The Dodge Challenger Message Board

Dave,

Following on from the great 'Shifter debate', my attention turns to rear gears. I was sold on a set of 3.91's, but having used your Spreadsheet to check RPM, it seems I'm seriously going to run out of steam with my tyres. 3.55's appear to be a much better proposition.

Just to double- double check, am I safe to assume the 1.08 trans ratio in your calculation is the "D"/ 3rd gear in a 727?

Also, I took a tape measure to my rear tyres last night, and got a diameter of 26.5 inches (this is for 275/60/15's, with good tread). Is your tyre diameter calculation pretty foolproof? It could just be I need 20lbs of air adding to the tyre I was measuring.

I'll stop asking annoying questions (fairly) soon.

Many Thanks

Tim.

ps- God bless your Spreadsheet. Already saved me the cost of a set of gears.

Tim, all depends on your Objective for the car - see the "interesting debate" on the MMA website about Gear selection.

Pete is quite right Tim. But let me expand on that.

The first thing you need to work out is how much power your engine will make and at what RPM it peaks at.

When you have a good idea how much power your engine will make and you know how heavy your car is with you in it you can use my excel spreadsheet to work out what sort of ET and MPH you can expect from your car.

Then your best bet it to combine a rear gear and tyre size that will give you that finish line speed at peak RPM.

The exact tyre size varies a little from manufacturer to manufacturer but if the sizes printed on the sidewall have any relationship with the truth then yes the size calculator is right. It is very hard to measure a tyre when it is on the car. Also what you need to know is what size it is when rotating at 2,000 rpm or so!

The final gearbox trans ratio of 1.08 is to take into account slippage in the torque converter. I have used 8% for this figure but it may be slightly less with a good unit.

Sorry. The wife was hassling me to go get a bottle of wine for dinner so I typed faster than I could think (and I type slow!).

OK what I should have said was you need to have a rear ratio that will get you a little way past peak H.P. at the finish line. How far past depends on how fast torque is falling off. But about where you would change gear at idealy.

If I apply my own theories to my own car I come up with needing something around 4.50:1 gears with my 29" tall slicks rather than the 3.91 I do have.

But my car is not intended for drag use only and it is a long drive to a strip so I want the little extra MPH and lower RPMs the 3.91s give me with the smaller street tyres.

So in effect, because of the tyre sizes, I have lower gears on the street than I do at the track! I might get taller street tyres when these wear out but I do like the tyre wall heights to be the same front and rear.

Another way to look at it is to say to yourself 'how often will this car be on the track'.

Think about what sort of top speed you want and rpm you want to cruise on Motorways at 70mph. What sort of milage will you get?

I think unless you are a serious racer you have to take all these factors into consideration.

Dave, then why did you go for such "Tall" Slicks - were you restricted for choice ?

Or, knowing you, were other aspects factored in ?

I do not know how "Aspect Ratio" works for Slicks.....

Dave,

Your second response was more along my original line of thought on this. I'm working (slowly)towards high- 12 second runs, whilst keeping the car steetable. Based on previous runs, I'm expecting trap speeds in the 110- 115mph area,depending on 60ft times, etc. I don't want to redline the engine at the top end, so the car needs to be able to run 120mph (or more) before it hits 6000RPM.
Also, as you point out, the car's going to get plenty of road use, and the 3.91's only give me 55- 60mph at 3000RPM. On long runs, I've a feeling the drivetrain is going to start getting a bit warm. The 3.55 gears seem more useable, overall.
'Course, this all looks good in print, but I'm still open to the voice(s) of experience.

Thanks again for your comments.

Tim.

I just make things up as I go along me!

My slicks are 28" tall! I remember now. Pete reminded me with his comment because what happened was I wanted 27" (just a fraction over the height of my street tyres) but they don't make them in that size so I had to go to 28" instead. My best gear for the strip works out at around the 4.10:1 mark but I would have to hit over 6500 rpm for sure so I think I will stick with the 3.91s for now. 70mph is about 3650rpm.

I would like to read the article that Pete is talking about but I guess I'm not familiar with the MMA website. could someone elaborate a little for me?

Go back to the title page for this message board. The one with the list of topics. At the very bottom you will find a link to the MMA website. Enter the site and click on 'Members Links'. The MMA message board is at the top of the page. It is exactly the same message system as this one. Go to the technical section and you will find threads on rear gears. Do a word search as well as some of the discussion was in another thread.

before I go off doing something stupid could someone tell me how much (if anything)I will gain by running 3.55 gears instead of 3.23 gears. in terms of 1/4 mile time and speed and the difference in RPM's for daily driving. Did you follow me?

If you read the above thread and the other threads it points to you will see that it all depends at what rpm your peak power happens at.

On my Challenger Home Page you will find a 'Maths Spreadsheet' which is a Microsoft Excel spreadshet containing tables that you can play about with by adding your own figures to see just what you are asking. Try it.

thanks