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Author: Subject: Gear ratios
J666AYP

posted on 15/4/20 at 12:04 AM Reply With Quote
Gear ratios

Anyone clued up on gear ratios? Have had a look online but its all technical mumbo jumbo that makes no sense to me.

And more importantly what effect does reducing wheel diamiter have on gearing/acceleration?

My gearbox ratios are:
1 - 3.86:1
2 - 2.18:1
3 - 1.38:1
4 - 1.00:1
5 - 0.80:1

Final drive 3.46:1

RPM @ 120kph: 2,800

The donor was built with 17's but it's now on 15's so I'm guessing it will accelerate faster and probably give me inaccurate readings on the speedo unless I go for a race type speedo that I can program for wheel diameter?

Also will the change in wheel diameter screw up my revs at speed? Don't fancy having the engine screaming at me at motorway speeds. Probably overthinking it but I have only increased wheel diameter in the past never reduced it.

Does not involve spark plugs or body filler so I'm pretty clueless.

Jay

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PorkChop

posted on 15/4/20 at 12:43 AM Reply With Quote
All gears do is multiply or divide the speed of your input (i.e. engine rpm) to give you an output (i.e. wheel rpm and therefore vehicle speed as your know your tyre circumference). That's it.

You're missing 3 pieces of info to answer your questions:

Max engine rpm
17" tyre size
15" tyre size

I threw a quick calculator together in Excel a year or two ago, dusted it off, plugged in your numbers and got:

Assuming 225/45/17 tyres, 7000 max rpm, top speeds in each gear are:

1 - 39 mph
2 - 69 mph
3 - 109 mph
4 - 150 mph
5 - 188 mph

With 120 kph coming in at 2779 rpm in 5th.

Changing to 195/50/15 tyres changes this to:

1 - 35 mph
2 - 63 mph
3 - 99 mph
4 - 137 mph
5 - 171 mph

With 120 kph coming in at 3061 rpm in 5th.

Your speedo will probably need recalibrating for IVA or you fit a programmable one like you said. If you have a speedo cable, it should be a straightforward job to get the original speedo recalibrated. However, I would check that the speedo cable routing in the car will not give you problems in the future by wearing / breaking easily.

HTH

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J666AYP

posted on 15/4/20 at 01:06 AM Reply With Quote
quote:
Originally posted by PorkChop
All gears do is multiply or divide the speed of your input (i.e. engine rpm) to give you an output (i.e. wheel rpm and therefore vehicle speed as your know your tyre circumference). That's it.

You're missing 3 pieces of info to answer your questions:

Max engine rpm
17" tyre size
15" tyre size

I threw a quick calculator together in Excel a year or two ago, dusted it off, plugged in your numbers and got:

Assuming 225/45/17 tyres, 7000 max rpm, top speeds in each gear are:

1 - 39 mph
2 - 69 mph
3 - 109 mph
4 - 150 mph
5 - 188 mph

With 120 kph coming in at 2779 rpm in 5th.

Changing to 195/50/15 tyres changes this to:

1 - 35 mph
2 - 63 mph
3 - 99 mph
4 - 137 mph
5 - 171 mph

With 120 kph coming in at 3061 rpm in 5th.

Your speedo will probably need recalibrating for IVA or you fit a programmable one like you said. If you have a speedo cable, it should be a straightforward job to get the original speedo recalibrated. However, I would check that the speedo cable routing in the car will not give you problems in the future by wearing / breaking easily.

HTH


That's perfect thank you. Everything I have found online looked like a maths lesson lol.

And as for the bits I missed, max RPM will be 7000 or somewhere in that ballpark. The standard 17's were 225/45/17, And on the 7 they will be 205/40/15 or 205/45/15 depending on tyre availability.

Thank you for the input.

Jay

[Edited on 15/4/20 by J666AYP]

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mcerd1

posted on 15/4/20 at 10:33 AM Reply With Quote
their are a few different spreadsheets around that can do it but I still like this little program:

https://locost7.info/gearcalc.php




but if all your doing is changing the tyre size then you can just do it on the % difference in the tyre size
225/45 R17 gives a approx. rolling diameter of 634.3mm

based on: (25.4*17)+(225*.45*2) - or in English the 17" converted to mm (x25.4) + twice the sidewall height, which is 0.45x the width (0.45 as its a 45% profile tyre)

so the others work out like this:
205/40 R15 -> (15*25.4)+(205*.40*2) = 545.0mm
205/45 R15 -> (15*25.4)+(205*.45*2) = 565.5mm



so for the difference these will make:
205/40 R15 -> 545.0 / 634.3 = 0.859 (or 85.9%)
205/45 R15 -> 565.5 / 634.3 = 0.891 (or 89.1%)

so then just pick any engine speed and gear that it was doing before and multiply it by the ratio to get your new road speed like so: (just picking your number from above)

for the 205/40R15 if you were doing 120kph in 5th and the engine was doing 2800rpm before then at the same engine speed you'd only be doing 0.859*120 = 103kph
or the other way round to do 120mph in 5th would mean the engine speed would need to be 2800/0.859 = 3260rpm


if you have actual data for the tyres your going to buy you can just sub that in instead of calculating it - doesn't matter if its rolling diameter, radius or circumference as long as you compare apples to apples

and of course none of this accounts for the power curve of your engine...

[Edited on 15/4/2020 by mcerd1]





-

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mcerd1

posted on 15/4/20 at 11:03 AM Reply With Quote
or in picture format:

original 225/45 R17




and the 205/40 R15 and 205/45 R15







-

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53Stix

posted on 15/4/20 at 12:17 PM Reply With Quote
I believe the most useful interpretation of this data is what's known as the Tractive Effort graph, which effectively scales your engine's torque output in each gear to the road speed.

It's reasonably straight forward to calculate or if it's helpful I can provide my own spreadsheet for you to plug your numbers in to.

The added bonus of this information is that it allows you to take a more informed view on gear change points for each gear.

Hope this helps!

-Rob

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J666AYP

posted on 15/4/20 at 03:50 PM Reply With Quote
Thanks gents, has put things into perspective for me.

My main aim was to eliminate the need for 3 gear changes before 60 as changing up at 50ish would bug the hell out of me.

Now have a good idea of what I need to work towards.

Jay

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J666AYP

posted on 15/4/20 at 03:52 PM Reply With Quote
quote:
Originally posted by 53Stix
I believe the most useful interpretation of this data is what's known as the Tractive Effort graph, which effectively scales your engine's torque output in each gear to the road speed.

It's reasonably straight forward to calculate or if it's helpful I can provide my own spreadsheet for you to plug your numbers in to.

The added bonus of this information is that it allows you to take a more informed view on gear change points for each gear.

Hope this helps!

-Rob


Rob,

That would be very helpful mate, thank you.

Jay

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53Stix

posted on 15/4/20 at 03:55 PM Reply With Quote
quote:
Originally posted by J666AYP
quote:
Originally posted by 53Stix
I believe the most useful interpretation of this data is what's known as the Tractive Effort graph, which effectively scales your engine's torque output in each gear to the road speed.

It's reasonably straight forward to calculate or if it's helpful I can provide my own spreadsheet for you to plug your numbers in to.

The added bonus of this information is that it allows you to take a more informed view on gear change points for each gear.

Hope this helps!

-Rob


Rob,

That would be very helpful mate, thank you.

Jay


Drop me a PM with your email address and I'll fire it over some time this evening.

-Rob

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J666AYP

posted on 15/4/20 at 03:59 PM Reply With Quote
quote:
Originally posted by 53Stix
quote:
Originally posted by J666AYP
quote:
Originally posted by 53Stix
I believe the most useful interpretation of this data is what's known as the Tractive Effort graph, which effectively scales your engine's torque output in each gear to the road speed.

It's reasonably straight forward to calculate or if it's helpful I can provide my own spreadsheet for you to plug your numbers in to.

The added bonus of this information is that it allows you to take a more informed view on gear change points for each gear.

Hope this helps!

-Rob


Rob,

That would be very helpful mate, thank you.

Jay


Drop me a PM with your email address and I'll fire it over some time this evening.

-Rob


Done mate , cheers

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53Stix

posted on 16/4/20 at 10:29 AM Reply With Quote
For anyone that's interested I've hosted my calc here so that people can take a copy and play with it.

For some reason the Tractive Effort graph itself doesn't seem to appear (for me) in the Google Docs version so do take a copy to get the full functionality.

-Rob

[Edited on 16/4/20 by 53Stix]

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