Speedo Drive Gear calculations - LOCK PLEASE

[takai]

Ok, just sorting out what speedo drive gear im going to need.

Car setup is as follows:
Gearbox: W58, from GA70 Supra
Diff: G312 == 3.9: ratio
Tyres: 225/50/16
Speedo reading: 60km/h GPS = 53km/h Speedo

Now im pretty sure the GA70 came with a 4.11:1 ratio diff, and as such would be a 33/10 speedo gear.
Im accounting for a few km/h inaccuracy built into the speedo, and as such assuming that 60km/h would be 57-58km/h speedo anyway.

Thinking i would want a 31/10 or 30/10 speedo drive gear to get the speeds roughly right. If anything i would want it to read low, i.e. 60km/h GPS == 62km/h speedo.

Am i thinking right, or have i just got it all fried up?

[PeteH]

Hi,

From reading the above I understand that you want to alter your speedo so that it will read faster than it does now for the same road speed. i.e. currently your speedo reads 53km/hr and you would like it to read 62km/hr for the same road speed of 60km/hr....correct???

If so this is an increase of roughly 17% indicated speed.

To achieve this you would have to change from your current 3.3:1 (33/10) gearing to 2.821:1 gearing. I think the closest available is 2.9091:1 (32/11) which will read 3% slower than you want. This won't be too bad as it should read an indicated 60km/hr at an actual road speed of 60km/hr on the information you supplied.

There might be a bit of rounding error in my calcs but I think it is roughly correct.

Cheers,

Pete

[takai]

Ergh, thats a pain, the 30/10 i was figuring would be close enough, but now it doesnt seem so. Really not overly keen on having to pull the extension housing to swap over the worm gear.

[takai]

Interestingly i just punched the raw data into Norbie's calculator and got this

[PeteH]

Hi,

Yeah Norbie's calculator is an awesome piece of work but I assume it relies on absolutes to reach the end figure. It may not allow for reading error in the speedo or any other vairable that could affect your GPS derived real world result.

In saying that there is the possibility of some error in the GPS you were using. Are you able to re-check your result using another GPS, or check your GPS against a known speedo?

Pete

[takai]

Yeah, GPS is reasonably accurate. Dead on in my wifes car, and pretty accurate before i did all the changes to my car.

Also reasonably accurate against my phone GPS, and a mate's GPS

[PeteH]

Hi,

Seems that your GPS is good enough to rely on then.

So for your result, based on the GPS and speedo reading, you may need something different than Norbie's calculator is telling you.

Also, it'd be good if someone else can do a double check on my calcs and see if I am correct. I can make mistakes.

Pete

[takai]

Yeah, as can i, hence this thread. Thanks for the help with it though, its quite useful. Especially when my brain is fried from too much work, and thinking about suspension design.

[PeteH]

Hi,

No worries.

It has been a nice distraction from the work I have on at the moment.

Pete

[glytch]

Heya

Just looking at that norbie.net speedo calculator - it doesn't seem to take into account rolling circumference of the tyres, just static. rolling circ varies from tyre to tyre but as a general rule 97% of the static circ is usually accepted.

I made a little spreadsheet that takes everything I could possibly think of into account to work out my cars variation (175/80/[email protected] to 205/60/[email protected])

using your info I get 33/10 (3.3:1) as the standard and 35/11 (3.18:1) as the required with almost no difference in real speed.

FYI, run norbie's again with your circ as 1924 (1983.6 * 0.97) and the values are the same as my spreadsheet.

Hope this helps some.

Luke

[PeteH]

Hi Luke,

Can you explain how you arrived at the above figure? I cannot understand how you came to that speedo drive ratio.

The way I interpret what Takai wants to do is change his indicated speed from 53km/hr to 62km/hr which is a change of 17%. To achieve this it would involve a corresponding change of 17% in the speedo drive ratio.

The figure you provide for the speedo drive change is only 3.7% different from the original.

Am I coming at this from the wrong angle?

Thanks,

Pete

[glytch]

Hi Luke,

Can you explain how you arrived at the above figure? I cannot understand how you came to that speedo drive ratio.

The way I interpret what Takai wants to do is change his indicated speed from 53km/hr to 62km/hr which is a change of 17%. To achieve this it would involve a corresponding change of 17% in the speedo drive ratio.

The figure you provide for the speedo drive change is only 3.7% different from the original.

Am I coming at this from the wrong angle?

Thanks,

Pete

Hi Pete -

Short answer - No you are not coming from this from the wrong angle,

Long answer - BUT - your calculations rely on two assumptions - 1) 225/50/16 is the standard rim and tyre size for the car and 2) the original diff really is 4.11.

Now if either of these assumptions are wrong - and I suspect they are - then your original ratio will be off.

The 3.18 speedo ratio only uses the information at hand - ie 637 revs per KM on the speedo, RC of the tyre and a 3.9 diff ratio.

Cheers

Luke

[PeteH]

Hi Luke,

I ignored the tyre size and diff ratio.

I arrived at my answer by taking the speedo and GPS readings supplied in the original post. And that the original speedo gear is 33/10.

The figures for speed supplied by Takai are real and the only assumption made is that the gearbox has a 33/10 speedo drive ratio. Which will be correct if the box is out of a GA70.

Pete

[takai]

Box is definately out of a GA70. I had to help the seller pull it out because he wasnt organised.

The one thing that your (glytch) and Norbies calculators have in common is that they dont take into account the inaccuracy of the OEM speedo in the car.
Ill be doing GPS calculations again in the near future (when i reinstall the exhaust)

[glytch]

Hi Luke,

I ignored the tyre size and diff ratio.

I arrived at my answer by taking the speedo and GPS readings supplied in the original post. And that the original speedo gear is 33/10.

The figures for speed supplied by Takai are real and the only assumption made is that the gearbox has a 33/10 speedo drive ratio. Which will be correct if the box is out of a GA70.

Pete

Hi Pete - Sorry I am not saying that you are assuming anything other than that 33/10 is the correct existing speedo ratio. And yes everything is only based on the information Takai gave us.

The ratio will only be correct if it came from a car with a 4.11 diff on 225/50/16 factory tyres. Norbies axle database doesn't have a GA70 listed but a GA71 seems to have also come with a 3.9, 4.3 and 4.55 diff ratio, so if the gearbox had any of those diffs on it originally then the speedo ratio will not be 33/10

The formula I used to work out the required speedo ratio is this:

speedo ratio = (1000/rolling circ) * Diff ratio / speedorevs

Entering the known info
1000 is meters per KM
Where Rolling circ is in m (ie 1.924)
Speedorevs = 637
Diff ratio = 3.9
Gives a ratio of approx 3.18. This is the ratio required for, everything being as is, the speedo to work correctly.

All I am saying is based upon the calculated speedo ratio required and your calculation using the GPS /speedo reading data, Takai is probably wrong about it having a 4.11 diff originally (or 225/50/16 is not a factory tyre though it seems to be AFAIK)

Cheers

Luke