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MG MGB GT V8 Factory Originals Technical - Rear End Ratios ...Again

Got my car on the road the other day and i'm pretty sure the rear end is about to fail!
I have the T-5 with .63 Overdrive, 15" wheels.I have a Ford 8" Rear end but the gears are 2.80.....Bit low i think. Looked through the archives and it seems the concensus points to 3.23 ish. Any ideas?
Dave Ross

dave, IMHO, i have a similar situation, but suggest you determine the Outside Diameter (OD) of your tires in inches and then you can choose the ratio that suits your needs.. i have 205-50-16 tires (24.07" OD) and run a 3.55 trac-loc in my 8.8 Ford. this puts the RPM at 2500 doing 80MPH on Interstate. My overdrive is also .63 in T-5Z.
i initally chose a 3.31 or 3.27 due to the std. T-5 ratio of .68 but replaced the trans due to a problem.
i made a chart (spreadsheet) of probable ratios, speeds, and RPM and chose on that basis.
Formula used: Engine RPM= V x T x R x N / 60
V= MPH, T =tire revs per mile (5280/tire dia x.2618), R = R/E ratio, N= final drive ratio
another factor to consider is around town driving, need enough gear that you can run 40 -50 MPH w/O bucking & jumping. i use 4 th. for this and then all is ok. the 3.27 or 3.31 would be too high in my application.
Good luck and great cruising!
kelly stevenson

Dave,
It all depends on the motor you are running and how much power, where the power comes in and what you want to do- best cruise or best acceleration.

Our standard V6 conversions, when using the same 5th gear ratio you have are best around 3.5 to 3.7 whereas using the V6 T5 Camaro with a .72 5th gear ratio work best with about a 3.2 to 3.4 ratio.

If you have a stock BOP/R that is not built up for high revving and this is just a driver, not a drag racer, then anywhere from 3.2 to a 3.7 would be in order. The 2.8 to 1 is better then the 3.9 to 1, but your 2.8 is terribly high (speed) and very low (geared) for 98% of the conversions done. I (guessing what motor you have!) would think a 3.3 or 3.4 would be good to go and allow great acceleration.

-BMC.
BMC Brian McCullough

I have found the free program published by Autodyne/Medatronics Corporation to be of great help.

It was written for users of their 5 speed conversions, but allows you to enter ALL the variables, such as:-

Gear ratios
Axle ratio
Overdrive ratio
Wheel size
Tyre aspect ratio

It enables you to calculate the RPM at any selected speed in each gear & store & print results.

I can be downloaded here:-

http://www.5speeds.com/rpm.htm





Nigel Steward

I'm running a 3.9 liter fuel injected (hotwire) engine with 9:1 compression and a fast road cam. Ported and polished heads and the RPI Optimax chip.
3.5 gears seem to be good. The stock 3.9 gears are to low, first gear is good for tractor pull events! Thanks for your input.
DTR Ross

Thank you Nigel this is very helpful info.
DTR Ross

You got some pretty good advice.
What really determines the axle ratio is the cam specs.
Your spec sheet should have a cruising rpm minimum rpm
Then figure out the minimum hwy speed 65? then use the formula that Kelly gave you.

If you cam is a 2200 with a minimum of 2000 from then the parameter is from 2200 to 2400 rpm <> for hwy use, then choose a gear ratio that puts you in this range
Bill Guzman

I have a 3.4 trac loc in my 8 inch,15 inch wheels,rover 5speed box,ratio is just fine.
Dave Lowe

I put together a similar spreedsheet myself a few years ago comparing the V6 & V* t5 boxes available. You can alster everything: wheel size, tire aspect ratio, rear end ratio, gear box ratios and it will give you a table indicating speed in gear in 1000 rpm increments for every gear.

http://mysite.verizon.net/vze2mtva/mgbgtrestoration/id3.html

You need Microsoft Excel to use it. Feel free to save it for your own use.

Regards,

Brian C.
Brian Corrigan

This thread was discussed between 15/05/2006 and 19/05/2006

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