Question: Lifted Rig's Estimated MPG

[minelson17]

I was just wondering if someone could confirm/deny my logic in regards to estimating my Miles Per Gallon on my lifted '92 Ex. Not that good MPG is my primary focus with this rig, but it'd still be nice to know.

Stock tire size = 28"

Newish tire size = 35" (a 20% increase in size)

If my logic is correct, the odometer clicks based on a certain number of revolutions assuming the stock tire size. Since the "orbit" has increased due to the tire size, or in other words the distance traveled per revolution has increased, so than has the distance traveled per number of miles tracked on my odometer.

Therefore, if my trip-o-meter shows 230 miles, in reality I have traveled 20% further (276 miles) on let's say 18.5 gallons (I know the tank size is actually 19.3, but I never run it completely dry).

So instead of dividing the 230 miles by 18.5 gallons for 12.4 MPG, I should actually divide 276 miles by 18.5 gallons for 14.9 MPG (let's call it 15).

Is that an accurate estimate?

If so, I think I can live with a '92 sitting on 35s (7+" of lift) with a cargo rack driving mainly city miles getting 15 MPG.

 

[Maniak]

Short answer: Your logic is correct...

Long answer:
Well, the way to do it without hurting your brain is to just change out the driven gear in the transfer case to one that is 20% larger than what you have now. That will negate the 20% change in tire size.

If you re-gear later (which you really should do) you then change the driven again. With 35" tires you really should be running 4.10's at a minimum and depending on the speed you drive you may even want to go up to 4.88's or possibly higher.

Those gears are < $10 and take 5 minutes to change on a lifted vehicle.

~Mark

 

[Tbars4]

...My X gets 11 city, 15 combined, and 17-18 max highway loaded or not..

..I have the 4:56 gears (which are best for the highway w/35's and spot on speedo reading), a4ld, and 35's but I believe it's the aerodynamics of my X that is where it gets it great highway gas mileage from..

 

[Maniak]

Our X is pretty similar, mileage wise.

4.10's with 33's but we have the 700r4 so the OD is slightly different.

Highway is around 16-17 mpg when driving < 65mph. Mixed is 15mpg or less. No idea what city mileage is.. I don't do enough city to know.

We are running a Detroit locker in the rear that doesn't unlock on the left side. That, in combination with the explorer having the aerodynamics of a billboard turned sideways makes for some lower mileage numbers. If I were to pull the spare off the roof I'm sure that would help the mileage some.

~Mark

 

[minelson17]

Gears

I have 4.88 Gears. And it's a manual tranny.

I assume the <$10 gear you're talking about is the driven gear in the transfer case? Cuz my ring and pinion set up was much more expensive than $10 haha.

 

[Maniak]

I have 4.88 Gears. And it's a manual tranny.

I assume the <$10 gear you're talking about is the driven gear in the transfer case? Cuz my ring and pinion set up was much more expensive than $10 haha.

The $10 gear is the driven gear in the xfer case.

Did you change your gears (diff gears) to 4.88 AND change the tire size at the same time?

If so, your logic is off.

What you need to know is what is the total percent off your speed is now compared to where it was when it was correct. To do that without using a GPS you need to know all the changes that were done to both the final drive gear ratio (transmission ratio, transfer case and axle ratios) and the tire size change.

~Mark

 

[minelson17]

I used to have a 4" lift with 33's...that's when I put the 4.88's in. I later put my current lift on with 35's.

And yes, as far as the speed goes, I would say it's about 20% off as well when compared with my GPS.

 

[Maniak]

Gotcha. So the only change since the last time the speedo was correct was the tires.

Then your on the right track. Look at how many teeth are on the driven gear now and just get one that is 20% smaller. That will speed up your speedo/odo to match how fast/far your really going.

~Mark

 

[minelson17]

Roger that, thanks for the insight. I thought I read elsewhere that those speedo adjustment parts can be hard to come by for Explorers. Not overly concerned about it, I just go my gps...and at close to 200k miles, and the rig being paid for, I'm going to be the last owner it ever has (if I last that long haha).

Thanks again.

 

[Maniak]

Roger that, thanks for the insight. I thought I read elsewhere that those speedo adjustment parts can be hard to come by for Explorers. Not overly concerned about it, I just go my gps...and at close to 200k miles, and the rig being paid for, I'm going to be the last owner it ever has (if I last that long haha).

Thanks again.

The local dealer still has them. I also see them on mustang sites so that should make it much easier to find. You driven gear in the transfer case is the same as the one for a manual trans.

colors and factory (ford) part numbers. http://www.explorerforum.com/forums/showthread.php?t=158739

~Mark

 

[Flip4ford]

I got 18-19 with the old explorer 4:56 and 33's-- my new explorer (100 thou more miles) gets only about 15 avg - same set up. I do have a larger MAF out of a 5.0 , headers- no cats and magnaflow exhaust (2.25) running 91 octane. On the new one.