Finding MPG w/ new gears and tires?

Mods, feel free to move this if it's in the wrong forum.

I started out with 3.27 gears and 235/75-15 tires (28.9). Now I have 4.88 gears and 37x12.50-16 tires. I've seen all the conversion calculators to find out new speed, gear ratio, etc, but nothing for having bigger tires AND new gears.

I filled up today and the trip meter said I drove 268.0 miles. It took 15.873 to fill it back up. Obviously, it would normally mean I got 16.88 mpg. But I don't know what ratio I need to multiply that by to find out my REAL mpg w/ the 37s and 4.88s. It still has the stock speedo gear also. Any help?

 

[ExplorerDMB]

It's kind of hard to really find a MPG calc that is accruate for such changes. Cause so many things play a role. Gear lube type (syn or regular), transfercase gearing, engine mods, exhaust, etc - which all contribute to fuel economy in one way or another.

MOST off-roaders don't care about fuel economy... I don't, but I still like to know. That is one of the reasons why I reprogrammed my 4WABS Module/GEM Module.

-Drew

 

[FROADER]

Dead Link Removed

You're pretty much as close to stock as you can get, but then you have to figure in the weight of the wheels and tires and all that crap.

 

[IZwack]

After doing an SAS, does MPG still matter?

But doing some quick calculations purely on just ratios based on your stock configurations (3.27 ratio running a 28.9" tire), you're gear ratio is actually on the high side (meaning 4.88 is a little too large).

From a quick set of calculations, you should see 14.2 % more torque. However, of course this is an over estimate because it doesnt take into consideration the fact that now theres more mass that must be moved/rotated by the engine due to the heavier tires/wheels and what not.

What's interesting is if you started out with the 3.73 rear end, you'ure current 4.88 setup with 37" tire would put you right back into stock ratio. So I guess you could look at it as if youre Explorer started with 3.73's But overall, I think the 4.88 puts you back close enough to stock config due to the added weight and rotational inertia.

As for how far off your speedometer is, this depends on where the speedometer reading is being taken. But I assume its being taken before the ring gear (so its being taken at the transfer case or at the tranmission), then your speedometer would be off by 14.2%.

 

[bmxking5]

Drew - I can't do that. I have a speedo gear so it's not elec. controlled like yours.

So there is no way to tell the adjustment from stock to find out what I'm actually getting for mpg? I know that when I added 31s, I just had to plug in some numbers into a formula and I found out what my actual mpg was since the speedo was off a little bit. I just wondered if I could find out what my mpg is now instead of thinking it was just "16.88" since I know it's not really that much (the speedo reads a little fast).

If I use the same formula as before (IIRC it was just new tire size/old tire size and that number was multplied by what the mpg reading was with the incorrect speedo. If I do that now though, it says I have 21.__ mpg because it hasn't factored in that I have new gears now (which causes the speedo to read fast instead of slow).

 

[ExplorerDMB]

I would talk to a local 4wheel shop. See if they can help you out on determining the MPG.

 

[Byrd91]

Here goes:

Say you first switched to 37" tires from the 29s and didn't change gears. The speedo reads of the driveshaft (in the tranfer case) so you are now going:

37/29=1.276= 127.6% of the original speed or 27.6% faster than the speedo reads. Speedo reading 100, you're going 127.6. Note that the pi can be dropped out because its on both the top and bottom.

Next, you change gears. So now.

3.27/4.88=.670 or 67%

Next, you combine to two. 127.6% x 67% = 1.276 x .67 = .855 or 85.5%. Your speed will be 85.5% of the speedo reading... when speed reads 100, you're going 85.5.

Now, the straight face test (double check or math). Should the speedo be reading high? Well you bigger tires are increasing your speed by 27% (logic tells us that at the same RPM we're going faster with bigger tires) and your gears are slowing (logic again) you down by...(100%-67%=) 33%, which is more than 27%. So yes, the overall effect should be now going slower than the speedo.

Oh, forgot the original question. Though you can't predict your mileage, you definitely can still check it. Mileage is (as you know):

miles / gallon = total number of miles / total number of gallons....with your new setup, your odometer is off my the same as your speedo. 100 miles on the odometer is really only 85.5. So you mileage will be .855 x (odometer reading) / number of gallons.

 

[bmxking5]

Thanks Byrd. I knew there was a more simple way to figure it out (not weighing tires, etc) So my milage was 14.436mpg this past tank. Not too bad for a little bit of wheelin' and a little bit of highway (4.88s cause high RPMs when you're driving 75 down the highway). I'm pretty happy with that. Thanks again.