Diff Ratio

[DN2168]

Hi all,
         At the moment I am sticking with the 205/60/13's that are on the car. My question is what would be a better diff ratio to run with these tyres? The car is tapped out at 100kph. The speedo is miles out, at 65mph it is doing 90kph.

I was thinking 3.36 or maybe 3.08. I just don't want to be wringing its neck the whole way to Orange & back (diff change is cheaper than engine rebuild).

What are your thoughts?

Dean.

[EffCee]

Dean,

There has been other posts in relation to this, it seems that the 3:36 is the diff of choice, though some do run the 3:08 and 3:55.

I had a 3:36 in my old FC many years ago and found that it worked fine, didn't like hills too much though, but hey that's what a gearbox is for.

My personal opinion is if the engine has plenty of get up and go, put in the 3:36 if it is a bit tired, then drop in a 3:55.

Keith

[DN2168]

Thanks Keith,
     the main part I am trying to work out is diff ratio in relation to tyre circuference. The 205/60 are 22.75", a 185/80 is 24.7" or there abouts. I just don't to go too far with my choice.

Dean.

[EffCee]

Dean,

I have attached acalculator to help out with working out tyre size and final diff ratio with engine RPM

Hope this helps

http://www.boosttown.com/gearbox_differential/speed_calculator.php

Keith

[CraigA]

For general use 3.36 is the right ratio for those tyres.

3.55 is OK as well but can become tiresome on freeway running.

[DN2168]

OK thanks guys, looks like 3.36 it is  .

Dean.

[CraigA]

Good choice for either a red or grey powered car using that tyre size.

My FC uses a 3.36 with a 149 red and its perfectly suited to the 175/75/13 tyre size. It cruises efffortlessly at 70mph and pulls easily in top gear from under 20mph. 

My XU1 uses a 3.55 with 175/70/13 tyres and its ok for zapping around but revs at about 3000rpm at 60mph.

[DN2168]

Just to throw a spanner in the works, I just got a flat tyre (I changed them on the rims & when the guy took them off he said a couple weren't too good in the bead area).

SO, looks like I am buying a new set of 185/70/13's. Would 3.36 still suit or should I go with 3.55. I was thinking 3.55 may be better with the taller tyre, or would 3.36 still be OK seeing as it is a bit smaller circumference than the standard size tyres the car came with.

The car will be mainly used on the open road.

Sorry for all the questions but I only want to change the diff gears once.

Dean.

[CraigA]

What engine? Redo grey, stock or modified?

[CraigA]

Try again.

What engine? Red or grey, stock or modified?

[freddyc]

dean I have 185/80 on my cars, wagon is slightly modified with 3:36 gears goes great. Sedan has tired std motor with 3:55 gears it also goes good but refs a bit on highway.

[Sheriff]

just a word of warning before you change your diff over, and don't make the same stupid mistake I did.   I assumed my car had the standard diff and bought a 3.55 and had it fitted.  When I picked it up mechanic asked me why I changed it, I said to lower the revs on the highway, and he replied that it already had a 3.55 in it, so pays to check...doh...   I have 185/70s on the car and still feel it revs a bit too much at highway speed, so will be changing the rears to 185/80s at first opportunity.  Mike

[NO NAME]

i have a red motor with a tri matic, 185/70/13, 3.08
 ive found taking off it looses traction and revs out if you are not gentle on the throttle but then when it gets traction the revs drop a little and then pull back up very quick then 2nd goes quick and top gear.
 i assumed 3.08 would be to high for take off but accelerating from 30-40 kph sometimes creates tyre smoke and accelerating at highway speeds shows a big rise in the front left corner of the car,
 i was thinking of fitting 185/80/13

[DN2168]

Craig,

motor is standard grey with extractors & 186 carby.

Dean.

[CraigA]

Then I would lean towards the 3.55 if you are using the 185/70/13 tyres.

The 3.36 will work but I reckon will blunt the 'performance' too much.

Others may have as different view or expereince to offer.

[FB_MAD]

If its of any help comparing different rolling tyre and rim combinations, the original 13 inch rims with crossplies had a 24 inch circumference.

I'm with Craig, 3.36 seems to be the limit with a grey especially with the original grey crash box as I find with the gearbox ratio's in the grey 3 speed taken into consideration and with these  differences between each gear, and any taller diff gearing than a 3.36 tends to be too wide for the engine to smoothly handle around town.

On the highway is different and you could ( maybe   ) stretch to a 3.08 but we don't always do highway miles do we and I think a grey with 3.08 would probably have to have a more higher throttle to maintain any given speed,therefore whilst revs would be down with a 3.08 you would have to maintain a higher throttle position and any fuel savings advantage with a 3.08 will be pretty much null and voided .

I have an FB sedan with standard 149 bored to 167 and 3 speed crashbox with 13's and 3.08 diff and I find the 3.08 is about the limit for my combo to smoothly handle around town driving but is very happy on the highway.

Modern tyres in 13 inch are usually around 22 inch ( 70 profile ) or so rolling diameter ( compared to the 24 inch R.D original crossplies ) and most blokes say that the 22 inch with 3.55 gives around the same revs and speedo accuracy as the original 24 inch tyres and 3.89 stock diff.

There are a few different ratio calculators out there and its fun to play with them.They'll give you revs versus speed with any given rolling tyre diameter entered, I'd put one up here but I can't bloody find the web address ATM but the one given above is a good one.

If you can get the 185/80's x 13  and they are in the rolling diameter of 24.7 as you said then that is near enough RD to the original combo so any change with that to either 3.55 or 3.36 will make a noticeable rev difference at a given speed.

You'l probably be looking around a 300 to 400 RPM drop with a 3.55 @ 60 mph and approx 600 to 700 rpm drop @ 60 mph with a 3.36 ( without working out on the calculator )

[DN2168]

3.55 it is .

This way I will be putting a higher ratio diff as well as taller tyres, but not going over the top.

I would like to thank all those who gave input, that is what makes this forum such a great place to hang out.

Cheers,
          Dean.

[DN2168]

3.55 is in & the world is a happy place. 

Dean.