Tire Size / Speedometer Question

[Gun Runner 5]

My HJ75 was imported with 15" rims and highly worn tires. I replaced them with 16" rims and I now run
235/85/16 tires. I passed two radar speed signs today ( courtesy of the local police department ) and at 110 kph the radar clocked me at 59-60 mph. My question, would the 16" rims and tires make that much difference or do I have a bogus speedometer?

 

[Shes Mad]

Your tires are now 31.7 inches tall. What size are the stock tires.

Should be able to do the math for the correction. Don't give me any of that rural Georgia upbringing either

My 33X10.50X15 tires yield 60MPH when my speedo registers 105 KPH. Should be going 65 MPH at 105 KPH. Yours speedo appears off by more than mine

 

[Gun Runner 5]

Your tires are now 31.7 inches tall. What size are the stock tires.

Should be able to do the math for the correction. Don't give me any of that rural Georgia upbringing either

My 33X10.50X15 tires yield 60MPH when my speedo registers 105 KPH. Should be going 65 MPH at 105 KPH. Yours speedo appears off by more than mine

I honestly don't remember what size tires the truck originally had because they were worn and dry rotted so they were quickly discarded. I would assume 29"-30 tires on 15"rims. So I guess I am now running 31.7" tires on 16" rims.

 

[John Galt]

Going from 15" tires to 16" tires will increase the speed by about 10%.
90kph on an accurate speedometer would be about 100 kph or 62 mph on 16s.
Ask the local PD where their spedo test interval is located, or find a fairly straight stretch of highway and measure the time between a few mileposts while holding a constant 100 kph indicated. 1 minute per mile is 60 mph.

 

[chappohj47]

I run 32 11.5 r15 and my speedo is out so 70kms on speedo is 75kms on a hj75

 

[Gun Runner 5]

Your tires are now 31.7 inches tall. What size are the stock tires.

Should be able to do the math for the correction. Don't give me any of that rural Georgia upbringing either

My 33X10.50X15 tires yield 60MPH when my speedo registers 105 KPH. Should be going 65 MPH at 105 KPH. Yours speedo appears off by more than mine

You're at 60 MPH with an indication of 105 KPH....
I'm at 60 MPH with an indication of 110 KPH .......
I guess I won't get many speeding tickets.

 

[cruiserdan]

Wheel diameter does not matter. It's the tire diameter that changes things.

The most prevelant size back then was 235/75/15. That tire averages around 29 inches tall. As already mentioned your current tire is 31.7 inches tall. That is roughly a 9% difference. The net result is your speedo (if it was "right" to begin with) should be reading 9% slower than your actual speed. An indicated 60 KPH should be traveling just over 65KPH actual speed.

 

[Gun Runner 5]

Wheel diameter does not matter. It's the tire diameter that changes things.

The most prevelant size back then was 235/75/15. That tire averages around 29 inches tall. As already mentioned your current tire is 31.7 inches tall. That is roughly a 9% difference. The net result is your speedo (if it was "right" to begin with) should be reading 9% slower than your actual speed. An indicated 60 KPH should be traveling just over 65KPH actual speed.

Based on the above information, my speedometer is only a couple of MPH's off.
Thanks.....

 

[89LandCruiser]

Www.4lo.com has a calculator

 

[Gun Runner 5]