speedometer accuracy w/larger tires.

[jonas]

running 15's on my '85 brat. just wondering what you guys do to get the speedometer to read accurately, or do you even care?

 

i'm fairly accurate (followed the woman around at various speeds, and just go with traffic) just guessing my speed, but i like things to be working correctly, needed or not. the little things tend to drive me nuts.

[nw_david]

im running 15's too, i just looked up a calculator for specific speeds, like when my spedo hits 75mph im really going 90mph

i compared my 205/65/15's to 170/70/13 (i think thats a close enough size)

http://home.netcom.com/~sgalaba/tiresize.htm

[idosubaru]

speedometers aren't very consistent. i've actually had a number of subaru's that have a MORE accurate speedometer when i increase the tire size. so i wouldn't rely on the tire size calculators for those.

 

best to carefully manage mile markers on a very long road trip or borrow someone's GPS. - mine have varied from 0% - 10% when i increased tire size, but the tire size calculator didn't necessarily predict it correctly.

 

for the highways - you'll notice going 105 miles instead of 100 - that would be about 5% off. meaning when you're speedo shows 50 you're actually going 52.5. do that a couple times and you'll narrow down the percent

[bheinen74]

I think mine shows about 47 or so with my 205/75/15 and that equals about 55. too soon to tell tho for sure. just overlay some stickers over the speedo face for correction lol

[jonas]

well, my speedometer bounces all over the place. really want to replace the cable 'cause it bugs the ************ outta me. now if i can't get it to read accurate, why bother i'm wondering. is there a certain gear/cable i can get to compensate for the tire size?

[TeamCF]

is there a certain gear/cable i can get to compensate for the tire size?

 

I know you can get kits for more popular rigs to adjust the "transmission to speedo" ratio after changing tire size. (or some people just change out the diff grear ratios)

 

But as for Subarus.... we are part of only a small percentage that put bigger tires on them. So Proly no off the shelf fix for it.

 

Could be a cable out of another model that might do the trick. But I don't know for sure.

[ahole]

I run 225/75R14's on my Loyale (26.8") and it's an easy-to-calcuate 10% slow on the speedo. It was 2.5-5% fast from the factory, which means that I don't really have 148,000 on the clock. It's closer to 141,000:banana: WHEEEEEEEE!

[Gloyale]

I have a cool Radar Detector/Jammer that also has GPS and can use the GPS to tell you you're speed.

 

I tested my GL Turbo running 27x8.50/14s and I was about 10% faster than indicated speed.

 

20=22

 

30=33

 

40=44

 

50=55

 

70=77

[edrach]

http://www.miata.net/garage/tirecalc.html

[nw_david]

http://www.miata.net/garage/tirecalc.html

thats a really good one

[The Beast I Drive]

with my 31's, when my speedo shows 55, Im doing about 70. its more accurate at lower speeds.

[lonnie]

today put 195.70.13 on my 85 brat. believe speedo will probably read 7% slow assuming stock tire at 165.70.13. does this sound right and is that the original tire size?

 

ps-do notice a significant improvement in ride quality on the road.

[Greenley]

assuming stock tire at 165.70.13. does this sound right and is that the original tire size?

 

On my '84 & '86 4wd ea81's, the stock tire size is a 185/70/13. I'm sure your brat is the same.

[Phizinza]

I used my GPS to figure out what speed was RPM on my tacho in top gear. As I usually always cruised in 5th even in town. The tacho never lied I found. MY speedo was bouncy and all over the place anyhow. Changed everyday.