Calculating Ratios to the Dash

[Shackleton]

Guys I'm trying to figure out what my speedo is out by. I'm running original RRC 16" wheels with standard spec tyres. 205/16/ are they 70?, early RRC axles (no idea of the ratio), '01 Defender R380 (it has 1.41 or 1.14 or something with ones 1's and 4's written on it and a '91 RRC electronic dash.

I was behind a friend who reckons they were showing 60kph when mine was showing 70kph.

Basically I'm trying to figure what kinda mpg I'm getting. The speedo seems to be reading 15 / 16% over then the trip counter is doing likewise.

She's showing 521 miles ona tank so far and hasn't run out yet, maybe another 20 / 30 miles in it. Thats about 440 if you subtract 16%.

Anyone good at this kinda thing?

George

[western]

all 90/110/Disco/RRC axles are 3.54:1 ratio

Transfer box should be 1.410:1 high range standard for 110/90.

tyres IIRC 205's are 70% aspect ratio, the speedo drive gear in the transfer box should be frc3312 Yellow ------ 22 teeth or on a 205 tyred RRC it's Red 24 Teeth. fairly easy to swap over to see which gives the most accurate reading.

[steve_d]

How many different ratio/colours are there?

I will have to go down this route having fitted 33" to my otherwise standard RRC auto running gear.

Thanks

Steve

[DaveSIIA]

Have a read through this thread. It covers all the bases.

[Orgasmic Farmer]

Standard tyres are 205/16 in 80% profile (they just never write the 80 on for some reason.) There are some useful online calculators that allow you to work out the speed changes if you change tyres sizes. Try

http://www.alloywheels.com/TyreCalc.asp

or

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

Best answer though is to get a TomTom sat nav and set it to show speed. I know that running standard tyres mine is showing 64 on the dash at a true 60 mph.

[Corgie Carrier]

Standard tyres are 205/16 in 80% profile (they just never write the 80 on for some reason.) There are some useful online calculators that allow you to work out the speed changes if you change tyres sizes. Try

http://www.alloywheels.com/TyreCalc.asp

or

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

Best answer though is to get a TomTom sat nav and set it to show speed. I know that running standard tyres mine is showing 64 on the dash at a true 60 mph.

Does anyone know of one of these converters that uses imperial sizes.

I have upgrades from 225/75 x 16 to 35 x 12.5 x 15 and have no idea what speed I am doing.

[Orgasmic Farmer]

The second one linked allows you to choose between imp or metric.

[headhunter]

Guys I'm trying to figure out what my speedo is out by. I'm running original RRC 16" wheels with standard spec tyres. 205/16/ are they 70?, early RRC axles (no idea of the ratio), '01 Defender R380 (it has 1.41 or 1.14 or something with ones 1's and 4's written on it and a '91 RRC electronic dash.

I was behind a friend who reckons they were showing 60kph when mine was showing 70kph.

Basically I'm trying to figure what kinda mpg I'm getting. The speedo seems to be reading 15 / 16% over then the trip counter is doing likewise.

She's showing 521 miles ona tank so far and hasn't run out yet, maybe another 20 / 30 miles in it. Thats about 440 if you subtract 16%.

Anyone good at this kinda thing?

George

My 110 on 265/75/16 AT's doing 70 mph on clock registers 65 mph on Tom Tom a 7.5% over reading.

However the mileometer vs Tom Tom under reads distance by 4.5-5 miles per 100 miles averaging 60 mph.

Very confusing when they deviate in the opposite direction, especially when one is trying to calculate accurate mpg!

John

[western]

How many different ratio/colours are there?

I will have to go down this route having fitted 33" to my otherwise standard RRC auto running gear.

Thanks

Steve

speedo drive gear list

[steve_d]

The second one linked allows you to choose between imp or metric.

Clicking 'standard' or 'metric just changes the readout from inches to mm.

There is nowhere you can select a tyre such as 33x12.5x15.

Will have a Google, there must be an American site that does it.

Steve

[steve_d]

Did not find another calculator to do the metric/imperial switch.

Had a play with the second calc above.

You can put in your existing metric size but with it set to standard the diameter etc. readings are in inches.

You can then play with the inputs of the 'new' tyre until you reach your required diameter in inches.

In my case I used 305 75 15 to force a diameter of 33inches dead.

It says my speedo will be 14.2% slow.

EPC says my RRC has a 24 tooth drive gear so I imagine I will need to move at least two sizes but I have no idea how to calculate it.

Have to go find some gears to try.

Steve

[Shackleton]

Holy carp

Was hoping to be spoon fed there for a minute

It finally ran out of juice on the way here (net cafe) and read 571 miles from the tank. Reckon the way I mounted the tank it's not quite level front to back so she runs out when there's still at least a few litres in there.

Anyway if my rough calcs are right and the speedo is reading 16 odd % over then that makes the 571 more like 475 miles and about 27mpg. Hoped for more then that (not much) but for the meantime and in lieu of a Tom Tom I'll do some tests side by side with the folks Merc and see what I come up with.

Oh and check all those lovely links you guys posted

G

EDIt: I should just give up - just remembered I swapped the speedo drive out of the outgoing Disco box onto the Defender box too

[Night Train]

I only saw this briefly the other day and thought I'd sort out a spread sheet for you. I then found the pages and pages of calculations I did in the past calculating overall ratios long hand my the old fashioned pencil and paper route and got lost looking at all my old designs and plans. I shall check out the links myself and save doing the work.

Oh well, back to marking school work.

[Shackleton]

SCORE! I'd sure appreciate it (I sounded all cowboy there)

You just reminded me I said I check on some wire colours for the air sensor in the air box of a Td5 - off to report my findings....