Not really sure where to put this. I have a 98 Dakota 4wd 5.2 and at 65 mph I run at 1800 rpm and get about 17 mpg highway. My wife has a 98 Durango 4wd 5.2 and at 65 mph she runs at 2200 rpm and gets about 14 mpg highway. Is this normal? Shouldnt the same motor run at the same rpm traveling at the same speed? I know the Durango is heavier. The only difference is I have a throttle body spacer on the Dakota.
It's possible the Durango has 3.92 gears and your Dakota has 3.55s. It's also possible the tach is inaccurate, I know mine is. Even two identical vehicles can get different mileage, that's just the way it is.
Honestly I didnt even think of the gears. How can I tell what gears are in each truck. Didnt think a TB spacer would really make that big of a difference.
Check the tags in the glovebox. If they aren't there any more, look at the rear end bolts. Usually one of them will have a tag that identifies the ratio.
there is also a difference in the amount of weight the same 318 is moving. the dakota is lighter is probably why its getting better mileage. the RPM difference is either rear end gears or possible tire size
Just looked and the Dakota is running 3.55 gears. The tag was gone from the Durangos glove box and I cant get under the truck right now. I should probably put the 3.55s in the Durango then. Thanks for the quick replys.
I'm pretty sure I have 3.92 gears in my 01 Durango 4.7
I have the factory tow package so I'm pretty sure the 3.92 are standard.
Going up to Austin, I got 18.4 mpg and on my way back, I got 16.2 mpg. I was going against the wind on the way back. Both times I had the truck locked in at 71mph cruse control.
I have oversized tires (265/75/16) and also the 02 JEEP TCM mod. Before my Durango ran at 2k RPM @ 65mph
Now it runs at 1600 RPM @ 65mph with the JEEP TCM mod.
Thats about right for that MPH with 3.55s, ~31" tall tires and the .67 OD of the 545RFEs 5th gear. I think my brothers truck pulls down 1,500 rpm @ 60 and roughly 1,900 rpm @ 80 mph. Crusing from Fort Worth to San Antonio at 75 mph with the cruise control set, the truck managed to get 24 mpg. At the time it had HO Cams, JBA headers, and Superchips 91 tuning.
Just wondering, why put the 3.55's in the Durango? Usually the 3.92's will get slightly better mileage than the 3.55's if mpg is your concern. It's not all about rpm, the 3.92's are less work for the engine even though you might run a couple hundred rpm more. Plus durango is heavier, it needs all the help the motor can get.
I guess Im just surprised that the Durango only gets 14mpg highway when I was expecting closer to 17. I didnt buy the Durango thinking I would get good mpg but I did think it would be better than this. The only reason I would think about3.55s in it is to make it more like my Dakota since it gets better mpgs. I looked into the GC TCM and I have 98 and a 5.2 not a 4.7 so I wouldnt be able to do this mod right?
If it has a fan clutch, check it to make sure it isn't seized. Mine was on my dakota and it was costing me 4-5mpg and making the trans shift high (loading engine).
You can go with a taller tire to knock down the rpms further if you have the clearance, but this will throw out the speedometer. If it has a limited slip, you can count how many times the driveshaft spins per 1 wheel revolution. I have had trouble doing it this way on open diffs though.
Dakota R/T runs a 29" diam tire, the Durango R/T's run a 30" tire. Durango SLT's can run up to a 31.6" tire allowing lower highway rpm.
For those that primarily drive highway, the 3.55 ratio will net better mileage, in primarily city driving it'll get worse mileage than the 3.92 gear set.
At 65mph my '00 R/T (all R/Ts came with 3.92 gears) turns a touch over 2000rpm. Going with smaller tires (Such as Dakota R/T tire size) will increase your RPMs. The standard R/T tire size on the Durango is 275/65R17, which is 30". However, some owners, or second or third owners will get a 265/60R17 for the lower cost, although it's not a factory size it will still fit on the R/T rim.
On SLT models, 3.92 gears were still optional with the towing package with either engine. The difference in towing between the 3.55 and 3.92 gears is roughly a ton of towing capacity.
Also, the 4.7L was essentially a four speed until 2003 (some 2002 models are 5 speed). The 45RFE could be flashed to gain the additional overdrive gear from 0.75 to 0.67 to lower highway RPM and effectively increase mileage in the process. I'm not sure what this Jeep TCM is or what it does, but I'll guess that it's a transmission flash to gain the additional overdrive gear.
The 318 and 360 V8s have an effective overdrive of 0.69, which when you take into account axle ratio and tire size all play a factor in engine rpm at any given vehicle speed. It's not fair to directly compare the 4.7L to the 5.2/5.9L as the transmissions don't use the same ratios.
Another tidbit is that the 44RE transmission (5.2L) has a 3:1 first gear where the 46RE (5.9L) has a 2.49:1 first gear ratio. The 45/545RFE use the same 3:1 first gear as they are effectively the same transmission, only the second overdrive is present on the 545RFE.
If you start swapping gears on your 4x4 remember you have to swap the front diff. gears too.
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