Speed sensor gear for bigger wheels?

[BossGreen]

I've been doing a ton of searching trying to find the proper way to recalibrate the speedometer/odometer for when I upgrade to bigger wheels and tires and all I've come up with is just people asking the location of the speed sensor and where to buy them. The most information I've came up with is to replace the speed sensor gear with one with a different number of teeth on the gear. I haven't be able to track down anywhere I could order a different speed sensor gear besides an oem one which does not help. Maybe I'm completely wrong and there's a different way to do this? If it helps it's going to be on an auto 1998 spyder gs-t

 

[91-GS]

How much of a difference will there be between outside diameter of stock tire VS new tires? Does not really matter if the wheel is larger. As long as tire diameter is the same, it will roll same distance per turn, and will give same speed/distance reading.

Let's say new tire is 2 inches larger in diameter than the old one. That means it will travel about 6 more inches per turn (diameter * Pi). That's extra foot every two miles, or 30 feel every 60 miles (60 MPH being 'average' travel speed on most highways). That extra 30 feet will throw off your speed by -0.006 MPH.

Personally, that does not sound like enough of a difference to worry about. Even if the diameter is larger by 6 inches, that's still only -0.02 MPH difference.

 

[boost97gst]

Something to note as well: Most cars that ive driven when compared to a gps speedo, are going SLOWER than what is indicated on the dash. Motorcycle mfrs were especially notorious for doing this so they could sell the "fastest" 600 or 1000 or whatever. You get the point. Speedos in cars are optimistic too, just not as much. Check it and seewhat you find.

 

[Seamus]

How much same speed/distance reading.

Let's say new tire is 2 inches larger in diameter than the old one. That means it will travel about 6 more inches per turn (diameter * Pi). That's extra foot every two miles, or 30 feel every 60 miles (60 MPH being 'average' travel speed on most highways). That extra 30 feet will throw off your .[/QUOTE

That can't be right. Having you OD change by 2 inches is a lot.

 

[2slow4now]

There are bunches of tire calculators to figure out what size tires you need for bigger wheels.
Tire Dimensions Made Simple - Discount Tire

For bigger wheels you go with a lower profile.
Stock size was 205/55/16. If you go with an 18" then a 225/40/18 would only have your mph off by 1/2 mph.
You could even adjust the needle on the speedo. Many have done that when they have taken apart the instrument cluster for swapping out gauge faces, changing colors, etc. Would I recommend doing that just to get within a few mph on your speedo due to different tire sizes? Nope not worth the trouble.

 

[boost97gst]

2" overall diameter increase is actually 5mph difference.

I run 235/40/18s. According to that size calculator im off by 1.37mph at 65.

 

[19Eclipse90]

The formula for circumference is 2(Pi)R not (Pi)D.

2(pi)R = (pi)D

It's early, we know.

 

[BossGreen]

Basically I'm just trying to go from the stock 16's on the spyder to a larger set of 17's or 18's but don't want my speedo terribly off.

There are bunches of tire calculators to figure out what size tires you need for bigger wheels.
Tire Dimensions Made Simple - Discount Tire

For bigger wheels you go with a lower profile.
Stock size was 205/55/16. If you go with an 18" then a 225/40/18 would only have your mph off by 1/2 mph.
You could even adjust the needle on the speedo. Many have done that when they have taken apart the instrument cluster for swapping out gauge faces, changing colors, etc. Would I recommend doing that just to get within a few mph on your speedo due to different tire sizes? Nope not worth the trouble.

Thanks that helps a lot! I don't know much about tires/wheels so I was thinking there'd still be like a 5 mph difference no matter what tires I put on 18" wheels. And wouldn't adjusting the speedo only work accurately at the speed you set it at?

 

[boost97gst]

^^^ Yes, most likely its non linear. Meaning it might be off 2mph at 30, 5mph at 60, and 9mph at 100. These are just hypothetical numbers. Even with 18s and a tire like mine i believe the inherent "optimism" of the factory speedo would actually FIX the speedo. Meaning at 70 you are actually doing 70. With stock diameter 70 might be more like 67 or thereabouts. I know brand new cars ive rented on business trips recently are dead on gps speed vs indicated speed but all my other cars are off with stock diameter tires. The 06 lincoln and the 99 expedition i have. I think it was a trend in all cars for awhile and only the newest have dead accurate speedos.
Just my opinion from what ive actually tested and measured.

 

[91-GS]

My calculations are way off. So much for doing math at 3 in the morning.

 

[2slow4now]

This tire calculator is pretty slick and it shows a picture comparison too.

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

 

[auker1]

When I got my car i was told the speedo was off by 10 mph because of bigger wheels. I have 18" rims with 225/40/18r coopers on and i tested the speedo with gps and it reads ten over. Online calculators confirm this. The weird thing is that my turbo timer reads the correct speed. Its not that it was setup that way cause even when i unhook my battery and reset everything, it still reads correct. Also on an episode of "Mighty Car Mods" on youtube (ik they are weird but do some cool stuff) they installed a speedo corrector what was made by Jaycar. Idk if its just in that country or if its in the states too.