Greater vehicle height and consequent greater air resistance at highway speeds.
- less efficient gearing because of larger tires (unless you regear to match)
- Extra weight of wheels/tires. bumpers, winches, armor, etc.
- Greater windage loss of aggressive tire tread when running AT or MT tires.
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@ ronrj - good summary, 100% agree. my g2's are 1lb heavier than the oe tire per the weights listed here - surprisingly similar. the wheels are 33lbs ea - a lot heavier (need to weight the stockers but guessing 25).
i think i'm going to put back on some oe size tires on the stock wheels, and go back to oe springs, lift the front via the 5100's perch settings a little and leave the rear stock. the fj spring is really bouncy / busy imo, i suspect that the firmer 5100 with the stock spring will be a better compromise. feels a little underdamped as it is. plus the benefit of dropping the nose a bit will improve aero. i suspect the 4r's oe ride height is really low in the front intentionally for this reason, as well as handling balance.
re exhaust and lower mpg - not surprised. the engine is tuned to a specific backpressure and this impacts exhaust temp which impacts velocity which impacts scavenging, etc. lowering it arbitrarily will have unknown consequences. my (completely uneducated, nonscientific) guess is you shifted power up a little, likely hurting torque which means more pedal to get the job done. these tranny's never let the motor rev up, you're always way low in rpm's so you never get hp just torque. really, you'd need to retune the ecu to account / optimize for the changes. it's just not as simple as 'freer flowing is better'. it flows what it is suposed to flow. plus the ecu's typically have set values for the hp they call for based on load, so the ecu isn't ever going to call for more power unless it's recalibrated (with minor vairance via short and long term trim / correction tables). at least that was the gist when i was working for an Audi tuner 10 years ago. these ecu's may be simpler but i'd wager the operating theory is simliar.
i think i'm going to put back on some oe size tires on the stock wheels, and go back to oe springs, lift the front via the 5100's perch settings a little and leave the rear stock. the fj spring is really bouncy / busy imo, i suspect that the firmer 5100 with the stock spring will be a better compromise. feels a little underdamped as it is. plus the benefit of dropping the nose a bit will improve aero. i suspect the 4r's oe ride height is really low in the front intentionally for this reason, as well as handling balance.
re exhaust and lower mpg - not surprised. the engine is tuned to a specific backpressure and this impacts exhaust temp which impacts velocity which impacts scavenging, etc. lowering it arbitrarily will have unknown consequences. my (completely uneducated, nonscientific) guess is you shifted power up a little, likely hurting torque which means more pedal to get the job done. these tranny's never let the motor rev up, you're always way low in rpm's so you never get hp just torque. really, you'd need to retune the ecu to account / optimize for the changes. it's just not as simple as 'freer flowing is better'. it flows what it is suposed to flow. plus the ecu's typically have set values for the hp they call for based on load, so the ecu isn't ever going to call for more power unless it's recalibrated (with minor vairance via short and long term trim / correction tables). at least that was the gist when i was working for an Audi tuner 10 years ago. these ecu's may be simpler but i'd wager the operating theory is simliar.
Rishi3
New member
That's what I thought too, but the day after the install I was getting 12 mpg, eventually getting it up to 13.5.
Rishi3
New member
Both, stock tires so no discrepancies
TheThunderRunner
New member
Yeah I wasn't expecting an mpg drop like that. It's because of the lack of back pressure causing me to lose low end power. So on the highway I usually get right around 20 mpg but around town it kills me. Probably should have done more research before changing. But at least it sounds great
king.cong.1119
New member
Also, not sure if this is covered, with larger tires, the actual traveled miles vs. the odometer will be different, therefore it will definitely drop your MPG just by the number but I believe around 5-10% of that is just the numbers being off from actual miles. But it still will drop about 2mpg-ish depend on tires. Mine dropped from 17.5 to 15mpg, I am not surprised.
yep going from 265/65 to 265/70 is a 3% difference. Calcuated mpg = 14.1, corrected about 14.5.
BlackWorksInc
New member
I would also mention the difference in rolling resistance, diameter, and gearing. You'll see bigger drops in the V6 than the V8 because of the difference in power band and gearing. Also you'll notice that they both mostly stabilize around the 12~14mpg mark on average for most people who are running a lot of armor, big tires, ect.
i used the math here: Bigger Tires and Gas Mileage: How to determine actual gas mileage?
using https://tiresize.com/calculator/ - shows 265/65 as 30.6" diam, and 275/70 as 32.2", so 4.9% difference.
