Is this hurting my engine ?

350 chev all stock with Edelbrock carb.

At 60mph it turns 1500 rpm in overdrive.

At 60mph it turns 2100 ,not in overdrive.

Rear gear ratio is 2:80 to 1 .

I have another rear with 3:92 to 1 with limited slip.

Does anybody know what the RPM would be in overdrive with just the rear change to the 3:92 to 1 ratio ?

Is it worth the effort to change rear ends ?
I am after good MPG , but i don't want to damage the engine if i am lugging it around.

This is in my 35 chevy so i'm not dragging much weight around and it get's up to free way speed with no effort at all and cruises with traffic easy.

Just a math question i can't do and concern about engine damage .

Thanks JIM

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Woops , correction on the limited slip rear i have , it's not a 3:92, it's a 3:73 , need to clean the crude off a little better next time.
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I have 2:41 rear gears with my 700r4 I turn 1400-1500 rpm at 70 mph.I have been driving it for 28000 miles and have had no problems and i am sure my 41 chev coupe probably weighs more than your 35.The worst thing is it is a little doggy in town.

 

Yeah mine's a little doggy in town too unless i just pull it out of overdrive. I would like the lower gears and limited slip in town and bad roads i drive on some times but want to keep good MPG. Thanks JIM

 

I have a similar setup in a OT car. It seems the faster I drive on the freeway the better my milage. The camshaft determines the powerband, and you need to stay in that sweet spot.

 

changing to the 3:92 will kill your gas mileage on the highway. In my experience, anything from 3:00 to 3:50 works good with an OD trans. Decent power and still pretty decent mileage. 3:23 has seemed to work out good for me.

 

I have a 3.55 with overdrive....I like it...20 mpg

 

Yeah I"m in the same boat cept I have a 200-4r with somewhere around that 2.80:1 rear. although better than it used to be when I had the th350. what size tires are you running out back?

 

3.92 gearset would result in 2100 rpm at 60mph in OD

 

Not a problem at all, plenty of cars out there with 2:73s and OD. Depending on the car I have had from the factory 2:73, 3:08, 3:73, 4:10 all with OD. Tire diameter also comes into play, I run a 4:10 with the 700r4 in the buick, 28" tall rear tires. Caddy I am doing now will have 3:08s with the 4l60E, also will have around 28" tall tires. Mid 90s caprices are 4000+ pounds and came with 2:73/3:08s, with OD, as long as the engine makes the torque to move the car it is not an issue.

 

Tires are 26" tall and i got the ratio wrong on the limited slip rear i was thinking of using , it's not a 3:92 it's a 3:73 . Mine is a 200/4r too.

 

Sorry i was wrong on the ratio ,it's 3:73 . Thanks

 

I'm not getting 20mpg and before i put the 350 in i had a 4.3L with the same Edelbrock ,headers and it was worse. Plus on hills i ran out of power and had to drop it out of OD.

 

Multiply 3.73 X the overdrive ratio and you will have the final drive ratio.I have a T5 .076 5th gear and the rear is 3.70..076X3.70 = 2.81 final drive.

 

The only downfall might be your engine might not pull much vacuum in OD at speed due to deep throttle opening. Vacuum helps to vaporize fuel at part throttle...And your carb's hi peed or power valve may be open due to the low vacuum. Some report better mileage and better response with slightly steeper gears..but there's always the variables

 

Yeah i wonder if i'm not getting in that power band . I haven't drove it enough to really know what the MPGs are but it's not very good at all. Guessing 15 tops if that much. Thanks

 

Good information , never thought of that . Well i might just go ahead and try the 3:73 gears , then i will see what happens. Would be nice to have the limited slip and it is a 8.8" where the 2:80 one is a 8" . Thanks to everybody for the help and thanks to anybody else the helps. JIM

 

I'm running 4.30's with a 700R4. At 70 mph I'm turning between 2600-2800 rpm. Driving at 55-60 mph I turn about 2200-2400 rpm and get about 18 mpg with 5xx horse.

 

Lugging is hard on parts and mileage.
I can't imagine your engine being built to build much torque @ 1500 rpms but it could have been.
Small runners, small valves, to keep the velocity up and a host of other stuff. A big part of this is cam selection and cylinder pressure at low rpms.

The key is to find out at what rpm your engine's torque curve flattens out at. If you'd look at a dyno graph the torque goes up rapidly then turns and flattens out. Staying on the same rpm as the graph will give you the best performance, mileage , and longevity. You move the cruise rpms with gears and tire size. If you know what's in there , you can plug that info into a computer simulation and get pretty close flywheel numbers.

 

Sounds like great numbers to me. I am replacing my six shooter in my 66 c-10 with a 350 combo and am concerned about my 4:11 rear. I will try it and see how the hiway rpm's look. I am sure I will need to think going for some higher numbers. Good luck dude. ~sololobo~

 

Tire size is also part of the equation. Try this: RPM= MPH X Gear Ratio X 336 / Tire height

Ie. 60 x 2.80 x 336 = 56448 / 26 = 2171 @ 60 with a 0.76 Od 2171 x o.76 = 1650

To find rear end ratio for desired cruising speed the equation is:

Gear ratio =Tire size x cruising rpm /cruise speed x336

 

Just drive faster

 

If your car drives fine I would not change anything. There are plenty of online calculators for tire size/diameter vs gear ratio. When starting from scratch I shoot for 2000 RPM at 60-65 MPH.

 

Man that has to feel like a slug driving around in OD with a 2.80 gear. I would hate that.

 

MPG will depend on several factors so what cam are you running? and what intake and carb?

 

As a general "rule of thumb", the "daily driver" GM overdrive automatics like axle ratios between 3.42 and 4.11.

 

350 Chevy all stock edelbrock carb ????
I can read minds too

 

i don't think so; i have 2.56 rear gear, 235/75 15 tires and turn 1800 @55 and 2100 @ 65.

2) Discuss further; i understand the torque curve thing, but if you're just cruising along at 55-65, then you don't need that much torque?

 

200 4R (.67 od)/3.73 gears/26" tire= 1939 rpms at 60 mph, 2262 at 70 mph.

Sounds like a sweet spot to me.

 

Your car prolly don t weigh 3000 lbs . That's a minor factor but a plus to the whole outlook . Bill aka Tnomoldw

 

I did weigh it once but don't remember it but it was under 3000 lbs. It does not just slug around I can pass traffic at a moments notice with no problems. Response is quick and it turns heads all the time. It was never intended as a race car , just a cruiser around town . It quickly picks up speed with no pinging on reg. gas.