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PostPosted: September 26, 2021, 11:13 am 
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As the sage Barbie once said, "Math is hard!" Well, it is for me anyway.. I'd think this would be straightforward, but I'll ask anyway to be sure (I've screwed up several times in the past trying to predict final RPMs) :roll: ...

I'm trying to calculate what my 'new' 6th-gear RPMs will be on a few different sets of tires.. everything else (motor/trans/diff ratio) will stay the same, so it's only tire size swaps that I'm looking at for this exercise... Here's where I stand with my BEC today:

--Current setup has me at 7,000 RPMs at 70 mph on 21.54" diameter tires...

--So I'm trying to figure my new RPMs at 70 mph on 22.29" diameter tires(?)

--And just for grins, what if I run 19.89" diameter tires... what RPM at 70 then?

So this simply boils down to a percentage of change, yes? i.e., increase tire size by 4%, and then <decrease> RPMs by the same 4%?

Thanks guys-

--ccrunner

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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

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"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: September 26, 2021, 3:10 pm 
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Yes, it's just not quite 4%. Closer to 3.48%.

22.29"= 6764.4 rpm
19.89" = 7580 rpm

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PostPosted: September 26, 2021, 5:02 pm 
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This is my go-to site for speed vs rpm calculations versus trying to do the math myself: http://www.apexgarage.com/tech/gear_ratios.shtml

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PostPosted: September 26, 2021, 6:18 pm 
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Just what I needed, and as always, very helpful.. thanks guys :cheers:

--ccrunner

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Dean

1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com


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PostPosted: September 27, 2021, 12:47 pm 
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Joined: April 23, 2006, 8:26 pm
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Keep in mind it's the rolling diameter that's important. It's doubly important on lightweight cars like Sevens because we run really low tire pressure. Because of that, the "diameter" of the tire will be about 1/2" smaller, and that's what should be used in the calculations. Measuring from the ground to the wheel center will be accurate.

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PostPosted: September 27, 2021, 7:42 pm 
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I'm not sure how much the od would grow (to inflate the flat spot-radials don't grow) at speed with heat.
I've used the formulas at face value and the speeds have been spot-on when compared to gps in a straight line and those mobile radar speed warnings.

A non lockup automatic typically looses about 5 mph through a standard converter.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: September 28, 2021, 12:45 pm 
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My go-to site is: http://www.grimmjeeper.com/gears.html

Also lets you play around with rear diffs and transmissions.

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PostPosted: September 28, 2021, 5:24 pm 
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I wrote a program and posted it here some years back. You are welcome to use it as you wish. You can find it here. And you will have to unzip it, as we don't allow excel spreadsheets to be posted directly here.

viewtopic.php?f=4&t=16175&p=179181#p179181

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