Slant Six Forum

5 speed gear ratios vs tire size
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Author:  gearhead [ Thu May 16, 2019 4:14 pm ]
Post subject:  5 speed gear ratios vs tire size

First, I am going to say sorry for all my long posts...that said, here is another one

This post is to help me make some of the great many decisions I need to make over the next few weeks, and hopefully lead to some feedback and information from other /6ers that have swapped to a 5 speed already.

In order to even get started upgrading my Valiants drivetrain, I need to decide which wheels, tires, transmission, rear gear ratio, flywheel, clutch, pressure plate, and bell housing I'm going to use for the build.

I've more or less decided to have Quicktime build me a custom bell housing, and I'm hoping to make it our dream bell housing and maybe even a group buy to bring the price down. That however, is a different post...this one is to help me choose the wheel and tire size, and which transmission and rear end gears for the project.

I recently acquired a Tremec 3650 transmission in like new condition from a low milage(38K) Mustang GT, trading some tuning work on a "real" 427 Cobra which belongs to the owner of the big Ford dealership in Salem, Oregon. Side note...on an engine stand in the garage next to the Cobra, was one of the ultra rare 427 dual overhead cam motors. Now that was cool to see and inspect. Anyway, I'm into this trans for enjoying an afternoon playing with someone elses million dollar hot rod, so I'm not married to it as a choice for this project. However, the car was owned by a little old lady and the transmission doesn't look like its ever seen a rainy day...and that is rare in Oregon.

So now the purpose of this post. I'm trying to find out what trans and rear end gears to use, and I'd like some input from those here who have swapped to a 5 speed already. I have searched the board and the internet for a few weeks researching the subject, and found many threads where these factors are discussed, but none that made me feel confident enough to make a decision without more input and study.

I am mostly concerned with the 1st gear ratio(3.38),(although the spread is important as well) relative to the overdrive ratio (.68). I'm worried that to get the cruise rpm I want, 1st gear will be too low. While surfing for info, an article was found that was pretty thorough, but then at the end made a claim that annoyed me to no end...they said that for a performance vehicle, one should shoot for a 1st gear final drive ratio of about 10:1. OK, this is fine, but what about tire size? If your car has 24" tires and 10:1 works fantastic for you, then if you install 26" tires, you've effectively changed it to 9.23:1.

So ultimately, I found a cool calculator that plots rpm vs. speed for all the ratios in your transmission, and I used that to make some comparisons. But first things first, because I don't want to make the same mistake twice. The first mistake was 20 years ago when I ordered a set of 15x7 sbp wheels for the Valiant, it never occurred to me that when I went to buy tires, the sizes I wanted weren't even available to buy, or were prohibitively expensive. The biggest tire I could find in 50 series was 225/50/15, and then only three companies make that size. Although I have been happy with the 225/50's

So this time I went to tirerack and made a chart of different tire sizes, calculate the diameter, and made notes about availability.

Here is the chart:


So one of the most important things when choosing a gear ratio is tire rollout (dia.*PI=circumfirence). Rollout is critical because it is the distance the car travels per axle revolution, so for any given gear ratios in your drivetrain, it always ends up in axle revolutions. If you have a short tire, you will travel a short distance per axle rev, a tall tire travels further. As an example of the effect of tire size, Imagine I have 2.94 gears and changed my current tire from 23.86" diameter(225/50/15) to 25.68"(245/45/17). Since the base circle of the tires is effectively the same as changing the base circle of a gear set, you can take the percentage difference in tire diameter and multiply it by the gear ratio to get the comparison. 23.86/25.68=.929 or .071% change which can be seen in the chart. 2.94*.929=2.73. A change from my old tires to new is the same as swapping to 2.73 gears.

Another way to look at it with a milder change in diameter, (there is still a significant affect on gearing) would be a change from 225/50/15 to 235/45/17. This would be 23.86 dia. and 25.33 dia. for a change of 1.47" in diameter and .058%. 1-.058 = .942, so if you were running 3.73 gears before the change it would be the equivalent of 3.73*.942=3.51. That tire change would be more of a change than buying a 3.55 gear set!

Now comes the difficult part, and I have to make many decisions at once or I might end up wasting much time and money.

I currently have sbp front disk brakes(KH) and a spb 8.75" 2.94 rearend in the car. I have a set of 3.23's but am willing to buy whatever gear set I need and sell the others to make this work. I would really like to run 16" wheels but the only good selection of tire is for one size, 245/50/16, which I might prefer because I am willing to give up a little handling for a more classic muscle car look. However, I would have to have custom wheels made unless I switch to large bolt pattern, and I'm not opposed to switching but have some questions perhaps for another thread. If I have custom wheels made, then I have a limited selection of 17" wheels should I decide to try them...if I switch to large bolt pattern I have better options...but...back to gear ratios.

After I had the tire diameters, I started playing with the gear ratio calculator. I started with a plot of what is in the car now. The plots will all show the rpm at 75mph, and the mph at 5500 rpm shift for each gear in the trans.


As you can see, driving to California on I-5 and trying to keep up with traffic taxes my poor little half million miler. As measured, my tires are actually a little shorter than theoretical, and I'm actually going 72 mph @ 3100 rpm as verified by my home made GPS speedometer and speed warning signs in the Siskiyou and Shasta mountains.

I am also worried about turning enough rpm at cruise speeds to get off the cam and run efficiently for fuel economy, so if I choose a 245/50/16 tire, I will need 3.73 gears to get into the range for fuel economy with the cam in my long term engine goals. The cam will be in the high 270 deg. @ .020", around 245 @ .050", based on previous experience, I believe I will need to turn at least 2200 or more rpm to get into the cams working range and not still loping.

In fact, this morning I went to check out one of my customers cars. Its a 1960 Vette with a built 383 and they put way too much cam in it for what he wanted. I built the distributor and he is disappointed with the fuel economy (14 mpg). It has 3.55 gears and relatively tall tires, with the Tremec 5 spd, he is only turning 1800 to 2000 at typical freeway speeds. I think it is still loping and the big holley can't generate an efficient signal at the venturi until higher rpm. I told him if he drove faster he'd probably get better mileage. Anyway, he is having an oxygen sensor bung welded into it today so I can hook up my wide band sensor and we can figure out whats really going on...I'll try to find out what gears are in his Tremec and tire size when I see him in a day or two to test the car. Then I can plot his and see what it looks like...

With a 245/50/16 tire and wheel combo, I will need 3.73 gears to turn 2500 rpm at 75 mph as can be seen in this plot.


One thing to notice is the 10 mph change between the first gear shift points. My little engine has never seen the high side of 4600, cam is long out of breath by then...

The final drive ratio of my current setup is 3.09*2.94=9.085, the tremec with 3.73's would be 3.38*3.73=12.61...However, corrected for the change in tire size to compare it to my original setup becomes this...23.86/25.65=.07, 1-.07=.93, 12.61*.93=11.72

That's a significantly lower first gear than I have now, even though the taller tires helped. Hmmm

So my next plot was based on tire diameter vs. tire availabilty vs. gears that might reduce the first gear ratio. If I just go with 17" wheels and the many tire choices available, then the choice of 235/45/17 tires and 3.55 gears yields this plot...


This shows that the first gear final drive ratio is still quite a bit lower than my current setup, because it is only 2 more mph at 5500 rpm, however, the overdrive would place me in nirvana. The math shows it all:

Final drive ratio = 3.38*3.55 = 11.999 or 12:1.

Tire drive ratio correction is from the chart at .058% 1-.058=.942. This means my current tire is 94.2% as large as the new tire. Multiplying the final drive ratio of 12:1 by 94.2% is the same as subtracting .058% of 12:1 from 12:1.(or 12-(12*.058). Anyway...12*.942= 11.3:1

I'm out of time and this is too long a post as it is...but I hope this provides some food for thought.

Of my many worries, I am often hung up on rate of change of rpm, and how it relates to engine design and tuning. The gears in the transmission, of course, other than power, determine the rate of change of engine rpm in each pass down the track or main straight. It is relatively easy to build an engine that performs well in the higher gears, where the engine is loaded heavy(such as dyno pulls), and just as easy to have that same engine perform poorly in gears where the rate of change of rpm is significantly greater. Hence the reason I'm worried about the 1st gear ratio, and the slope of the graph relative to the other gears.

There are some possible different choices for gears in this transmission, but not nearly as many as other tranny's.

What I would be interested in most, and I think based on the forums threads I've searched, many others would be interested in, is some real world data from others on this board.

It would be nice to know, what rear end gears, transmission gears, and tire sizes are making people happy or unhappy for any reason. I could plot and post what is brought to attention, and then maybe we can form a consensus, or at least guidelines, for what works well in different applications.

In my next post I will link the gear ratio site. But to post the plots, I must take a screen shot, then photoshop to edit unwanted info on screen, then convert to jpg and post.

So if any of you post your gear and tire data, I will plot it and post it here in this thread.

Got to go...Blazers starting game two soon.


Author:  Rick Covalt [ Thu May 16, 2019 6:29 pm ]
Post subject:  Re: 5 speed gear ratios vs tire size

Great to see you posting Gearhead!

Just a little personal opinion on the Belhousing side. If you get a custom one done be sure they make it for use with a 10 1/2: clutch. There are no clutches worth a hoot in the 9 1/2 - 10" range for a slant 6. Going to the V8 size flywheel with the high slant starter would be ideal.

Good luck!

Author:  sixinthehead [ Thu May 16, 2019 8:26 pm ]
Post subject:  Re: 5 speed gear ratios vs tire size

Now, There is some thorough planning :)

Here are a few opinions and observations:

-Be careful not to paralyze yourself with decimal points.
-Pick the tire size first based on fitment, look 8) , and availability.
-Then pick an axle ratio based on making your highest transmission gear useful. This depends on vehicle weight, torque curve, local topography, and driving style. Let the lower gears fall where they may.

My experience was with a 65 Valiant and a mild 170 (BBD, Comp 252).
I put an A535 (3.79, 2.29, 1.48, 1.00, .7) from a Dakota in it and tried 2.76, 3.23, and 3.55 gears behind it. I had 225/50/15 tires on cop rims on the back.
All the gears were useful with any of the chosen ratios, but the 3.55s were 'track' oriented and the 2.76 were more 'street', just as you might expect. The short tires helped the 2.76s at cruise.
Smaller or worn out engines like/need help getting off the line, so the lower first gears as well as the closer gear spacing commonly found in 5-speeds make for a more enjoyable (and, dare I say, "modern") driving experience.
Oh, and better burnouts :twisted:

Author:  Dart270 [ Fri May 17, 2019 3:55 am ]
Post subject:  Re: 5 speed gear ratios vs tire size

I would run 3.55s or 3.23s. You would probably prefer 3.55s, Karl. They will be fine for your mild motor and will work well with a hotter one too. You will likely want to go to 16 or 17" tires/rims since 15s are getting more scarce. You can get quite cheap 16X7" or 16X7.5" rims for late 90s to 2010 or so Mustangs or Ranger/Explorer. They have nice offset and work well with 225/50-16s. Just a suggestion.


Author:  Exner Geek [ Fri May 17, 2019 8:29 am ]
Post subject:  Re: 5 speed gear ratios vs tire size

I didn't take the time to try to read everything you have written here, but if your objective is to have a car that will be fast on the 1/4 mile as well as cruise all day, I would try to trade the transmission you have for one with a 2.92 first gear which also came on Mustangs of that vintage. It has better spacing between gears to minimize RPM drops, assuming you are going to use the first four gears on the drag strip. The big drop going into 5th isn't that important.

Author:  Joshie225 [ Fri May 17, 2019 9:38 pm ]
Post subject:  Re: 5 speed gear ratios vs tire size

Okay, some generalities...

The 10:1 overall low-gear torque multiplication works well for small-block V8 cars with average tire diameters. Average being 25-28". A 236/60-15 is a 26.1" tire so let's use a 26" tire for the sake of discussion. I also want to say that most HP small block cars didn't get this much torque multiplication as delivered. Try 2.66 1st gear and 3.23 axle for 8.6:1 in low.

If we factor in tire size as you have the 10:1 divided by the tire radius in inches of 13" gives a factor of .77. Higher than that is more torque multiplication, lower=less. 12:1 gearing is .923" with the same 13" radius, 26" diameter tire. And I consider that for a light street car to be as much torque multiplication as you want. Any more means very little time in 1st gear and maybe too much wheel spin.

Some real world examples...

4-speed slants got a 3.09 1st gear, 3.23 axle ratio and 7.00-13 tires about 24.8" in diameter. That's 9.98:1 overall in low factored to .805 so a little more torque than the example above, but also only 215 gross ft/lbs torque from a 225.

My 3600 lb Brand C car has 25.6" tires and an overall 1st gear ratio of 9.246:1. A factor of .722 so not far off the previous baseline factor of .77. The car develops 330 ft/lbs torque so it moves out smartly.

My 4000 lb Brand N pickup has 29" tires and a 14.64:1 overall low gear. That's a factor of 1.01, a relatively low 1st gear which makes a lot of sense in a pickup that only makes 171 ft/lbs torque. It's not fast.

Author:  Dart270 [ Sat May 18, 2019 3:07 am ]
Post subject:  Re: 5 speed gear ratios vs tire size

I have run 3.91 gears in my 64 Dart forever with the 2.95 1st T5 trans. Works great. It has the higher 5th (0.63). With 3.55s you should have similar performance. Seymour is right you have a bigger 1-2 drop and 2.95 1st is better, but 3.38 is not bad. I ran the 3.36 1st T5 for a while and did not feel it was a big deal. Yes, 3.38 with 3.55 or shorter gears will feel low, so maybe 3.23s are better.

As was well said above, choose your tires based on the overall car performance/appearance desires, get the diameter, then decide on rear gear. If you end up with 24" tires, put in 3.23s. If you have 26", put in 3.55s.


Author:  sixinthehead [ Sat May 18, 2019 4:54 am ]
Post subject:  Re: 5 speed gear ratios vs tire size

1995 Dakota Magnum 318 (295 lb/ft @3000), NP3500 (4.0, 2.3, 1.4, 1, .73), 3.23, 215/75-15 (27.5 dia).
First gear is very useful even in normal driving, even though it seems too low 'by the numbers'.

Most modern vehicles (with a zillion gears) have very low first gear ratios.
Don't be afraid of it :wink:

Author:  gearhead [ Wed May 22, 2019 7:15 pm ]
Post subject:  Re: 5 speed gear ratios vs tire size

Hi Guys, sorry for the delay in posting, but this week turned crazy. My dad had to go back in the hospital and I have been coordinating with my sisters how best to deal with the situation. I'm going to have to go back to Ca. early next week, and I need to fix our Camry(needs timing chain) so I don't blow another transmission adapter worth of cash on a rental car.... It seems like every time I sit down for a minute, the phone rings with a new demand on my time.

That said, I did do some graphics for the gear sets posted, and have been thinking quite a bit about what I see. Also, I went and tuned on the 60 Vette after installing my wide band O2 sensor. Turned out to be nothing related to gear ratios or cam loping...just way too rich. Idling at 11.5, part throttle at 12-13, down the freeway at 12.5. I changed the jets from 68 to 65(Quick Fuel carb) and the power valve from 6.5 to 5.0, readjusted the idle mixture, and the car came around and ran fantastic. Idles at 13.7-14, part throttle accel 13.5-14, full throttle a bit rich at 12.5, but cruise just right at 14.5-15.5.

Anyhow, I will post the graphics I did for the various gear combinations mentioned, there is definitely food for thought here, but I have no time for my usual dissertations. Just wanted everyone to know I haven't abandoned this thread.

So Sixinthehead posted the various ratios he has used in his truck with a much lower first gear than most cars. I did calculate the tire diameter for his ND3500 graphic, he said his tire size was 27.5 but the math says different. He is more than likely right, since my own tire measurements don't reflect the math, however, for consistency, I am using only calculated diameters based on standard tire sizing. The first three graphics reflect the 225/50/15 tires he said he was running at the time.

Here is his plot while running 2.76 gears...


And here with 3.23 gears...


And here with 3.55's...


And here with 3.23 gears and a NP3500 tranny...


Here is an interesting comparison, my 70 Chev 3/4 ton with the truck 4spd. Meaning unusable for daily driving granny gear in first. I'm stuck driving this every day so I no what low gears are...


And last but not least, Lou's ratios. He didn't give a tire size, but I know from various posts I've read that he runs 225/45/17 at least some of the time, so I used that here. Lou, if you think a different tire size would be a better representation, just let me know and I will change it or add another graphic...


I've been thinking hard on all of this, and have more to say, but I just don't have time at the moment and must get moving again, got to run to NW Portland and borrow my bosses large bead blast cabinet for a job needs finishing yesterday...

Here is the link to the calculator, I'm sure there are many different ones, but I found this one and liked the presentation...

Gear ratio calculator

Wish I had more time, but this week has really been taxing...


Author:  sixinthehead [ Wed May 22, 2019 7:35 pm ]
Post subject:  Re: 5 speed gear ratios vs tire size

Sorry to hear that things have been crazy for you.
Hope it gets better.

Just for clarity, my second post was/is my current '95 Dakota but the first was about the Valiant that had a different ('89) Dakota trans and the various axle ratios.
I just mentioned the truck's factory setup for comparison and a subjective impression of a low (but not 'granny' range) first gear.

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