rear ratio to match peak torque??

[RapidRide2]

I have been reading on here that is best for economy to have peak torque at your desired cruising rpm. My desired rpm is between 65-70 mph. An average mostly stock sb chevy achieves peak torque at or around 22-2300 rpm. With a 700r4 and 235/75/15 tire the chart from bowtie overdrives calls for a 3.90-4.11 rear ratio. This seems somewhat steep??? However it DOES put me right around 22-2300 rpm at 65-70 mph. My question is why have factories for many, many years,,, circa 82' all the way up to now still put 2.56's 3.08's 3.42's etc. behind overdrive cars and trucks??? If a taller (numerically) gear ratio would have given the best mpg on small block cars and trucks???? seems to me you would see alot more 3.73-4.11 factory rearend gears. Thanks to all who reply.

[N2TRUX]

My guess would be that factories assume that most people take off slow and don't use the torque multiplication factor. I think they are more concerned with getting you at a lower rpm while driving highway miles.

This is a great theory for passenger cars, but doesn't make much sense for trucks. Even if you don't hual loads all the time just the fact that itshaped like a brick requires more torque to get it moving.

The manufacturers are coming around though. On my 04 2wd QC Dodge I wanted the 20" tires. If you check that option box, it comes with 3.90 gears....

[cliffsta]

yeah that does seem to not make sense. In my truck, i have the TH350, a non overdrive tranny, and I'm glad i have the 2.56 gears in it, cause it stays at low rpms all the time, helping MPG. But in an OD vehicle, you would think maunfacturers would use quicker gear.

[MylilBowTie]

I thought I had it bad with my old 2.73 gears, guess I could have had worse. I have 3.08 gears right now its better than the 2.73 gear. 55@2150 65@2500 - th400 - 235/75/15 tires - 15.3 mpg on the last hwy trip about 200 round trip running 65 for the most part.

[leastbay]

i have a th350 and 3.92 rears the rpms stay pretty low and it has tons of acceleration after 60 mph or so but it burns tons of gas

[Tom]

I have a 2004R in my 79 with a 3.07 rear. At 80mph Im doing 2000rpm. Even with the vortec 350 which has a ton of low end torque its still lugging the engine a bit. Im going to 3.73's as soon as I can and I can almost promise a raise in MPG.

[Destructo]

I'v been trying to find the perfect balance for my truck, when towing in 3rd I don't want to run over 3k rpm but I want to keep the rpm up in OD so I can still use it without having to downshift a lot. So I figured 4.10 gears for my 33" tires will put me around 2krpm in OD at 65mph and 3k rpm in 3rd at 65mph, should be good for economy in OD and good for towing power in 3rd.

[86 Red_Bowtie]

Originally Posted by Tom

I have a 2004R in my 79 with a 3.07 rear. At 80mph Im doing 2000rpm. Even with the vortec 350 which has a ton of low end torque its still lugging the engine a bit. Im going to 3.73's as soon as I can and I can almost promise a raise in MPG.

I went from 3.08 to 3.73 in my truck with a 700r4 in it and my gas mileage went from 12 mpg to 15 mpg and i have more lowend torque.

[Destructo]

My truck came off the dealers lot with 3.08 gears, 305 engine, 700r4, and 31x10.5" tires as optional equipment, its not too quick even with a good 350 in it, I am running 3.42 geared axles now and 33x12.5" tires, which if you do the calculations is almost exactly the same as 3.08s and 31's, I plan on running 4.10s, cruise rpm now at 70mph is still under 2k rpm, about like 1800-1900 and when a hill comes up it has to downshift and struggles to push the brick through the air at 70mph hehe.

[N2TRUX]

Here's something for y'all to play with....

Gear Calculators

[RapidRide2]

What could be cooler??? more low end with better m.p.g. I guess technically,, under normal driving experiences you wouldn't have to push the gas as much with a lower gear because of the multiplier factor. Any other "real world" experiences out there???????

[gchemist]

I have peak torque for my cruising speed. It takes more than gears sometimes. I have a 700R4, 255/60R15 (26") rear tires and 3.08. Cruising speed 70 MPH ar 1950 RPMs. Max torque for a stock 305 ~2000 PRMs. MPG with a tonneau cover =~17-18
Same application for my city driving!! I drive only in 3rd. 40-50 MPH in the city limits. Cruise RPM ~1800-2200, still near peak torque!! A locking torque converter at 1800 RPMs helps too.

[tkeduk8r]

hmmmm.. htis is an awesome topic... more people should understand this, but i think most people assume that the lower RPM on the highway, the higher the MPG... (myself included until just recently).. im glad to hear your stories because i am swapping an a 305 in place of a 250-6 and a 2004r behind it... I run 3.73's and 31x10.5s... my parts truck has 3.08's and i had considered using them, but I think i will be much happier with the 3.73's.. ripping and tearing starts wed nite
--Mike

[RapidRide2]

Hey g chemist,, what kind of mods so you have done to your engine either for economy or power??? I am getting rid of a 2003 gmc that is payed off (will pocket the money before resale value declines anymore) I would be tickled with 17 m.pg. on an 83' gmc. My 03' is getting 17.5 w/ a 5.3 3.73.

[gchemist]

If I had a 5.3 I would chose 3.55 gears.
What have I not done!! It's all about aerodynamics, tuning and weight. Lowered truck, air dam, removed the dumbo ears, remove all the side trim, removed all cab trim, removed the rear bumper, removed the spare tire carrier, 12 bolt upgrade, rebuilt 700R4($1100), new brakes, new font bearings and such, tonneau cover, aluminum radiator, dual electric fans, new carb with spacer, timing, sparkplugs every 6 months, 94 amp alternator, 4.5" air cleaner, removed the heat riser, high flow cat, Flowmaster 40 series and a light foot. That's does not happen very often though.