I now have 3.50 gears in the 32 Roadster. I am thinking of changing to 2.75 to lower RPM. would this be to hard on it in town driving?. How much would RPM drop at 80 MPH?. The car has a th350 and the tires are 235 75R-15.
Need to Know if you are using A regulare or overdrive trans and whatr size(in inches) tires. and could let you know in a couple of hours.
I wouldn't say it'll be "hard" on it - that depends on you!! It will take some fun out of it though. As for the motor speed drop it's pretty much a straight ratio (ignorring parasitic losses) so basically do this: 2.75 divide by 3.5 then multiply by your OLD RPM. 2.75/3.5*rpm 2.75/3.5=.786 Say at 80 mph you were reving 5,000 rpm with the 2.75 gears you'd now be reving 3928 rpm [5,000(.786)] Also consider that your rear gear ratio is directly multiplying your available torque output of the motor. So ruducing your gear by 25% WILL reduce your torque by the same - there's no free lunch.
Figure about a 22% drop in RPM. Take the gear ratio you want and multipy it times the RPM you turn now, then divide that number by the gear you have now and it will tell you your new RPM. For example; 2.75 (new gear) x 3000 (RPM at 80 mph) / 3.5 (old gear) = 2357 RPM. We use this formula in racing all the time, execpt modified to find gear ratio at the RPM we want. It's a simple ratio equation (RPM wanted / RPM at present = Gear needed / Gear at present.
Thanks. learned alot (try to learn something every day) .the road rpm would be great, but how would it run in town?.
In the 'real world',with a 3 speed TH350,the diffrence would be small. Around town you would have to put your foot in it more. I have a 2.70 rear in my A (sbc,. p.g.).I am also using 235/75 15" tires. At 80 m.p.h. the tach is reading 2600 r.p.m. I get about 20 m.p.g. on the highway,but lose the burnout fun Sparky
======== Thats what I want. I dont need to prove anything to the young group. Just try to help when I can. LATER
Why don't you just swap the tranny out to an overdrive? 700 r4 will require a lot of mods but a 2004r (like they used in grand nationals) will fit with only a crossmember move. The overall length is the same so you can run the same d-shaft. Just a thought to ponder.
Skipping the math - which has been adequately explained - imho the 2.75 is too high geared for in-town driving. When I started driving my torquey 462" Buick powered 32 roadster I ran a 2.75 diff with 30 1/2" tall tires. (Your 235's are about 28" tall.) It worked ok, but the gearing was too high and the engine was lugging at in-town speeds. Which was fine with the torquey engine, but the big cam wasn't as happy as it could have been. I went to a 3.70 and the car's been a lot more fun to drive. Even so, the 3.70 with the tall tires is a touch low for highway driving. A 3.50 diff is about a 5% change and that would help during highway driving, but for this car a 3.25 would be just about right. A good compromise for your car would be a 3.00 or 3.25 diff depending on where you do most of your driving - highway or town. If you're running a Ford 9" be aware there are a couple of different pinion lenghths out there. About a 1" difference in length. I had a very nice, electronically balanced driveshaft in the car and when I swapped to the 3.70's I had to build another one because the optimum length was now an inch short. You may be able to make up for the length bit with a pinion yoke swap, but that's not a sure thing far as I know. Just something to be aware of.
you would be more happy with a 3.00 gear than a 2.75.i have made that swap and have been happy with the performance still.any thing less than that would make it feel like a dog on and off the freeway. -danny
My 235X75X15 tire is 29 inches tall.... So 3.50gear X 70mph with a 29 inch tall tire is 2,838 RPM A 2.75 gear X 70mph with a 29 inch tall tire is 2,230 RPM In town at 35 MPH your RPM with the 2.75 would be.. 1,115 RPMS OR sort of sluggish 1st gear with the 3.50 is ( 2.52 X 3.50 ) = 8.83 1st gear with the 2.75 is ( 2.52 X 2.75 ) = 6.93 .
Here's a quick crunch. The program I use is probably a little off it uses a standard diameter for your tire and it varies some by manufaxcturer. @ 80 with a 3.50 gear you should be turning around 3259 RPM with a 2.75 gear you would be truning around 2560 RPM I like a 3.26 gear my self in a lighter vehicle for basic (mixed) driveing around town and hiway that would put you at 3035. But its just a personal preferance. Remember when you are chooseing gears you want to keep your mill in the power band if you want to get decent mileage. I try to keep mine right at the bottom of the torque curve. If you get out of your torque curve or lug you motor you will actually get worse mileage. If you do a lot of around town driveing the higher gear will actually hurt your mileage.
Also, if you find your self geared a little low(lugging the engine) you can always drop it down into second gear for those times when your caught in slow moving traffic. Then just slide it back into drive when the speed picks up.. I find myself doing this occasionally with the 3.08 that's in the '32. I love the highway RPM with this gear as I'm frequently traveling at 70+ on the expressway and it is right in the torgue curve for the cam I'm running. Frank
My '78 malibu I drove to high school had a 2.41 gear from the factory. With a mild-ish 355/th350 and 28" tires, my in town fuel economy almost doubled by switching to a 3.42 gear, and only lost a couple of MPG on the big road.
I have a number of 8" rear ends, that I have had in and out of my 40 Ford coupe many times over the years. I'm running a dual quad, very healthy 355, 350 turbo trans. It's all relative; I have two posi center sections, a 4:11 and a 3:55, the 4:11 is obviously the most fun but the 3:55 is the best all around gear. I also have a 3:00 and a 2:79 open. Until this past year, I had the 4:11 in for a long time, but I never traveled and I purposely put it in to see what the new engine would do at the drag strip. The year before I went to the Nationals at Louisville with the 3:55..................horrible, turning approximately 3100 RPM on the highway, right at the point where my progressive linkage wants to open the front carb, used a bunch of gas unnecessarily. I enjoyed a little performance gain with the 3:00 rather then the 2:79 but when I jerked the 3:55 out I said the hell with it, and put the 2:79 back in. Staying with traffic on the Interstate @ about 70 MPH, I'm turning about 2100 RPMs. I run 235X15X70s, a little shorter then the 75s series tires...we're comperable. The car still runs pretty strong from the bottom up, your roadster is most likely a little lighter then I am, so I would say, put the Bonneville gear in and save some gas money, it's a good trade off with the current cost of gas. Shoe
two cars we have are running those highway gears... 55 Chevy, blown big block, switch pitch TH400 (1500/3000 stall at the flick of a switch on the shifter), 2.75 gears in a 9", engine has a 230 degrees @ .050" lift cam. Works great in town, works great on the highway, in high stall from a stop it will smoke the tires if I give it some gas, in low stall at part throttle it takes off "right now". Gets crappy mileage in town, 12 on the highway. 70 camaro (with traditional aluminum slots), bone stock 250 straight 6, pg, 2.76 rear gears, it's a real dog off the line but eventually gets up to speed. Gets close to 20 mpg. moral of the story? if you run gears in the 2s, then build a torquey motor for it. Or, if you have a torquey motor, you don't need to gear it to have fun on the street. But for Duster's car in the original post I'd run 3.00s.
I'm actually running 3.08s in the Pusher. 28" tire. I turn about 2800 to 3000 on the highway most of the time. Normally get between 16 and 18 depending on the wind conditions etc. Same mileage loaded or not loaded. its actually pretty funny I have a friend that has a '54 chebby truck with a tallish gear an OD and an inline. he gets about 12 not loaded. Gears too tall for the mill. The trick is keeping it in the powerband. If you don't have enough torque to pull the gear its going to do two things for you, trash your mileage and eventually trash your mill.
This is all excellent advice. I started with 3.73's which were strong on the street but hell on the highway. Wanted tall ones like your 2.75 idea but only found a 3.08. Turned out to be a good compromise (but I still wonder if I'd like 3.25's the best??).
I had 3.25s in my 55 for a little while, it's a bit much for a big block on the highway at 75. Would be fine for a smaller engine.
i have the same setup your talking about in my 28 sedan highboy....at 80 or so ....i am usually at 2500ish .....around town (or country) i usually can run 60 to 65 at 2000 or less.....and the car will run 12.60's leaving at 1000 rpm....and shifting at 5000 rpm ...and going thru in 2nd gear.....of course that was with some old plugs and a points distributor that would break up around 4800 or so...i had thought of swapping in a set of 350's....just to try and put it in the 11's.....hahah brandon
Read all post. think I will run 3.00 and not the 2.75. the thing is I can get the 2.75 for $150 cash. still will wait for the 3.00 to come along and run the 3.50 for now. thanks all LATER
I am running a 2.75 9 inch in my 32 roadster. As everyone has mentioned, it is not much for lighting the tires with stop and go traffic. I have kept the gears since I drive to California from Texas every year. For the highway, they keep the rpm's, temp, noise very tolerable. In town there is a compromise. Slight feathering of the clutch is normal. No lighting of the tires.
Coincidence. Going through this exercise with a friends roadster with 29" rear rubber. He's got a built (not race car built) sbf that is doin' nothing, turns out he has a 9" centre with very low 3 gears (doesn't know exactly but we did a rough count turning the wheels). As an experiement he threw on some smaller diameter wheel/tyres off a late model and wow, really woke up the engine, but we can tell there's more there yet. So he's tossing up over a 4:11 or 3:7. Trans is C4. He has to do some thing because it's a slug up to the speed limit and doesn't start to wake up til over 60 mph, at which point, game over.
The lowest gear I have run in my 55 Chevy is 4:88, normally run gears in the mid 5's , course it is a circle track car. You would be surprised how tight you can wind a 250 six banger. Like my wife, she ain't happy unless shes screaming.
The Jalopy Journal
