Speedo inaccurate before or after or always?

[BeardedMarine]

I wish I had checked the speedo before I got new wheels and tires. I replaced the 17in wheels with 18in. At first I tried 235-60-18 but didn't like them with a 7in wide rim, but the speedo was somewhat accurate with them. So I put some 225-60-18 tires on which are the same height as the 225-65-17 but now the speedo is off by 3mph at 70mph. Has it always been off? I dunno. Anyone else have this problem? Are there and speedo correction units available?

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[SP1966]

Speedos expose it all, there is nothing hidden... Oh, wait you meant...

 

[BeardedMarine]

Speedos expose it all, there is nothing hidden... Oh, wait you meant...

 

[Cactus Mav]

Maybe can be adjusted with Forscan?

 

[paneubert]

Maybe can be adjusted with Forscan?

It can......to a limited extent. You adjust the tire parameters in ForScan, not the speedometer parameters. But if you enter a tire value that is outside of the range they like, it causes all sorts of errors and drama.

 

[Mike1597]

 

[BeardedMarine]

But, I have 225-60-18 tires on there. Almost identical to stock.

 

[Carlitos_92]

Are you high, or low? My FX4 reads high by ~1 mph at 40mph, based on stationary radar, which rounds up to more or less 3%. I have seen the same trend at double that speed.

 

[Mike1597]

But, I have 225-60-18 tires on there. Almost identical to stock.

Yes, you are correct, I read your tire size wrong.
Also, tread wear will affect speedo accuracy.

Different brand tires can have different revolutions per mile for the same size tire. If you look at the tire specs on Tire Rack you will see the manufacturers stated rev/mile.

From Tire Rack

Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

 

[MakinDoForNow]

But, I have 225-60-18 tires on there. Almost identical to stock.

Generally the speedo will report mph higher than actual. Many drive 2-4 mph faster than posted speed limits to correct across different brands. The posed charts also may differ in that the tire is measured on a specific rim (a tire mounted on 18"x7" rim will not have exactly same circumference as on a 18"x8" rim.). Plus the tire is most likely measured at max inflation pressure (40 psi on a 225/60/r18 primacy but a 235/60/r18 is a light truck tire size and has max inflation of 51psi. Those vary by tire manufacturers). But rev per mile will vary slightly if you are running 35 psi and tire was measured at 40 psi. It is probably likely there is a correction field somewhere in programming that when the mounted tire size/brand is entered that "multiplier" for that size is used by computer to calculate speedo read out. The computer then could even calculate change in tire pressure without having tpms readout. Also correct for tread wear. My 225/60/18 primacy are about 3+/32" tread and my readout mph is off about 4 but I don't check it I just drive.

 

[BeardedMarine]

Are you high, or low? My FX4 reads high by ~1 mph at 40mph, based on stationary radar, which rounds up to more or less 3%. I have seen the same trend at double that speed.

It reads low, so the GPS says 70 the speedo says 67.

 

[BeardedMarine]

Yes, you are correct, I read your tire size wrong.
Also, tread wear will affect speedo accuracy.

Different brand tires can have different revolutions per mile for the same size tire. If you look at the tire specs on Tire Rack you will see the manufacturers stated rev/mile.

From Tire Rack

Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

Generally the speedo will report mph higher than actual. Many drive 2-4 mph faster than posted speed limits to correct across different brands. The posed charts also may differ in that the tire is measured on a specific rim (a tire mounted on 18"x7" rim will not have exactly same circumference as on a 18"x8" rim.). Plus the tire is most likely measured at max inflation pressure (40 psi on a 225/60/r18 primacy but a 235/60/r18 is a light truck tire size and has max inflation of 51psi. Those vary by tire manufacturers). But rev per mile will vary slightly if you are running 35 psi and tire was measured at 40 psi. It is probably likely there is a correction field somewhere in programming that when the mounted tire size/brand is entered that "multiplier" for that size is used by computer to calculate speedo read out. The computer then could even calculate change in tire pressure without having tpms readout. Also correct for tread wear. My 225/60/18 primacy are about 3+/32" tread and my readout mph is off about 4 but I don't check it I just drive.

I still have the old tires and rims. I'll do some measurements tomorrow. Thanks for all the info.

 

[Joe Strummer]

Speedos expose it all, there is nothing hidden... Oh, wait you meant...

Or budgie Smugglers as they are known in the UK.

 

[HeyBales]

I've found in Forscan Lite for display 3 different VSS readings off different modules.

1 matches the speedo display.
1 matches the displayed speed on cruise (XLT so no ACC), which doesn't match the speedo, low by 1-2 mph depending on speed.
1 matches one of those, can't recall which.

And sadly haven't done GPS speed to see which is closest.
Stock tires.

 

[BeardedMarine]

I've found in Forscan Lite for display 3 different VSS readings off different modules.

1 matches the speedo display.
1 matches the displayed speed on cruise (XLT so no ACC), which doesn't match the speedo, low by 1-2 mph depending on speed.
1 matches one of those, can't recall which.

And sadly haven't done GPS speed to see which is closest.
Stock tires.

I know nothing about Forscan.

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