speedo accuracy

Submitted: Monday, Dec 30, 2002 at 01:00
ThreadID: 2737 Views:1251 Replies:7 FollowUps:7
This Thread has been Archived
G'day all, I've just been trying to work out how much my speedo is out, since i put 285/75/16 tyres on. It originally had ?/70/16 tyres. So I've got 3 questions.(A) How much is my speedo out? (B) Am i driving more klms then the odometer reads? (C) Do I need to tell my insurance company about them? Thanks in advance. Andrew.
Back Expand Un-Read 0 Moderator

Reply By: CJ - Monday, Dec 30, 2002 at 01:00

Monday, Dec 30, 2002 at 01:00
I have checked the accuracy of my my speedo for KPH using my GPS - 7 satellites must be accurate ?? The trip distance I have checked using the 5km roadside distance measure signs that are now on a lot of main highways. My theroy is - if the tyre is bigger you will get a smaller reading over a measured distance. Someone else may be able to confirm this.
AnswerID: 10316

Reply By: rodeoowner - Monday, Dec 30, 2002 at 01:00

Monday, Dec 30, 2002 at 01:00
Mathematically, here goes.....

If you went from 265-70-16 (LC size?) to 285-75-16, then...

you have gone from a tyre diameter of approx. 777mm to 834mm. Divide 834 by 777 and you get approx 1.07.

Therefore you speedo will be approximately 7% out. ie If your speedo says you are travelling at 100kph, you will actually be doing 107kph.

These are round figures, but close enough.

Yes you will be travelling more kms than your odometer says.

Should you contact your insurance company? Up to you. There are different laws in every state. I'll leave this one alone as I'm sure plenty of others will give you their opinion.

PS If you live in Tassie, you are legal!
AnswerID: 10324

Follow Up By: Allyn - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Please correct me if I am wrong but if the new tyre is larger then it is surely going to take longer to complete a full revolution and therefore your speedo will be reading less kilometres than actually travelled.
I have just switched to 285/75R16's and whilst I haven't yet confirmed this with my GPS a recent long journey is indicative that my odometer is reading somewhere around 4% below what it was previously..
0
FollowupID: 5417

Follow Up By: Rodeoowner - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Allyn, I don't pretend to be an expert. I wen throo this when I changed tyre size. My understanding is that the speedo/odometer measures a tyre/wheel revolution. If it measures 1 revolution, with the tyre size larger by 7%, then the actual distance travelled is 7% more. The 7% figure is purely a mathematical calculation. That figure will change slightly again with tyre wear.
0
FollowupID: 5420

Follow Up By: Rodeoowner - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Allyn, I just read your reply again. We actually agree with each other!
0
FollowupID: 5421

Follow Up By: Andrew - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Thanks guys, spot on, just the insurance query know!
0
FollowupID: 5427

Reply By: Wayne - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Hi Andrew,
You could try this site out for your tire calcs. "www.miata.net/garage/tirecalc.html" Don't know about the insurance query.
Ciao,
Wayne.
AnswerID: 10342

Follow Up By: Andrew - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Cheers Wayne, I looked up the site and it confirms the other responses, good way to keep the klms down on the old speedo!
0
FollowupID: 5426

Reply By: Truckster - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Do you know someone with a GPS???

Take them for a drive, and mark with Liquid paper the correct markings on the speedo for the correct speeds.. Otherwise get it recalibrated

GQ/GU's you can buy different gears for the speedo for under $20 to correct your speedo. Change them on the side of the gearbox.
AnswerID: 10354

Follow Up By: Andrew - Tuesday, Dec 31, 2002 at 01:00

Tuesday, Dec 31, 2002 at 01:00
Thanks everyone. Truckster, no i dont know anyone with a gps unfortunatly. However the tyres are on a gq and i would be interested in changing the gears to correct the speedo. Where do you get them from cheaply? are they easy to change? Is it possible to compare a 5th gear RPM of a standard gq set up, to mine? If so mine is a gq 2.8 td and at 100 kph does 2600 rpm. 110 kph does 2800 rpm. Does this sound close to other 2.8 gq's? Happy new year everyone!!!
0
FollowupID: 5423

Reply By: Truckster - Wednesday, Jan 01, 2003 at 01:00

Wednesday, Jan 01, 2003 at 01:00
Andrew,

Nissan Dealers sell the gears, Ive posted here the conversion table before, so have a search of the archives.

they are supposed to be 20 min job to change. I havent done it yet, too lazy!
AnswerID: 10393

Reply By: John - Thursday, Jan 02, 2003 at 01:00

Thursday, Jan 02, 2003 at 01:00
Andrew
How do you find the 285/75/16 (33") on the poor old 2.8. With my 32" tyres she runs away a bit on hills. Not real happy about it. Was pretty good with 31". Checked my speed with a GPS just 2 days ago. With my 32s at 100k on the clock the GPS said 102k. BUT at 60k on the clock the GPS said almost 65k. The indicative and true speeds do not seem to be linear. Be aware of this if you go for one of the $20 gears Truckster was talking about. May make one speed accurate but the others still may not be. If I went by the 60k check and bought a gear I would be underspeed at 100k. Do your GPS checks and just remember how far out you are for 60/80/100 etc.
AnswerID: 10434

Follow Up By: Andrew - Thursday, Jan 02, 2003 at 01:00

Thursday, Jan 02, 2003 at 01:00
G'day John,
Most of the time the 33's are good. It can make hill starts a bit of an effort. I have found myself needing to use the brakes more than i would like in low range 1st, because of the extra run-on. But the advantages of higher ground clearance and better top end gearing probably out way the cons. 6 for one, 1/2 dozen for others! I'll probably leave the speedo gearing as is, as i said previously, good way to keep the klms down!
0
FollowupID: 5483

Reply By: Truckster - Thursday, Jan 02, 2003 at 01:00

Thursday, Jan 02, 2003 at 01:00
Andrew,

the Rockhopper low range gears and things have come down in price a lot now, www.bigballsoffroad.com have them cheaper. They were $3200! now $1500 I think.

And I know what you mean, I run 33's and 35's, downhills are eye opening...
AnswerID: 10445

Sponsored Links

Popular Products (13)