37" Tires w/ 4.56 gears & mileage question...

[V_P_O]

Ok, so i put 37/13/22 Toyo M/Ts on my 07 6.0 truck along w/ 4.56 gears. I corrected my speedo via a Superlift Tru-Speed. I have AFE stage 2, MBRP ... , SCT w/ Looney, Royal purple (motor, trans. case, F&R diffs w/ Mag-Hytec covers. I also have the accu-fab power elbow, w/ a Retrax bedcover.

At 70 mph I hit about 2150 rpms. I drove over 700 miles this weekend & barely got 12.8 mpg. I am kinda upset at that. I also use 1oz. of TCW-3 oil per gallon, w/ RBP Additive. I also put 3oz of Acetone per every 10 gallons. of diesel.

Do i need to drop down to 4.30 *or* 4.10 gears ?? Everywhere i read it was always 37s w/ 4.56s w/ 6.0 diesel. Give me your input guys please....

PS i think my EGR might be leaking as when i romp on it for a while, it doesnt take long to get hot, and spews coolant like no other. Will this affect mileage, if the EGR is bad. I havent seen any white smoke, *or* coolant in the diesel, so i think my headgaskets are bad yet...

[PGreenSVT]

That's a little more rpm than I would want to run at that speed.4.30's would probably drop it to about 2K. Depending on the use of your truck and whether you tow and what you tow lower gears might be better for you. Personally I like to be around 2k at 80mph.

[ralphinnj]

For starters, those gears will not run 2150 RPMs at 70mph. There is a calculator on the other site (that has the word Garage in it). You can see for 4.56's and 37" tall, 2500RPM is about 60MPH and 3000RPM is about 72MPH, so you are probably turning 2800RPM give or take. Might not have the speedo corrections in there properly. Something is off.

I assume you do a lot of highway driving. If so, and if you don't care that much about take off acceleration, then a lower gear (like a 4.10) would help fuel economy.

Also, are you using the odometer for calculation purposes? Not the overhead, we all know that is crap. The odometer. If your re-calibrate is wrong, it will all be off. I would look at a map and check the true distance you travelled (or use Mapquest and run the route). You know for sure how many gallons you used (make sure you fill to the top every time) and you can check on the distance and then calculate MPG. 12.8 on 37's with 4.56's isn't all that bad if you ask me. But something is wrong f you see 2150RPM at 70.

Ralph

[V_P_O]

Originally Posted by ralphinnj

For starters, those gears will not run 2150 RPMs at 70mph. There is a calculator on the other site (that has the word Garage in it). You can see for 4.56's and 37" tall, 2500RPM is about 60MPH and 3000RPM is about 72MPH, so you are probably turning 2800RPM give or take. Might not have the speedo corrections in there properly. Something is off.

I assume you do a lot of highway driving. If so, and if you don't care that much about take off acceleration, then a lower gear (like a 4.10) would help fuel economy.

Also, are you using the odometer for calculation purposes? Not the overhead, we all know that is crap. The odometer. If your re-calibrate is wrong, it will all be off. I would look at a map and check the true distance you travelled (or use Mapquest and run the route). You know for sure how many gallons you used (make sure you fill to the top every time) and you can check on the distance and then calculate MPG. 12.8 on 37's with 4.56's isn't all that bad if you ask me. But something is wrong f you see 2150RPM at 70.

Ralph

Ralph,

I suppose my new Kenwood Navigation is off as well as my portable Garmin ?? My speedo is PERFECT. I bet that calculator does not take into effects of OVERDRIVE. W/ 3.73s i was hitting about 1700 on the hwy. I know what its hitting & i know what kind of mileage I am getting because i hand-calculated it. Now back to my orig. question.

[ralphinnj]

You're right about the overdrive. With that correction aded, 68.997MPH=2000RPM, so 2150 for 70MPH is close (but it should read exactly 2029RPM or so, so if you really are seeing 2150, something is off; sometimes 37" tires aren't exactly that size....so I would say you are in the right ball park in OD). 12.8 with big tires isn't that bad. Also, the rule on gears is that it is almost never worth changing one size. If you go down, go to 4.10's.

As far as your prom queen puking goes, that won't help fuel economy.....you may be losing compression because you have a moderately blown head gasket. I say gasket and not a hot spot on your EGR cooler because EGR cooler puking is usually lower volumes. You makie it sound like it really heaves, which is a blown head gasket. I should know, I had one. Also, if your EGR valve is sooted up, that means it won't close all the way, which means you aren't getting pure air (it always recycles, which is not what it is designed to do). It is designed to recycle at low RPM to keep NOx emissions in check. At highway speeds and higher RPM's, it is supposed to be closed (no recycle) and therefore pure air going in.

Whether to change gears depends on what your normal driving habits are. If you do a lot of highway and fuel economy is important, then go down to at least 4.10's, maybe even 3.73's. If you drive a mix, it is a tough call. If you drive more short haul, lot's of stop and go, and/or you tow a lot, then stay with the 4.56's. Going to a lower gear will hurt fuel economy in stop and go situations (because you have no mechanical advantage, so you work hard to get yourself moving).

I am building a setup (see other post near the bottom the the 1st page "I'm at it again") with 37's and I am running 4.88's. All that matters to me is performance.

Ralph

[ralphinnj]

I ran another calculation. To truly see 2150RPM at 70MPH with a 4.56 rear in OD (on a 5R110; the OD ratio is 0.7, so you have an effective straight through gear of 3.192), you need about a 35" tire. Are you positive on the actual tire height being 37"?

If you give some other RPM vs. ground speed, I can tell you for sure. One point usually has some error in it; be good to have 50, 60, 70 and 80MPH engine speeds.

Ralph

[FastMatt]

I have the same Tires 37x12.50R22 Toyo MT's on 22x11" wheels, and 4.56 gears on my 2004 Harly F350. I get about 14mph at 85mph not towing, and 9.5 MPH towing ~8000lb (enclosed car tralier), and 8 MPH towing 16,000lb (5th wheel camper, and the camper towing a jetski and tralier) I tow at 80mph.

This is with the Banks "big Hoss" bundle and AFE stage 2 intake. I drive and Tow with the 6 gun on setting #3.

These are true MPG #'s not the ones from the overhead lie-o-meter....

I puke coolent when towing the 16,000lb op a mountain at WOT. The truck has done this from the time I baught it new and it was stock. Now nearly 5 yrs later it still does it, does not seem to have gotten any worse it has just always done it.

[stevedunham]

I've got about the same set up as you and see 1950 RPM at 65MPH... People forget that the RPM calculators don't take into account for tranny slippage, wind and tire resistance, etc... My calculated RPM's are about 150 - 200 below my actual RPM's at highway speeds... I get about 15.5 MPG's at 75 MPH for long steady highway crusing... Much over that, and the MPG's drop off alot... I average 14.6 in city driving... I also tow a 14' Offroad Utility Trailer and towing it doesn't effect my MPG's that I notice... It's only about 1000 lbs. unloaded though... See my pics to see what it looks like... Your numbers seem low to me as if there is something going south with the engine effecting the MPG's... I can tell you of all the mods I've done and tried, the most significant improvement I got to raise MPG's came from airing up the 37's to 60 PSI front and 48 rear... And it was free...

[ralphinnj]

Steve, thanks for the input. I didn't realize about slippage (although I would think if it was locked, it should not slip).

Ralph

[leros]

I have an 04 Excursion with 37's and 4:30 gears, I am running an XCal 2 and primarily run the Xtreme Street unless I'm towing then I run a Tow aggr tune. I am also running a 4" Banks exhaust, a Turbonetics intercooler, and a Fluidampr. Have very little puking a while back (Dec 06) and had studs put in with new gaskets here> Head Gaskets and Studs Photo Gallery by Steve Orel at pbase.com

Depending on how heavy my foot is, I get 10.5 to 13 in town, 15.5 to 16.7 on the highway doing 75mph, and 8.5 to 12 towing at 60 to 70mph pulling about 10K. My Ex is also alot heavier than your truck.

My RPM's are:

1850 @ 65mph
2000 @ 70mph
2200 @ 75mph
2300 @ 80mph

[rstaichi]

My truck in sig has never gotten over 14mpg and I towed my 3000 pound trailer over the pass and got 11.

I have always read about people getting good mileage but mine nave has and probably never will.

But at least its almost paid off

[nitrogen]

he might have "37" inch tires but that doesn't take into account the difference in diameter caused by weight. if we are going to be picky, you need to precisely measure the loaded tire radius from the ground to center of wheel. that kind of goes with what Steve said about fully inflating.ummmm tranny slippage, yes if you don't have a lockup torque converter. with a lockup slippage would be like slipping the clutch on a manual and we all know how good that works

[TWINDIESELS]

I have a 05 with a six speed on 37 with 8 inch lift s with 3.73 gears and have seen 18-19 on the free way, It crusies at 1850 rpm at 70 I like it but going to 35s . I towed 10k from the bay area to lake tahoe and average 14 not bad. I can run third gear up to 70 mph no issues at 1190 egts

MOds edge evo power elbow 5 inch exhaust intake 6 speed etc

[late_chief]

i'm running 37's and i know if my speedo says I'm driving 70 i'm actually going 79, there for not only are you going faster than what you think your mileage is better since you are covering more ground even though your speedo doesn't say so...

[JW350]

Originally Posted by V_P_O

...I also use 1oz. of TCW-3 oil per gallon, w/ RBP Additive. I also put 3oz of Acetone per every 10 gallons. of diesel...

That's an interesting chemistry experiment.