I am getting low mpg wondering if tires are cause???

[XLT]

Well for starters i have a 1998 explorer with the SOHC. Its got about 115,000 miles on and still seems to be running strong. I have replaced fuel filter, air filter, new motorcraft platinum spark plugs, pvc, and new motorcraft spark plug wires. So i usually get about 14.5 up to my best is 17.5mpg but usually average about 15.5mpg. I drive about 75 to 80% city. Heres where i think my problem is. The last owner must have been feeling cheap and didnt put the stock size tires on the vehicle. I have 225/70r16 while stock is 255/70r16. Plus i have the 4.10 gears (stock) on my sport but i will say it does get up and move if i need it to. So on the highway doing about 70 my rpms read around 2750rpm. So do you think my gas milage would improve if i jumped up to a 255/70r16 ore even 265/70r16? Thanks.

 

[corkey]

yes it would benefit from the bigger tires , as the tires will make less revolutions per mile, and the rpm's will drop,, thus the engine turns less , and uses less fuel,

 

[Cobraguy]

If you have smaller tires than came with the car, then your speedo is reading fast unless it was reprogrammed too. That means it will show you went further than you actually traveled...which means your mpg will be artificially high if you calculate off the odometer. So actually, with no speedo calibration, if you calculate off the odometer, there should be no change at all in your mileage between the two size tires. Of course, there probably is, but that can only be calculated off ACTUAL miles traveled and not off your odometer. Make sense?

 

[XLT]

The thing is the last owner was a female and i doubt she knew about recalibrating it (not becuase she was female, but she and her husband didnt strike me as the type of people who would know about that. Becuase i noticed that when i am doing 60 and drive by one of the radar screens on the highway it says i am doing a few mph slower.

 

[corkey]

If you have smaller tires than came with the car, then your speedo is reading fast unless it was reprogrammed too. That means it will show you went further than you actually traveled...which means your mpg will be artificially high if you calculate off the odometer. So actually, with no speedo calibration, if you calculate off the odometer, there should be no change at all in your mileage between the two size tires. Of course, there probably is, but that can only be calculated off ACTUAL miles traveled and not off your odometer. Make sense?

the tire is not as big around , so there are more revolutions per mile, so the engine turns more to get the tires to go the same distance ,but you actually get there slower, therefore, more gas, , sure it will show you went further , but if you watch the road signs or mile markers, it is not as far as the odometer says,

 

[X~FACTOR]

14 to 17 mpg is about right.

 

[gijoecam]

So i usually get about 14.5 up to my best is 17.5mpg but usually average about 15.5mpg. I drive about 75 to 80% city.

14 to 17 mpg is about right.

Ditto... Sounds pretty normal to me regardless of tire size or gearing. That's what an Explorer gets in the city.

edit: And before Aldive chimes in here stating that he gets 25mpg in the city, I should qualify my statement: That's what any normal un-modified Explorer should expect to get in the city.

 

[toypaseo]

:scratch: Nobody mentioned yet that a larger tire is more rotational mass, so it takes more power to turn it...

 

[gijoecam]

:scratch: Nobody mentioned yet that a larger tire is more rotational mass, so it takes more power to turn it...

Because he said he's running a smaller tire than original...

last owner must have been feeling cheap and didnt put the stock size tires on the vehicle. I have 225/70r16 while stock is 255/70r16.

 

[Lazzman]

I have pretty much the same set up, 98' sport, 5 speed auto, 4.10, 4wd. I have the original size tires the P255/70R16.

I turn a hair below 2500rpm on the highway at 70mph.

Check your EGR system- I was getting around 20mpg all around average driving it easy, then the milage went down to about 14.5mpg. At the same time I was getting a faint surging below 2000rpm in overdrive.

I thought it was the plugs because there was no CEL, while replacing the plugs I bumped the EGR tube several times with my rachet when tightening the front spark plug.

That must have enhanced the problem, after I drove the truck for a mile or so the check engine light came on, turned out to be a bad DPFE sensor.

Even if you do not have a light on you should still scan for a code.

 

[Turdle]

Your torque converter is not locking up-

You should be around 2400 rpm's at 70 mph



You might try resetting the PCM by disconnecting the battery for a while. Then, take it for a drive and see if it made the transmission shift correctly. If not, I would have this checked-
Pleas, re verify the speed and engine speed, on level ground after you've reached 70 mph and are cruising steady.

From a dead stop, while accelerating smoothly to 70 mph you should feel 5 "shifts" from the transmission.

 

[gijoecam]

Your torque converter is not locking up-

You should be around 2400 rpm's at 70 mph

He didn't mention if that was an indicated 70 on the speedo or a true 70 via GPS or some other means. At a true 70, running smaller than stock tires, I would expect elevated RPMs.... If it's running those RPMs at 70 mph indicated, then I'd agree with you that the TC may not be locking.... But now that I think about it, I believe I run at around 2600@70. When the TC unlocks, it's up around 3k IIRC...

 

[XLT]

Alright thanks for the replies i will try unhooking the battery and then doing another 70mph run and get the rpm's and post back.

 

[XLT]

Alright i just took it for a drive and this is what i got.

Going 70mph according to my speedo i am at about 2600-2650 rpm
Going 40 mph according to my speedo i am at about 2000rpm

Does that sounds right? I just figured it was good since i have 4.10 gears and tires that are about an inch less in diameter then stock.

 

[XLT]

Tonight on my ride home which is about 30 miles i notice the rpms were at about 2550 to 2600 rpms while going 70mph. While at 60mph my rpms are at about 2250. So maybe i am about right.
I think i will look into the DPFE yet and see how that looks. I got about 114,000 on her and the orginal DPFE. Plus it will give me something to tinker with for a little Bit

 

[corkey]

on my gps, it says at 60 i am turning just about 1950, to 2000 rpm at 70 , about 2250rpm ,i get about 24 miles to the gallon , most of the time on the highway,.,
i have a bit bigger a tire than stock, they came with 225 74 15, i have 235 75 15, so the tire is just a bit bigger,,
that brought my rpm down and increased my fuel mileage,

 

[coreman021]

I know when i go 80 on the turnpike i am exactly at 3000 rpm!!