my step van has 4:10 gears and a friend of mine has the same van and is going to replace the "ring and pinions" and put in a different set of gears, i think he said they are 3:21 but i don't know for sure if that is correct. He said it will nearly double the fuel economy, or at least bring it up to 15 mph instead of 10 mpg. He also said it will bring the rpm's down on the engine and make it last longer - supposedly this engine is at 2700 rpm at full throttle, but with the new gears it will be 1800 rpm, and much better for the engine.
Does anyone know if the part about fuel economy is really true?

I would like to do this but can only afford the cash for the parts, not the labor; and my friend says you need a transmission place to do it. Is this true? There is no such thing as a real mechanic where i live, except for me (and i am not really a mechanic).
thanks,
Paul

Fuel economy is directly related to engine speed and load. The lower the rpm at cruise speed, the better the fuel economy. Load carrying capability is another matter. The greater the load, the more fuel it will require to move the load. Most trucks, set up to haul/tow, have lower gears. The 4.10 axle ratio was the gear of choice for heavy hauling.

If the vehicle is used mostly as a "passenger car", taller gears will slow the engine down at cruising speed and help with both longevity and fuel economy. Power will suffer when trying to tow heavy loads but, I would use gearing to provide the desired overall result for the vehicle.

If towing a travel trailer for two weeks vacation, once every summer is the only towing you plan to do, I would go with the taller gear for better fuel economy under normal driving conditions. In other words, don't gear the vehicle to do a job two weeks a year and suffer the other fifty weeks.

Installing and "setting up" a ring and pinion is a job that requires some special skill. It can be done by a novice if you follow the directions carefully. It is a "feel thing", when it comes to setting the pattern of the gears and pre-load of the bearings. It is difficult to explain but the correct meshing of the gears is important to gear life and quiet operation. Some gear sets will work well when set up "close" or "in the ballpark". Others need to be exactly "spot on" and are more difficult to get the pattern to read correctly.

The pinion bearings need to be pre-loaded to operate correctly. Again, it's hard to explain but if too tight, they will overheat and turn blue. Too loose and they will make noise and destroy themselves prematurely. Read the service manual section on setting up the axle gears and read the instruction sheets that usually accompany the new parts. More info is available from the internet or a specialty 4X4 shop. Hope this gets you started, good luck.

A taller gear set will help bring down the RPM's and will improve the fuel usage SOMEWHAT. If you keep the same size tires you have now you will feel a noticable difference going down the highway. You may lose some pep around town and on towing. As far a installation, if you have never done a few sets of gears and dont have any of the specality tools I would not attempt it on your own. I have helped do a few sets, and it does require some experience to get it right, even now I would get help to do a set. I would ask around and find a shop that does them on a REGULAR basis , not " I have done a few in the past". Around here the off the street price on labor is about $300-400 bucks, not including parts.

Fred482: You posted while I was typing, but you did a better job explaining things.

c.r., You're absolutely right! Don't try this at home, kids! I missed my mark with the "novice" comment. It does require a "feel" for setting up a gear set correctly. The only way to gain the experience is to set up lots of them. I've done that and it seems easier, the more of them that I do.

I suggest a 4X4 shop because, around here, they are the most experienced and seem to be the most reasonable on price. The general repair shops just don't do enough to be recommended. Fred

Don't be surprised if you don't get a big jump in MPG. I have a 4.56 in my 1986 C30 dually and the rpm's are definitely higher. I carry a heavy camper about 99 % of the time. If I had a 4.10 I would have to down shift to 2nd gear on every grade.

My suburban hiway mileage went from 16 mpg with the THM400 to 20.3 mpg with the 700R4. RPM makes all the difference. I'm speaking of light load, hiway speed, average including some town driving. The round trip is about 450 miles and includes some hills. I had to downshift to "D" on the hills.

My suburban hiway mileage went from 16 mpg with the THM400 to 20.3 mpg with the 700R4. RPM makes all the difference. I'm speaking of light load, hiway speed, average including some town driving. The round trip is about 450 miles and includes some hills. I had to downshift to "D" on the hills.

Fred, approx what RPM's are you running like that, to get that said MPG?

with mine im getting about 18-20 depending on whats happening. running the setup in the signature, with 3.73/700R4. at 55 im at 1500RPMs in overdrive...and I think around 1800RPMS at the same speed, in 3rd. I cant quite remember, i will have to double check.

just wondering since you have a heavier truck with lower gears, same trans and the same MPG.

also seems hard for me to get in that prefered range of 1800-2800RPMs

TF, ignore me if this appears twice! I typed an answer, hit reply and it disappeared!! I would guess your turning a few less RPM than I due to the different gearing. I don't have a clue as to my highway cruise RPM. I've never put a tach on it. The 700 is what the suburban came from the factory with. Someone put a 400 in it before I bought it. I had a nice '87 core, so I built one and put it in to save fuel and the engine. The suburban wasn't as bad as the pickup on the highway because of the larger tires but it still sounded like it was wingin' the heck out of it at 65 mph.

When I get around to building up the 599 block I've been sitting on for eight years, it will go in the suburban and the take-out will go in the truck. The engine in the suburban is a "Goodwrench TargetMaster" with about 100,000 on it. It's tight and runs good. The truck is the original old "red" engine with lots of miles. It's tired but still running. I would like to build another 700R4 and put it in the truck along with the engine. I have no idea when that'll happen!

The nice thing about the 700 in the suburban is that the TCC locks up in 3rd and 4th. I can pull it into "Drive" and still have the lockup act like another gear. It comes on about 45 mph in 3rd. This is good in the hills. It will come out of lockup going up a steep grade, but still stay in 3rd gear, unless you really stand on it. Crusing on relatively level road, OD works well. It likes 65 mph and feels nice at that speed. I've checked the speed with the GPS and adjusted the speedo drive and driven gears to make everything accurate again, after the trans change. I would guess the engine is in that 2200 RPM "happy range" at 65 - 70 mph.

TF, ignore me if this appears twice! I typed an answer, hit reply and it disappeared!! The suburban wasn't as bad as the pickup on the highway because of the larger tires but it still sounded like it was wingin' the heck out of it at 65 mph.

thanks Fred, and it only appeared once. I guess its gone for good.

Ive heard this before, about larger tires. can you explain why that is? i do realize that height of the tire effects the overall gear ratio when combined with axle/transmission/transfer case, but why does it make it better? I always thought that a smaller tire would take less to turn, so would be less energy/fuel spent. please help me out on this. if it helps im running 245/75/R16E. I was going to go 265, but it wasnt available in that tire yet.

my 700r4 acts like yours, with the lockups. seems each one transmission locks up at different times. mine is hard to notice in 3rd and 4th(od), but I did the brake test and its there. I just dont notice while driving.

TF, my TCC is more noticeable because of the higher pressures. I've modified the pump pressure relief spring and, the shift kit kicks the pressure up also. I have the HD convertor, which has some hotshot clutch material (kevlar, I think) on the TCC clutch plate(s). You can definately "feel" mine at lockup.

The only explainationI have for the tire size is: Measure the circumference of both tires. The taller tire rolls several inches farther down the road with each revolution. If the engine is over-reving with the small tire, the larger tire slows the engine down, therefore allowing it to move the truck the same distance on less fuel.

If the gearing were tall, to start with, different story. If the engine were already "working hard" to move the given load, increasing the distance to move the load by putting on the taller tire would have a negative effect on mileage. It would make matters worse. But, since mine was already too low geared for the hiway load, the engine was happier and used less fuel to do the same job because I slowed it down, got it back in it's economy range of RPM.

The I.P. is throttle position sensitive as well as RPM sensitive. The farther you open the throttle, the more fuel passes through the metering valve. By not needing to push the throttle down as far it uses less fuel to go the same speed, with the larger tire.

Make sense? Don't know, just my theory, Fred

All I can say is that the tire size and rear end ratio have to be tied together. I put 7.50 X 16 rims/tires on a Ford instead the wide smaller dia. tires and everything suffered. Ford knew what they were doing is all that I can say.

Our dually with a 4.56 rear end makes the diesel engine run at very high RPMs at 65 MPH. I used the RPM calculator many years ago and it seems that it was around 2800 RPMs.