Rear Gear Ratio

[Nikotrans]

I'm doing a build on 1957 Chevy stepside short bed. I have a URE 450 HP 383 engine ordered and was looking at a TKO 600 5 speed tranny:

First Gear Ratio: 2.87:1
Second Gear Ratio: 1.89:1
Third Gear Ratio: 1.28:1
Fourth Gear Ratio: 1.00:1
Fifth Gear Ratio: 0.64:1

I was considering 18 - 20 inch wheels.

My question is what would be a good rear end ratio for good performance and decent gas mileage?

Thanks in advance.

[ol_skool_chevy]

in the 3.50- 3.73 range would be my guess

Im sure others will have .02 worth also

[NEWFISHER]

That 383 makes good torque, so in my opinion 3:50 for a cruiser and easier on fuel unless you are going to tow with it and then the 3:73.

[OrrieG]

This link will help, you put in variables and it solves for engine rpm, mph, trans ratio, gear ratio and tire diameter. It does not account for wind resistance, etc. as speed increases. I don't know if it account for auto vs manual differences.

http://www.csgnetwork.com/speedcalc.html

[dadz34]

Wheel diameter plays very little in the calculations. However overall tire diameter is the missing key variable you need to consider. Not to be cliche, but this is "where the rubber meets the road". Your final drive ratio is the only consideration you need to measure in order to meet your goals of "efficient drivability".

The previous mentioned website is a good way to calculate multiple options. I created an Excel workbook to do this 14-15 years ago when determining the optimum rear end ratio to install for my '34 Ford Tudor behind a 383 small block with a ZF-6 Speed (.49 gear or 51% reduction...29.5" tire, 4.11 gears, 100mph = 2,300rpms). You really need to decide what your typical driving will be (highway, street light-to-street light, or light cruising). Then compare your engine design (torque motor or RPM) and add the complimenting rear end ratio which will make the most of your choice. The higher torque you have (383, 396, 402), the lower your tranny final gear can be. In contrast, the more de-stroked your engine is (302, 327, etc), the higher your final gear should be because your engine won't be able to handle the load effectively and will force you to up shift or you will dangerously "lug" the engine.

Kudos to you for asking now rather than later. Most realize too late they made the wrong choice.
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[_Ogre]

driving a task force truck is like driving a brick down the road, no aerodynamics at all.
i am able to get 20.1 mpg with mine using a ramjet 350, 700r4 and a 3.27 rearend i also run a tall P235/70R15 tire
usually a 383 is not good for mpg, efi will get better mileage than a carb, so will keeping your foot out of it

[Nikotrans]

I appreciate all of the great posts. Based on inputs, and general consensus, I went with a 3.50 rear end ratio.

I hope with this rear gear ratio and my engine and trans combination I’ll get good performance and decent fuel economy for my weekend cruiser.

[dadz34]

Let me know how tall your tire is and I will tell you exactly what RPM you will run based upon that rear end ratio.
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[judd55]

Perhaps too late to weigh in on this now, it sounds like you have pulled the trigger already ...............but just in case.

I had 3:42's with a 700R and went to 3:73's ........... I find the drivability improved and did not affect mileage one way or the other. I found unless doing freeway type speeds (70 and up), it tended to lug in O.D. and I would find myself knocking it down a gear. Now I find it in the sweet spot more often in OD at normal hwy speeds (50 and up)...........

I based my decision on my 55 Ponch, it had a a T-5, 5 speed with a 3:70 set in it. I found it to be a perfect combo, lots of pull in the lower gears, would bury the needle in OD. and get around 19 MPG if I drove sensibly (which was seldom the case)

You will have a final gear ratio of 2.24 with 3:50's or 2.38 with 3:73's, little more than 5% difference, (give or take a little, depending upon tire size) those both are still very long legs, the only reason I didn't go with 4:11's is the 700R has a really deep first gear 3.06 (I don't think it would spend much time in that gear) You on the other hand would be able to get away with it, beings how the TKO has a 2.87 first gear and .64 OD compared to the 700R's .70 OD ............ 4:11's would still yield a 2.63 final drive, almost identical to my 3:73's at 2.61

Let's think about this a moment, lots cars in the 60's came with 3:73's in 4 spd cars with no O/D and auto's with 3.08's for mileage, you will have better than that with the TKO/4:11 combo and even more fun down stairs than the old Muscle cars with their 2.20 or so 1st gears (You will have the best of both worlds, that's what the TKO's are made for) Yah, It will still be drivable with the 3:50's but it will suck big time in O/D unless your on the freeway doing 80 MPH all day long....... Trust me, 3:50's are on the low side for a .64 O/D, they may even be a detriment to mileage by not putting your motor in the sweet spot at moderate hwy spds in O/D......... lugging is not good.

Just asking here ..........Is mileage really that big of a deal here, is this a daily driver? If it's not........... I wouldn't let mileage alone be a deciding factor in gear choice, driveability, sweet spot and the fun factor would be my main focus, besides mileage obviousy wasn't a factor in the engine choice (450 hp 383's are not generally reknowned for their fuel conservation)

I suppose the question is , where do you do most of your driving, If it's Hwy I'd do the 3:73 ................around town and back country roads 4:11's but that's me !!!

YMMV but don't let that stop you from having a blast

That's my 2 cents for what it's worth ............

Cheers, Judd

[derrickmanx1]

Originally Posted by judd55

Perhaps too late to weigh in on this now, it sounds like you have pulled the trigger already ...............but just in case.

I had 3:42's with a 700R and went to 3:73's ........... I find the drivability improved and did not affect mileage one way or the other. I found unless doing freeway type speeds (70 and up), it tended to lug in O.D. and I would find myself knocking it down a gear. Now I find it in the sweet spot more often in OD at normal hwy speeds (50 and up)...........

I based my decision on my 55 Ponch, it had a a T-5, 5 speed with a 3:70 set in it. I found it to be a perfect combo, lots of pull in the lower gears, would bury the needle in OD. and get around 19 MPG if I drove sensibly (which was seldom the case)

You will have a final gear ratio of 2.24 with 3:50's or 2.38 with 3:73's, little more than 5% difference, (give or take a little, depending upon tire size) those both are still very long legs, the only reason I didn't go with 4:11's is the 700R has a really deep first gear 3.06 (I don't think it would spend much time in that gear) You on the other hand would be able to get away with it, beings how the TKO has a 2.87 first gear and .64 OD compared to the 700R's .70 OD ............ 4:11's would still yield a 2.63 final drive, almost identical to my 3:73's at 2.61

Let's think about this a moment, lots cars in the 60's came with 3:73's in 4 spd cars with no O/D and auto's with 3.08's for mileage, you will have better than that with the TKO/4:11 combo and even more fun down stairs than the old Muscle cars with their 2.20 or so 1st gears (You will have the best of both worlds, that's what the TKO's are made for) Yah, It will still be drivable with the 3:50's but it will suck big time in O/D unless your on the freeway doing 80 MPH all day long....... Trust me, 3:50's are on the low side for a .64 O/D, they may even be a detriment to mileage by not putting your motor in the sweet spot at moderate hwy spds in O/D......... lugging is not good.

Just asking here ..........Is mileage really that big of a deal here, is this a daily driver? If it's not........... I wouldn't let mileage alone be a deciding factor in gear choice, driveability, sweet spot and the fun factor would be my main focus, besides mileage obviousy wasn't a factor in the engine choice (450 hp 383's are not generally reknowned for their fuel conservation)

I suppose the question is , where do you do most of your driving, If it's Hwy I'd do the 3:73 ................around town and back country roads 4:11's but that's me !!!

YMMV but don't let that stop you from having a blast

That's my 2 cents for what it's worth ............

Cheers, Judd

What he said...... but 4.11's will be more fun.

[bk2life]

Originally Posted by Nikotrans

I was considering 18 - 20 inch wheels..

how tall are the tires? wheel size means nothing.

and how much are you realisticaly going to drivet it? 3mpgs sucks, but if you drive the truck 500 miles per year, 3mpgs isnt a big deal..

[solidaxel]

I have one of each and what you have chosen will work great.

The TKO will be a great selection with your engine and your tire bill will be as much as the cost of gas in Cali. just make sure it all hooks up and have fun.

I do like the 4L60E better than the TKO because it selects gears better and faster than I can think, and it is always in the right gear !!!

[_Ogre]

Originally Posted by bk2life

... but if you drive the truck 500 miles per year, 3mpgs isnt a big deal..

any one that only drives their classic 500 miles per year should sell it

[Stepside Jim]

I have to agree with the 3.50 gear.

Reason being.... I have a Chevy car, I used a turbo 350 for years it has a 3.55 rear gear.

I was ready to go to a standard transmission, I was going to settle with a Super T-10. As I did some studying I saw where there was a install kit for the TKO transmission to fit my car. Hmmmm. about another $1000.00.

Best extra $1000.00 I ever spent.

I compared the first gear of the turbo 350 at about 2.50 ratio. The TKO trans I bought was 3.27. I went to the Kiesler engineering site and they had a chart for comparison. This ratio change gave me the first gear ratio comparable to a 4.56 rear end. This has totally changed my burn out ability like never before. Some will say it requires too much shifting, just start out in second gear I say.

Now when you get to the over drive, no comparison..... I was at 2200 rpm around 65 mph before. Now at 2200 rpm I'm around 90 mph.

What I'm saying is... the range of the 5 speed trans transforms the 3.50 rear end to a great drag rear then to a great highway gear. You should be pleased with your choice.

[Nikotrans]

Well, I did pull the trigger on the rear end ration (3.50), and was ready to pull the trigger on the TKO 600 with .64 overdrive, but after talking to the Hurst Driveline technician about the transmission, I’m thinking I should have gone with a higher rear end ratio (3.70 or 3.90).

His concern is that with a standard 255/70 15 tire and my 3.50 rear end, my rpms would be around 1800 at 70 mph – a little low in his opinion – even with the Chevy 383 450/450 stroker engine. At 1800 rpm the engine might bog down during typical highway driving in overdrive – something I’d like to avoid.

Part of my problem is I haven’t settled on rims and tires yet. I want to fill up the wheel wells as best I can (without tubbing) and look like a “muscle truck” with a slight rake (wider tires and higher in rear). Yeah, I know I was asked about tire size before, but I thought I could worry about that later rather than sooner. It seems I didn’t give tire size the due attention it deserved when putting together my power train combo.

So, until I figure out my tire size, I’m putting the transmission on hold. I’d also like to wait for the dyno sheet on my engine so I’ll have a better idea of my engines sweet spot (I should have that next week). Once I have those pieces to the puzzle, I might have to ask my rear end builder to swap my rear end gears to something a little higher to get closer to an rpm range that my engine will perform best in.

The Hurst Driveline technician said I could go with a TKO 600 with .82 overdrive to compensate for my existing 3.50 rear end ratio, but without knowing what my tire size is going to be, it still might not be the optimum setup.

Having said all that, I would appreciate your thoughts about wheel/tire sizes to get the look I’m after and maximize performance. Then I’ll reengage the rear end issue.

Thanks for listening.

(This build is becoming twice as hard and twice as expensive – why do I still enjoy doing it?)

[solidaxel]

You will be fine with the 3:50 gear.

Just remember you are one shift from FUN with a 5 speed.

It will be much easier to change rear tires (height) than rear end gears IF you find your first choice not to your likeing.

With all that HP you will be changing tires very soon anyway!!!

Go with your first choice you will be happy as most of your driving will be in 4th gear in the city, I do not go into 5th until at least 55 MPH, by then I am always looking in the rear view mirror!

[bowt1ed]

Got any pics?

[solidaxel]

Originally Posted by bowt1ed

Got any pics?

Of what, a gear ratio?

[bowt1ed]

...or maybe Nikotrans' truck....

[Nikotrans]

I only have pictures of an old truck my dad owned and then passed on to me on my 16th birthday. As you can see, the truck is currently in pieces. When I first got the truck, my dad helped me put in a small block 283 and a 4 spd T-10 so I could give it a little more attitude.

I drove it for a few years, but being a teenager, those years were rough on the truck. I did my best to maintain it, but it accumulated a few dings and dents and eventually experienced some major engine trouble. By this time I was married, had a house, and other vehicles to drive, so the old truck was parked in my garage.

While the truck sat in the garage, my father and I made plans to do a complete restoration – some needed parts were acquired, but other things in life seemed to fill our time and kept us from getting serious about working on the old truck.

After my father passed, I lost all interest in getting the truck restored. It sat for a few more years before I began to think about restoring it. Actually, it was my son who got me motivated to do something with the old truck. He kept asking if I was going to anything with it. If I wasn’t, then he would. Then it hit me – instead of being a project for my dad and me, it would now be a project for my son and me. With a renewed purpose and commitment, my son and I began the project. We initially thought about an original restoration, but since the original engine and transmission were long gone, we decided on a restification – an old looking truck with plenty of muscle and modern amenities.

As you can see, we discovered lots of rust, lots of dents, and lots other issues during the build. We currently have an assortment of old parts lying around the garage and many new parts still in boxes. This build is taking twice as much work and costing twice as much as we initially envisioned. Because this build went beyond our expertise, we also enlisted the help of a local restoration shop to keep us on the right track. However, as with most of the projects on this site, this project is a labor of love – a true second generation father and son bonding project.

To get back to the original topic of this post, I’m still trying to finalize the tire size and rear end ratio of this build, so here’s another question for those of you who have completed projects. What’s the widest tire I can run on the rear without tubbing or fender modifications? I was thinking about running 295 /45R /20s. As I mentioned, I’m looking for the old school muscle car look – wider in the back with a slight rake.

OBTW – doing this build with my son has helped us bond, but it has also led to a lot of spirited discussions and compromises. For example, he wanted a slammed truck on 22s. I said I’d go with 20s if it would sit like a muscle car. Color choice is also a good discussion, but its all part of the process. I can certainly live with these minor disagreements if it means staying close to my son (and in honor of my late father).


What the truck looked like after pulling it out of the garage.

Behind the cab rust.

New panal installed.

Rear view of the cab with new lower panels installed.

[Nikotrans]

More pictures of my ongoing build.


Old firewall.


New firewall.


Semi boxed frame with TCI front end installed.

[Nikotrans]

New parts yet to be installed.


New 19 gallon side fill aluminum gas tank.


New firewall mounted 4 disc brake master cylinder/booster.


New 4 core aluminum radiator.


New rear fenders.

As I said, this project is a labor of love – a true second generation father and son bonding project. We hope to have it completed sometime next year.

[solidaxel]

WOW, you bit hard !!

I have had all sizes on the rear of my AD's and I found the 275 to rub inside and outside of the rear fenders and box, the 255 gives me the best clearence (with a rear sway bar) on the bed and fenders with 8" wheels.
Those trucks are very light on the rear so a LOT of tire will help with the HP.