Best gear ratio/mpg combo?

[welpro222]

At the moment I have a 1983 K5 6.2 700r4 3.08. I get around 16-18 city and 20+ freeway. I usally drive in 3rd at 50 mph, and 4th(overdrive) on the freeway at 70mph. With the 3.08 gearing it seems to lug the motor alot. Even cruising in 3rd at 50mph, i'm turning 1669 rpm and my TCC Lockup kicks in and lowers it to around 1450 (Yeah I have Lockup in 3rd too). On the freeway it turns 1636 at 70 in overdrive and lower when TCC locks up.

If you do the math with 3.73 gearing its 2021 RPM at 50 in 3rd gear and lower when TCC Locks up. 1981 RPMs at 70 in 4th and lower with TCC Lockup.

It seems to me that 3.73 gearing would put me in the 1800 RPM range and give better power and MPG.

 

[Cucvnut]

with the OD yes it would and better wheeling gears also.

 

[CUCVFAN]

I had a gasser K10 with 3.08's and swapped it to 3.73's and they were perfect for towing, but still got decent (if you know mid-80's Chevy gassers, you know that should be read as "semi-lousy") mileage with the OD.

 

[airmech]

I'm getting the same mileage with 4.56 gears/33x10 tires and 700r4. I would assume the 3.73 gears would increase the mileage some but not drastically. I think the 4.10 would be about perfect with my set up but not worth the expense for the little difference I would see. With the Blazer I think the 3.73 is your best all-around gear. You'll see much better driveability, but not too much more economy.

 

[DriverSideImpact1008]

You'll probably see a little increase in city mileage (lower gears=less effort to get up to speed), as well as being quicker off the line (for a 6.2).
I doubt you'd see much difference on the highway, but maybe slightly better mpg since the engine wont be lugging down as much.
You should expect to see less variance in your mileage...your City and Highway mpg's will be closer together. And I'm sure at the very least drivability will improve while not hurting fuel economy.



*Edit* Keep in mind tires play a big role in fuel economy. Size, hardness or softness, tread pattern, etc. all plays a big part.

 

[jdemaris]

At the moment I have a 1983 K5 6.2 700r4 3.08. I get around 16-18 city and 20+ freeway. I usally drive in 3rd at 50 mph, and 4th(overdrive) on the freeway at 70mph. With the 3.08 gearing it seems to lug the motor alot. Even cruising in 3rd at 50mph, i'm turning 1669 rpm and my TCC Lockup kicks in and lowers it to around 1450 (Yeah I have Lockup in 3rd too). On the freeway it turns 1636 at 70 in overdrive and lower when TCC locks up.

.

6.2 is most efficent within the 1800-2100 RPM range.

Actual fuel mileage is going to vary depending on how hard the engine has to work on hills, and with altitude you're driving in.

I've got over 20 diesel Blazers and trucks with all sorts of combinations of gearing.

One example is my favorite K5 diesel mini-motorhome. 86 K5 Blazer with a pop-up roof camper body. Weighs 7500 lbs. and is driven a lot in the Adirondack mountains.

When it was a bare Blazer, with stock 235/75-15" tires, 3.08 axles and 700R4 trans it got a best of 21 MPG. Even on slight hills, it would shift back and forth out of 4th OD to 3rd and lugged a big.

I then drove it for a year with the lockup totally unhooked. Drove much nicer but best mileage dropped to 19 MPG. No more lugging though.

I then put wide 33" tires on it and mileage dropped to around 18 MPG.

I then put stock 15" tires on again, along with the extra weight and wind resistance of the camper-body. I also hooked up the lockup again. Got a best of 16 MPG.

I then ripped out all the axles and installed 3/4, 8 lug axles with 3.73 ratio. I also installed a 4L60 trans that only has lockup in 4th, and NOT in 3rd. The 16" E-rated truck tires are narrow and 30" diameter. I also added a Banks turbo. I'm now getting 16 MPG as before, but with a lot more power and I can cruise most hills in 4th OD.

I'd say a bare K5 Blazer with no turbo and overdrive/lockkup does best with 3.73s.

With a turbo, overdrive and lockup 3.44s are probably the best overall.

My best fuel mileage 6.2 is my 1982 K10 4WD pickup with 3.08s and a four speed manual trans with overdrive. It is pure stock just as it left the showroom floor. I've gotten 24 MPG on flat highways with high-sulfur fuel. It is useless though for any heavy work. Good plow truck though since I can use low-range when pushing snow.

I would't want 4.10s in anything unless it's a tow vehicle. I've got two diesel Ford F250s with 4.10a and wish they had better low RPM gearing. My 92 Dodge-Cummins has 3.50 axles and will outpull my Fords and gets near 20 MPG. Pretty good for a heavy 3/4 ton truck with 4WD, extended cab and an 8 foot bed.

 

[kenz5380]

I've got 3.73s in my M1008 and I am very happy with that choice. I don't have overdrive, but I can still cruise the highway pretty easily. I've had as much as 1500lbs in the back and have not had any issues. Aside from the gear change my truck is more or less stock and I am very happy with the performance.

 

[jdemaris]

I've got 3.73s in my M1008 and I am very happy with that choice. I don't have overdrive, but I can still cruise the highway pretty easily. I've had as much as 1500lbs in the back and have not had any issues. Aside from the gear change my truck is more or less stock and I am very happy with the performance.

3.73s are nice until you have to travel a fast highway somewhere. I drove an 87 3/4 ton diesel Suburban for years. 3.73 axles and the TH400 trans. No overdrive and 10-15% torque-converter slippage at cruising speed instead of lockup. I got used to the noise and hardly noticed at speeds of 65 MPH.

But, when I got to some Interstates posted at 75 MPH and the general flow of traffic was 80 MPH? The Suburban was awful. Engine screaming and at 85 MPH - felt it was near its limit. I later changed to a 4L60 and it drives like a totally different vehicle. Much better take off in low gear, and much lower RPMs at high speeds. Better at both ends. I can drive all day at 85 MPH and it's smooth and quiet (until I get a ticket).

Only downside to the 4L60 is it needs to be beefed up to be near as rugged as the TH400.

 

[Iceman3005]

I have 4.56's gears still with the 700r4 and stock tires, I run about 55-60 mph to work and back, I get about 22-24 MPG.

 

[jdemaris]

I have 4.56's gears still with the 700r4 and stock tires, I run about 55-60 mph to work and back, I get about 22-24 MPG.

I assume you're in the flatlands of the lower P of Michigan?

I own several properties in northern Michigan just below the Mac Bridge. All my diesel trucks get 2 MPG better there (as compared to the mountains of New York).

I had my 94 Ford F250 turbo-diesel here in NY for 10 years and 15 was the absolute best MPGs it ever got. When I later took it to Michigan and left it there, I drive it ever summer and get 17 MPG consistently. MY 92 Dodge W250 diesel got 21 MPG when used it one summer there.

Only downside to northern Michigan is they salt the dirt roads all summer long.

 

[kenz5380]

.73s are nice until you have to travel a fast highway somewhere. I drove an 87 3/4 ton diesel Suburban for years. 3.73 axles and the TH400 trans. No overdrive and 10-15% torque-converter slippage at cruising speed instead of lockup. I got used to the noise and hardly noticed at speeds of 65 MPH.

But, when I got to some Interstates posted at 75 MPH and the general flow of traffic was 80 MPH? The Suburban was awful. Engine screaming and at 85 MPH - felt it was near its limit. I later changed to a 4L60 and it drives like a totally different vehicle. Much better take off in low gear, and much lower RPMs at high speeds. Better at both ends. I can drive all day at 85 MPH and it's smooth and quiet (until I get a ticket).

Only downside to the 4L60 is it needs to be beefed up to be near as rugged as the TH400.

I get what you're saying here. I've been told that a 4L80E is the way to go. What do you think about that? I've also thought about putting a Gear Vendors Overdrive behind my TH400. What do you think about those mods?

 

[jj]

I have a 2002 one ton GMC Savana van with a 6.5 turbo and a 4L80e and 3.73 gears. That makes a 2.80:1 final drive ratio. I get almost 19mpg running light in eastern PA at around the 65mph speed limit. Out west, loaded, at the 75mph speed limits, we got mid 17's. That seems to me to be just about the perfect set-up. The 245/75r16 tires on the van are about the same size as the 31-10.5-15's on the Blazer. Trying to tow, will of course, change the equation some.

 

[jdemaris]

I get what you're saying here. I've been told that a 4L80E is the way to go. What do you think about that? I've also thought about putting a Gear Vendors Overdrive behind my TH400. What do you think about those mods?

4L80E is the lockup-overdrive version of the TH400. Very rugged. Problem is the size of it and the expense to install it. You'd have to get an aftermarket electronic controller which is very expensive. Or if lucky might find a stand-alone transmission controller GM used in the first couple of years when the first 4L80Es were used with diesels.

Problem with the Gear-Vendors overdrive against the TH400 is you are still going to have the 10-15% slippage from the torque-converter. If you had the HD manual trans, Ranger makes a great overdrive unit that goes before the trans.

The 700R4 (renamed 4L60 in 1991) is the overdrive lockup version of the lighter-duty TH350. Needs no electronic controllers and can be made very rugged with updated parts. But, it's not going to fit a transfercase that originally had a TH400 hooked to it. Different splines means you'd need a different transfercase or new shaft in the case. Normally no big deal since used transfercases can be found for $50 or $100.

 

[armytruck63]

No overdrive and 10-15% torque-converter slippage at cruising speed instead of lockup.

This sounds like a lot of slippage to me. I've always heard that slip should be 5% or less, ideally 2 to 3% at cruising speed. Is your transmission fluid toasted?

 

[jdemaris]

This sounds like a lot of slippage to me. I've always heard that slip should be 5% or less, ideally 2 to 3% at cruising speed. Is your transmission fluid toasted?

8%-=10% is an standard average figure/range for a properly working torque-converter. It's going to vary with load, speed, wind resistance, etc.

Easiest enough to check if a vehicle has a tach. I've got a Blazer and a Suburban with 6.2 diesels, tachs, and 4L60s. Both slip about the same. If I unhook the lock-up at 75 MPH, RPMs increase by 250-300 RPM with each. That's 9-10%.

It's the reason why non-lockup torque converters are often referred to as "oil heaters." It's also why they've been eliminated in production vehicles made for highway use.

 

[axeman]

found this as possible overdrive. nice that they have adapters. i dont know the rating of the od unit though.Maximum Overdrive Transmission Auto Parts