Why regear if RPM are still high @ 70? | Ford Explorer Forums - Serious Explorations

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Why regear if RPM are still high @ 70?

tweakedlogic

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November 4, 2003
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City, State
memphis, tn
Year, Model & Trim Level
'92 XLT
I have 31s with stock gears w/5 speed. I took a very long trip last weekend with no stereo (damn thieves). So I had plenty of time to contemplate all the things I have done and want to do with the X.
At 70 MPH (according to Tom Tom) I'm turning around 2500 RPM. That seems high to me. If I re-geared for the tires it would turn even faster, 2800 maybe 3000 RPM.
My Rx7 turns 2500 RPM @ 70, but it has a 9000 RPM red line and very little torque below 3000.
My wife's car pulls about 2000 @ 70 (auto Malibu).

So I ask the question, aside from torque multiplication, why re-gear if the engine has to turn faster for the same speed? Wouldn't that hurt fuel economy more than the big tires?
This weekend I was playing in the Arkansas mountains with a 17' canoe strapped to the top. And lots of gear and coolers in the back. I was able to drive up every dirt and gravel roads with steep grades in second gear @ 20-25 MPH with no issues... except for the broken exhaust mount, but that doesn't count. ;)
I know I only have 31s, but I have 33s waiting for their wheels. 33s may make more a difference I'm sure. Perhaps bogging may be more of an issue.
 



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So I ask the question, aside from torque multiplication, why re-gear if the engine has to turn faster for the same speed? Wouldn't that hurt fuel economy more than the big tires?
Yes if you live in a flat area and you accelerate very slowly. But for general driving, regear is needed because the vehicle is outside of its powerband with oversize tires. This is especially true for undersized engines - which is true for the 4.0L v6 engine applied to a 4dr Explorer. Every time the transmission has to drop out of overdrive because its not producing enough torque (such as on a highway on ramp for example where you have to accelerate to highway speed), the vehicle is probably operating at sub-optimal conditions as far as MPGs is concerned - which sometimes is due to the change in air-fuel ratio and most likely, the injectors are actually dumping more fuel than necessary. If you've hypermiled with someone, you'll notice that they usually take on ramps faster than most people. This is to conserve momentum so that the vehicle has to accelerate less in order to obtain highway speed (at the consequence of tire wear and maybe some chassis components like ball joints, bearings, etc..). So the goal for high MPG is to stay in overdrive but still have enough torque to counter opposing forces such as hills, wind drag, and so on. But when you put larger tires on the vehicle, you decrease the torque which means the vehicle is more likely to drop out of OD and therefore operate at sub-optimum air-fuel-ratio conditions. And for the record, for a stock configured vehicle, the optimum MPG is usually attained a few MPH after the vehicle acquires overdrive. So if the vehicle shifts into overdrive at 50 mph, then the optimum MPG will probably be around 53-55 mph. Others will argue that going 70 mph gets the best MPG - but they are probably taking advantage of momentum to get them up hills and such. If they maintain about 55 mph and drop a few MPH before apexing the hill w/o dropping out of OD (without bogging the engine), then they will probably get better MPG.
 






Regearing with an increase in tire diameter will put the engine back in its powerband.

Look at it like this. Lets say your truck came with 3:73's and 31" tires stock. 4:11's and 33's, 4:56's and 35's, 4:88's and 37's etc would have it spinning the same rpm at the same highway speed as the factory 3:73/31".

Of course, none of this takes into account the added unsprung weight of a larger wheel/tire package- this means you may want to go lower. Whats going to get better fuel mileage? Foot almost on the floor at 2500rpms or foot barely touching the pedal at 3000? Foot barely touching the pedal usually wins... forget about how high the engine is spinning.
 






Sorry guys, but i disagree with the general consensus that he is geared improperly.

it sounds to me like his gearing is perfect. remember 4.0 OHV's are a bit of a torque engine. at 70mph his engine is right at its torque band, 2500rpm. as for accelerating, the only difference lower gear ratios would make, would be a slight gain at sub-3,000RPM speeds in first gear. after that, all it will do is change the shift points (to slightly higher speeds), the engine will still be in it's power band at all times, as the ratios between transmission gear's are the same. I dont suspect his foot is nearly on the floor at 2500rpm cruising 70mph. (and up a hill he should of course be in a lower gear) my '93 cruises nicely at any speed over 55mph with stock (3.73 i think) gearing and 31's.

his 31's aren't that much taller than stock tires, i bet a 2wd explorer (taller gears) would run about the same RPM at any given speed as he is running with his 31's.

i'm also going to have to go against the grain and say that i don't think a 4.0 is underpowered for an explorer either if the smaller engine can do the same job it'll usually use less fuel than a larger one. besides, i've had mine over 100mph, so there's plenty of cruise-torque there for 70mph cruising.
 






From the rpm's the op said he was turning on the highway and only running 31's I wouldn't change the gear ratio. I don't think it would be cost effective. If you jump up to 33's you'll notice a big difference, My overdrive was useless unless I was going 70 mph with 33's and stock gearing, but the slightest upgrade it would end up kicking out of od. I regeared mine to 4.56 and loved it!

If you are planning on going with 33" tires, you will probably end up with 35" tires, then dana 44 narrow solid axle swap, then dana 44 full width solid axle swap, then dana 60 swap, so on and so on. I am in the in the dana 44 full width axle swap stage and skipped the narrow stage cause everyone pretty much wishes they would have left their rig full width who completed the narrow stage. I am completely off topic and ranting haha!

What I was getting at is gear for the 35's if you think you are going to get 35's and then just get them because you'll save alot of $$$$ down the road. It all depends on your plans for the explorer too... if it is your daily driver and how you off road it and what not.

I also wouldn't call the 4.0 underpowered. It isn't a high revving v8 though. It has low end grunt and is a reliable motor.
 






I could give you the technicial reasons to regear. When you regear to compensate for larger lires you do return the engine to its stock powerband. Giving an X taller gears make it a lot more fun to drive. True it wont make it faster but it will make accelerate faster. If you drive a lot on the highway, then you may not need to regear. If you drive city and or offroad a lot then regearing is a good idea.
 






What gears does it have now?
Who's telling you to regear?
 






Sorry guys, but i disagree with the general consensus that he is geared improperly.

it sounds to me like his gearing is perfect. remember 4.0 OHV's are a bit of a torque engine. at 70mph his engine is right at its torque band, 2500rpm. as for accelerating, the only difference lower gear ratios would make, would be a slight gain at sub-3,000RPM speeds in first gear. after that, all it will do is change the shift points (to slightly higher speeds), the engine will still be in it's power band at all times, as the ratios between transmission gear's are the same. I dont suspect his foot is nearly on the floor at 2500rpm cruising 70mph. (and up a hill he should of course be in a lower gear) my '93 cruises nicely at any speed over 55mph with stock (3.73 i think) gearing and 31's.

his 31's aren't that much taller than stock tires, i bet a 2wd explorer (taller gears) would run about the same RPM at any given speed as he is running with his 31's.

i'm also going to have to go against the grain and say that i don't think a 4.0 is underpowered for an explorer either if the smaller engine can do the same job it'll usually use less fuel than a larger one. besides, i've had mine over 100mph, so there's plenty of cruise-torque there for 70mph cruising.
I was not saying HE was improperly geared, just answering the question as to why regear. :)
 






I have the stock gears that came with the 5 speed. I don't ahve an auto, I hate them. I'm guessing it 3.73 but the tag is damaged and unreadable.

No one has told me to regear. but it is a common thing in this forum. I can't speak about 31s with an auto. But my 5 speed with ported intake and 2.25" cat back pulls nicely and only on the steepest grades do i have to down shift to 4th. That was true before the 31s though. I haul trailers sometimes loaded with other cars or trucks with this X. Granted only in 4th. But I would do that even if I had re-geared. Overdrive is not for towing.

and no my foot is not on the floor at 70. I would guestimate @ 1/4 travel. Maybe 1/3. Using my Tom Tom this weekend I estimated my fuel economy @ 17.5 MPG. That is the A/C running the whole time, and a 17' canoe on top and about 250 pounds of gear and dog in the back. half and half straight flat interstate and curvy hills. I think that's pretty good. I would like it to be better though.

I do plan on 33s later. but there are other things I need to do first. so I'm looking at next year. I'm going to cut and turn the front and 4" SOA. as well as a few strength/reliability mods. The largest tire I might ever install will be 34/10.5. I don't want a huge truck, I want a reliable DD trail rig / utility truck. So I'm thinking if I like it like this, I might want to try 4.10 or 4.33 gears with the 33s. My only complaint about not re-gearing is the speedo is off.

I thought this might be a good discussion since most people think you have to re-gear.
 






If you runn 33's, you'll notice the difference a LOT. Makes for a tall 1st gear with 3.73's.

2500 RPM is not bad for the 4.0. They are pretty comfortable at even 2800 RPM at highway speed.

If you're rocking 3.73's, you're fine with 31's. For sure if you're not into off-roading.

People are constantly talking about re-gearing on here because large tires and stock gears eliminates you're low end crawl. I had my stock 3.45's and my 35" tires on before I finished my re-gear and it SUCKED. Even in 4 low... just no crawl at all. Now I have 4.88's and it's got lots of power, crawls decent (although I want a doubler...) and still cruises at 70 under 2800 RPM. I love it.
 






Just for whaat it is worth, my Ranger (Level II) has a 5-speed, 31-inch tires, SOHC and 4.10 axle ration stock from the factory. Not to good at highway milage, but a hoot to drive.
 






2600-2800 RPM at 70 is actually the ideal cruise RPM range for these engines, both in terms of MPG and power. With your 31" tires you're pretty close to the lower end of that range. You can imagine though with the bigger tires, you're no longer going to be anywhere near that range at all anymore, and is the reason for regearing. ;)
 






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