If I go from 4-10 to 3-42 What mpg will I get??

[Vandermax]

I have two 4-10 turbo burbs, and I average 16 to 17 mpg in them. One of my trucks is 2 wheel drive, so I thought I could try putting a 342 in it, for as cheap as posible, and then hope for 3 mpg increase???? Anyone have a 342 burb??? What mpg do you get???? Thanks for the info!!!

[john8662]

This is my next project after I get the burb running again. I want to go from 4.10 to 3.42. I don't feel that getting an additional 3mpg is unreasonable. I would venture to say that it should be capable of getting 19 very consistantly. What size tires? Mine will be 265/75 when I make this change, currently 245/75.

J

[Vandermax]

I have 265, 75 16 on my trucks!!

[Warren96]

Acording to the info on the ''Diesel economics and fuel mileage" article on the Diesel Page it states that a 6.5TD engine RPM[4L80E trans] and typical mileage at 65mph - 3.08 gearing 1690rpm [23]- 3.42 gearing 1880rpm [21] - 3.83 gearing 2050rpm [17]-4.10 gearing 2250rpm [15] I hope this info helps,good luck!

[4348rd]

my surburban is 3:08 with a 700r4 i can do 23mpg , non turbo 6.5 and a 255/70/15 tire

[Warren96]

How about a little more info on these 4.10 burbs? Stick or auto 4L80E or 700R4?

[john8662]

I got 16mpg with the 245's and 4.10 gears, thats why I feel that I could get the 19-20 if i really drive easy on the highway.

I did see a .5mpg gain when swapping out the steel rims for aluminum rims (no joke).

J

[surfbeetle]

My burb is a 1500 with the eight lug axle, according to the dealer its rear end ratio is not the 4.10s, I think he said that it was 3.73s. This was the only difference between the '97 2500s and the '97 1500s. I have gotten over 20 mpg on the highway using cruise control at speeds of 65 to 70. Around town it gets down to around 13.

[Joe Duffiney]

If I remember correctly 2500's came with either 3:73 or 4:10. The 1500 was only available with 3:42. This was 94-99.

[DA BIG ONE]

Consider, height of truck, weight, tire diameter, transmission condition and control (auto), is tranny slipping, converter slipping, how about that clutch?

I've done many mods at a great expense, the truch is a beast, however, I found the point where I actually lost mpg's driving around town because of tranny lockup changes resulting from my changing out final drive gears.
3.42:1 was great w/245/75/16 tires, I went to 285/75/16 and developed a shift point/lockup issue at speds under say 45mph, over not an issue, but loss of mpg's tells me it was a bad idea the bigger tires. I can get it back to the sweet spot by going to 3.73:1 gears.

My issue is the NP-246 t-case which has 3 speed sensors, is intergrated into ABS and other controller and thereby eliminates aftermarket tranny controllers that could fix the shift point/lockup issues.

So, I'm stuck w/going to 3.73:1 to fix my issue.

[MTTwister]

I have a 96 k2500 - 3.73. Running the oem 245/75/16's. I was considering going to a 255/85/16 to get an effective ratio of about 3.42. After reading your shift point / lock-up issues, I'm wondering if I should go with the 265/74/16's.
( Also a minor concern over potential handling problems with the taller tire - wall flex, etc... )

Any thoughts? Thanks!

[JohnC]

My '93 2WD pickup with 3.42 got about 22 mpg highway by itself and 17 towing a 5000 lb horse trailer. I wrecked the truck and am selling the axle. If you buy it I guaranty you'll get the same or better mileage....

(just kidding about the guaranty... the axle has 50K miles, is a 10.5" ff locker)

[DmaxMaverick]

Just increasing tire size won't give you the same result as a gear ratio change. Larger tires increase rolling resistance and mass. They can improve mileage by increasing the final drive ratio, but there will be a loss due to the additional power required to turn them. A 3.42 with 245's has a similar final drive ratio as a 3.73 with 285's (or 255's), but they are not equal. Tire tread style also has an effect on rolling resistance. Tire ratings also effect it, as higher rated tires are heavier. Also, larger tires can dramatically effect ride, as they increase your unsprung weight.

[Bobbie Martin]

We have a 1995 2WD Sub with a 3.42 axle. We have owned since new and we usually get 16-17 around town and 20 on the road. I have seen 21, but that is about the highest.

[DA BIG ONE]

I have a 96 k2500 - 3.73. Running the oem 245/75/16's. I was considering going to a 255/85/16 to get an effective ratio of about 3.42. After reading your shift point / lock-up issues, I'm wondering if I should go with the 265/74/16's.
( Also a minor concern over potential handling problems with the taller tire - wall flex, etc... )

Any thoughts? Thanks!

I have a NP246 AutoTrac so the shift point and lockup issues, other t-cases do not intergrate into ABS so stand alone controllers can be had for them.

Tire height has not caused any real issues, however on off ramps one should always slow down w/larger diameter tires no matter what.

Do remember to calbrate VSSB w/any change.

[bluuzman]

I've got 4.1 final gear ratio on my '95 suburban K2500 and have been considering changing the gear ratios to 3.73 or even higher. As I'm doing mostly highway cruising without any load or towing, I'm pretty convinced the truck would do much better with higher gear ratio.

Anyway, I asked an opinion from an experienced technician here and he said it would not be worth due to extensive amount of labour and all the costs compared to possible minor mpg improvement.

Is there anyone who might have performed this job and would be willing to share his experience and comments? Is it really a big job? At least the rear differential should be fairly simple to do. How about the front?

-ari

[Warren96]

When I went from 245/75's to 255/85's, there are a lot of issues to consider.The only thing no one has mentioned yet is the fact the loading floor on your tailgate is higher.Not a problem for me, but anyone helping me load a bulky or heavy load expects the floor to be lower and always hits the floor with the load.What about when you are working on your truck? (changing a fuel filter?)You have to reach that much further to get the job done!You want to up your MPG?Driving slower with a diesel,with your RPM's less than 2000 saves me a bunch! Just my .02c

[moondoggie]

Good Day!

When I bought my pickup, hardware was all stock (4L80-E AT, 4.10 gears), so with 8% oversize tires my effective ratio was 3.80. After a couple years ownership, I changed to 4% oversize tires & installed a Gear Vendors Aux. OD. Assuming the GV is engaged (which is always is when I'm > 50 mph or so) my effective ratio is 3.08. I'd guess I improved ~ 2 mpg, with extremely good record-keeping involved. (ALL mpg figures corrected for odo error.)

Blessings!

[16gaSxS]

I've got 4.1 final gear ratio on my '95 suburban K2500 and have been considering changing the gear ratios to 3.73 or even higher. As I'm doing mostly highway cruising without any load or towing, I'm pretty convinced the truck would do much better with higher gear ratio.

Anyway, I asked an opinion from an experienced technician here and he said it would not be worth due to extensive amount of labour and all the costs compared to possible minor mpg improvement.

Is there anyone who might have performed this job and would be willing to share his experience and comments? Is it really a big job? At least the rear differential should be fairly simple to do. How about the front?

-ari
I did the change from 3.73 to 3.42 at about 3,000 miles, about 1997 or early 1998 cost me about $800 for 2x4 rear axle only of course. I figure it took 60,000 miles to to pay me back @ $1.35 a gallon and I have put an additional 110,000 miles on since and price of diesel is now over double that. I had it done by a recomended shop. I tow 5,000 lbs regular with no problem and have towed heavier. I wouldn't want to tow much over 7,000 lbs regular with it even tho I have towed about 11,000 lbs a number of times.

You can run the numbers, assume you pickup 2-3 mpg going from a 4.10 to 3.42 axle if you can live with a reduced towing. Figure your current cost for fuel then you can get your "break even" miles you need to own the truck. You then have to determine if you will keep your truck long enough to have it make sense to change.

[16gaSxS]

I have a 96 k2500 - 3.73. Running the oem 245/75/16's. I was considering going to a 255/85/16 to get an effective ratio of about 3.42. After reading your shift point / lock-up issues, I'm wondering if I should go with the 265/74/16's.
( Also a minor concern over potential handling problems with the taller tire - wall flex, etc... )

Any thoughts? Thanks!

Tom;

When I swapped gearing TCC lock up speed increased about 5 mph to about 50~51 mph where before it was 45~46 mph, same RPM's. It really is not a problem as on the hiway most of the time I'm going at least 55 mph. If your ratio was 3.21 lock up would be I estimate 53~54 mph and with 3.08's lockup should be around 56~57 mph.

The larger tires may help some but not as much as gearing because of what DMax maverick said. Plus the added "wind resistance" of a higher off the road truck and more surface of larger tires against the wind. ie DRAG.

[More Power]

The practical sort usually looks at just a potential fuel economy increase when ruminating over a gearing swap.

Just to throw this out, an often overlooked aspect of a swap to taller gearing is how much nicer to drive the truck will be on the highway - with lower engine rpms and much less noise.

Back in the 90's, I was finally convinced to replace the non-OD auto in a 6.2 truck I owned with a performance diesel-built TH700R4 and lockup TC. I was initially hoping for just a fuel economy increase, but after driving the truck with the OD (the truck had 3.42 gears already), I discovered I would do the swap again even if there was no fuel economy increase. The truck became so much nicer to drive. The engine sound simply disappeared in overdrive... By the way, that 6.2L truck went from 17-19 mpg to 22-24 mpg. :)

Jim

[DA BIG ONE]

I've got 4.1 final gear ratio on my '95 suburban K2500 and have been considering changing the gear ratios to 3.73 or even higher. As I'm doing mostly highway cruising without any load or towing, I'm pretty convinced the truck would do much better with higher gear ratio.

Anyway, I asked an opinion from an experienced technician here and he said it would not be worth due to extensive amount of labour and all the costs compared to possible minor mpg improvement.

Is there anyone who might have performed this job and would be willing to share his experience and comments? Is it really a big job? At least the rear differential should be fairly simple to do. How about the front?

-ari

I have a 1999 and wrote about my mods as they were finished, problems too, but the best overall mileage (high 20's) increase was w/3.42:1 gear sets, and stocked sized tires. There are other factors I have to live with but quality of fuel is the big factor, I find that I have to add booster/lube at higher levels where I live in the sub-tropics, as I go north the fuel gets better.

[moondoggie]

Good Day!

What More Power said. If Scotty beamed you into my pickup at highway speed, no way would you know it's a diesel. I know to some hard-core diesel folks that's blasphemy, but I am reasonably sure the bit of hearing loss I've experienced is partially due to my 82.

Blessings!

(signature in previous post)

[bluuzman]

I have a 1999 and wrote about my mods as they were finished, problems too, but the best overall mileage (high 20's) increase was w/3.42:1 gear sets, and stocked sized tires. There are other factors I have to live with but quality of fuel is the big factor, I find that I have to add booster/lube at higher levels where I live in the sub-tropics, as I go north the fuel gets better.


Hi, you say you wrote about your mods. Where one might found your article?

If you change your gearing, can't it be compensated to get TCC lockup etc operating as they used to?

Lock-up around 50mph is however just fine. Now it takes place at 45 without cruise and around 42 with cruise on (may sound strange but the cruise affects the TCC lock-up somehow in my truck)

*

The fuel here is very good by any standard. I have seen slight difference, however, between "arctic/winter quality" vs "std. quality". You know, the temp may decrease well below -30C here and std diesel would be just frozen ;)

The fuel is also expensive, we pay currently about USD 4.6 @ gallon of diesel ($1.25/litre), just for your information. Gas is about 35% more expensive - over USD6@gallon.

BR, Ari

[DA BIG ONE]

Hi, you say you wrote about your mods. Where one might found your article?

If you change your gearing, can't it be compensated to get TCC lockup etc operating as they used to?

Lock-up around 50mph is however just fine. Now it takes place at 45 without cruise and around 42 with cruise on (may sound strange but the cruise affects the TCC lock-up somehow in my truck)

*

The fuel here is very good by any standard. I have seen slight difference, however, between "arctic/winter quality" vs "std. quality". You know, the temp may decrease well below -30C here and std diesel would be just frozen ;)

The fuel is also expensive, we pay currently about USD 4.6 @ gallon of diesel ($1.25/litre), just for your information. Gas is about 35% more expensive - over USD6@gallon.

BR, Ari

Just search da big one, you'll find all kinds things I had done at shops that went bad had to finally fix myself.

As for changes in lockup, shift points or aftermarket tranny controllers There are no options whatsoever for "1999 6.5td w/4L80e and NP246 t-case (autotrac)" because of the 3 speed sensors to monitor frt/back/driveshaft & road speed then sending data to autotrac controller, and intergrated into ABS. I searched, and searched, and searched some more couldn't even get someone to do a bata unit for the combo. Tried hacking using aftermarket speed recalibration devices w/o any luck, however, any other t-case is no problem to correct.

[rjwest]

Comment: The PCM senses Drive shaft speed for TCC control and shift point control, It can not be changed without an add on Trans controller .
Also calibrating the VSSB will have no effect on the PCM
control of the transmission.

If with your current gear ratio ( rear gear & tire diameter )
you are driving at speeds just above your TCC lock up speed,
Changing to a higher gear ratio( gears or tires ) will most
lickly lower MPG ( TCC unlock, delayed Gear Changes )

changing from 4.10 to 3.73 on my 96 Dually was not cost effective. ( lost MPG when loaded )

[bsevans]

When I ordered my 95 4x4 Tahoe from the factory, the stock ratio available was 3.73 and 3.42 could be ordered as an option which I did. For the first 9 year I keep meticulous records of mileage at every fill up. I was getting 16-17 around town and at 75mph it has been 20-21 mpg. I have a roof rack that carries my spare tire and it, along with the brush guard and winch; have probably hurt my mileage by 2 mpg on the freeway. I could see doing this change on a 2 wheel drive but think the cost to do both axles on a 4x4 would be cost prohibitive from a material and labor stand point.

[JSteward]

I looked all over my gear box and can see no indication of what gear ratio my truck has. Is it possible to determine what this stock gear ratio is from the VIN or by looking in another location?

Thanks

[DmaxMaverick]

I looked all over my gear box and can see no indication of what gear ratio my truck has. Is it possible to determine what this stock gear ratio is from the VIN or by looking in another location?

Thanks

Look at the sticker in your glove box. There'll be a list of RPO codes that shows what was OEM on the truck, including the gear ratio. Compare what you have with the list HERE (http://www.thedieselpage.com/members/rpocodes.htm).

[JSteward]

Thank you!

[bcbigfoot]

I have a 93 c2500 PU longbox extended cab,4l80e trans with 3.42 and 265/75 16 tires, so at 60mph rpms are 1700. Miliage at 70mph is 19mpg (us gal.). I think you would see min of 1 mpg better with Sub. Also combined city highway figure is 16mpg. I think there are outher benefits of lowering your rpms, longer engine life, drivability on the highway, truck idles along flawlessly when not pulling a trailer.