ok....my OME lift is on, does this mean my MPH reading is off now?

[JonB]

BobKid previously said:

My FJ is actually closer to actual after upgrading to 33" tires than it was beforehand. I'm reading about 1.5 MPH lower than actual speed now, versus ~3 MPH high with 31" tires.

Most all cars that I've checked come from the factory with the speedometer reading 2 or 3% high. I know Honda got in big trouble for this about 5 years ago. It makes your vechicle get out of warranty faster. On the FJ we've found that 285 or 33" tires puts the speedo spot on. My Chevy 2500HD is the exact same way.

JonB

[FJ JAY]

I am still struggling with how a 2" lift drops mpg. The FJ is already a brick as far as drag coeficient goes. Will 2" really make that big a diff? Now, taller, wider tires are a differant story. That was when my mileage dropped.

[VooDude]

FJ JAY previously said:

I am still struggling with how a 2" lift drops mpg. The FJ is already a brick as far as drag coeficient goes. Will 2" really make that big a diff? Now, taller, wider tires are a differant story. That was when my mileage dropped.

I'm in no regards an expert, but am in grad school for mechanical engineering right now, and had a Fluid Mechanics class about 6 months ago...

The lift changes the flow around the vehicle. It's the same concept that indy cars stay low to the ground, and people put "spoilers" (might not be the right word) similar to a ground kit on the front end. It keeps the flow out from underneath the vehicle, where it causes drag on the whole bottom of the vehicle. So, keeping a vehicle lower to the ground in the front has flow effects as well as stability (center of mass or center of gravity).

Also, most people with lifts are "leveling" the FJ's bunny rabbit stance, and thus changing the flow again. It seems minimal, but it might change the flow a little bit. Add into that equation that the stance and the flow underneath the vehicle are now changed and you now have 2 effects that change the drag. Increased drag equals decreased fuel efficiency.

Hopefully it helps, and is nearly correct. Obviously, one would have to do some experiments or modeling to figure it all out, but it comes down to the change in drag based on new stance and flow.

[BobKid]

VooDude previously said:

I'm in no regards an expert, but am in grad school for mechanical engineering right now, and had a Fluid Mechanics class about 6 months ago...

The lift changes the flow around the vehicle. It's the same concept that indy cars stay low to the ground, and people put "spoilers" (might not be the right word) similar to a ground kit on the front end. It keeps the flow out from underneath the vehicle, where it causes drag on the whole bottom of the vehicle. So, keeping a vehicle lower to the ground in the front has flow effects as well as stability (center of mass or center of gravity).

Also, most people with lifts are "leveling" the FJ's bunny rabbit stance, and thus changing the flow again. It seems minimal, but it might change the flow a little bit. Add into that equation that the stance and the flow underneath the vehicle are now changed and you now have 2 effects that change the drag. Increased drag equals decreased fuel efficiency.

Hopefully it helps, and is nearly correct. Obviously, one would have to do some experiments or modeling to figure it all out, but it comes down to the change in drag based on new stance and flow.

and if all else fails, lift springs and shocks are usually heavier than the OEM components; and more pounds always = less MPG.

[RomaBoy]

Very well said VooDude. BobKid, I never have checked the weight difference, but how much heavier do you think aftermakert springs and shocks are? I bet not that much.

[JRock]

RomaBoy previously said:

Very well said VooDude. BobKid, I never have checked the weight difference, but how much heavier do you think aftermakert springs and shocks are? I bet not that much.

An OME HD coil is nearly twice the weight of a stock coil. Hence the increased boing factor.

[VooDude]

RomaBoy previously said:

Very well said VooDude. BobKid, I never have checked the weight difference, but how much heavier do you think aftermakert springs and shocks are? I bet not that much.

Thanks. Add the weight to the equation too! Everyone knows the tires make a difference, but even the tread pattern and tire "cross section" can all play a factor... for example, with my all pro setup, from the front, you can see the entire front tire... under a stock bumper, that's something you wouldn't get, maybe only see 1/2...

[BuddyG]

BobKid previously said:

My FJ is actually closer to actual after upgrading to 33" tires than it was beforehand. I'm reading about 1.5 MPH lower than actual speed now, versus ~3 MPH high with 31" tires.

I just finished my third long trip and have been wondering about the discrepancy between FJ speedometer and GPS reported speeds. I still have the stock tires on mine and the GPS was showing somewhere between 3-5 MPH less than the speedometer. The odometer was spot on according to the GPS and the mile-markers used for aerial speed enforcement (big white lines painted across the highway). The only thing I could figure was that Toyota, knowing I might speed a little above the limit, intentionally designed the speedometer so it would be off. So I set the cruise for 73 (according to the GPS) in a 70 MPH zone and waited for the highway patrol. I found one and noted that the Toyota was registering damn-near 80 MPH and he didn't come after me. I have to consider a few possibilities:
1) GPS is wrong, FJ is correct, and highway patrolman owns a FJ Cruiser
2) GPS is wrong, FJ is wrong, and true speed is somewhere in between
3) GPS is correct, FJ adds 3-4 MPH to my actual speed so I wont get a ticket, and my 3 MPH over the limit wasn't worth the patrolman's time

I even went so far as to attempt some math. (1/60)*75 = 1.25 miles in one minute. So I set the cruise at 75 on the GPS and started the clock. Since the FJ odometer only does tenths of a mile, I had to guess where the half way point was between 1.2 and 1.3 miles and it was pretty close if not exactly 1.25 miles. If I was really traveling 78 MPH or more (as the FJ reports), then It would have exceeded 1.3 miles; (1/60)*78 = 1.3 and (1/60)*79 = 1.3166. Next time, I'm going to try it over 10 minutes to see if I go 12.5 miles, 13.1 miles or whatever. I just need 10 minutes without interference from the other drivers, hills, or anything else that will affect speed.

Has anybody else had a chance to experience this anomaly?

BTW, the GPS also shows my Jeep as being off, but it has bigger tires and no mods to the odometer to compensate. Mileage and speed are both consistently 10% off, but I still love her cuz she likes to do it topless.

[DominicG]

Compare stock to your new sizes and see effects on your speed

Tire Size Calculator - tire & wheel plus sizing

[homedad]

The GPS is correct......every time!!! Regardless of anything....