Does a broken speed sensor affect MPG?

[Jay_G]

Basically, iv got a ph2 172. Speed sensor is in its way out as speedo is constantly dropping out on trips, and doesn't work for the whole trip.

My question is what problems would that cause? Asking as 40 quid saw me about 150 miles if that.
Cheers

 

[DannyR]

It would only affect if your going faster than you believe, hence using more fuel.

But no, sounds like you have two problems.

 

[Jay_G]

It would only affect if your going faster than you believe, hence using more fuel.

But no, sounds like you have two problems.

Well on the motorway, averaging about 60-70 mph, iv done 60 miles and used about quarter of a tank of vpower.

 

[DannyR]

Definitely another problem then.

Lambda or coolant sensor

 

[Dan@SJM]

Yes it would.

If your speedo is cutting out. It doesn't see that you are covering distance but you are using fuel. Thus less miles per gallon.

 

[DannyR]

That wouldn't actually mean lower MPG, only a lower figure. Fuel gauge would still read what was in the tank.

 

[Jay_G]

Quick question. Will a ph1 speedo sensor fit my ph2? Only ask as I know the boxes are different.

Cheers

 

[DannyR]

Yes, as long as you definitely have a gearbox speedo sensor model and not an ABS speedo driven model.

If you don't have CC and ESP you have a gearbox speed sensor.

 

[Dan@SJM]

150 miles of his speedo working though..

 

[DannyR]

Yes, so it looks like less but in theory it's actually not, it's just not recording all the miles being driven.

 

[aucky]

Its only the trip computer that reads low mpg.

 

[Jay_G]

Well on the way home (same trip as above, 120 miles = half a tank), I averaged at about 70mph (on way there it was 60), and even had a stint of faster speeds, and I used quarter of tank the whole journey. It was about 70 miles before it dropped half a bar. Strange car!!

 

[Rob]

Speedo not working will have 0 affect on actual petrol used, like none.

You've got another issue there somewhere!

 

[Dan@SJM]

What I'm saying is, if he is manually or even using the mpg read out, it wont be correct. as the car says he has covered 30 miles to 1 gallon. but, because the speedo has not been reading correctly, it will have covered say 40 miles to that 1 gallon. the MPG readout will say the 30 instead of 40.

get it fixed and see if there is abnormal fuel useage.

 

[Jay_G]

MPG read out says something like 3 ATM. Iv completely ignored that, iv just worked out mileage from satnav (120 mile trip, 60 miles left to go, iv done 60 miles, used quarter of tank in that time)

Speedo does not work at all

 

[Dan519]

Speedo not working will have 0 affect on actual petrol used, like none.

You've got another issue there somewhere!

it can actually cause havoc with a couple of ECU versions. Get all sorts of misfires and poor running, which effects economy a lot

 

[Rob]

Must differ a lot if it's gearbox or ABS fed then?

My ph1 was gearbox fed and didn't affect it in the slightest.

 

[Dan519]

seems to be mostly early ph2's that are effected

 

[Jay_G]

Interesting Dan.

Also, iv noticed a few times when the speedo came back on while driving. No throttle difference, but if cruising, I could feel the engine "tighten up" (poor use of word tbh) when it came on, I knew it wasn't me as I felt it and looked down and speedo was back on. A few times that's happened.

Wonder if it has anything to do withtp the fact it won't rev past 5k sometimes...


It is definetly gearbox driven tho, Ant had a look

 

[KFRRS94]

What I'm saying is, if he is manually or even using the mpg read out, it wont be correct. as the car says he has covered 30 miles to 1 gallon. but, because the speedo has not been reading correctly, it will have covered say 40 miles to that 1 gallon. the MPG readout will say the 30 instead of 40.

get it fixed and see if there is abnormal fuel useage.

Yeah what your saying is correct, but OP is saying the pertol needle itself is dropping quicker. Not the MPG read out. The sensor in the tank would still show the right amount of fuel because it is there. So the high fuel consumption must be a result of another fault.

Edit: but as other Dan has pointed out it can cause ECU issues then yeah. Could be right there mate

 

[Chip-mk1]

Our early phase2 172 with a speed sensor was unaffected when it had a faulty speed sender. Neither was our ph1 172. I can't see why it would effect it given it has no impact on timing or fuelling (on ours anyway) and those are what effects economy.
Trying to work out on the fuel gauge is a bad plan though. Go by what you fill at the pump not try and guess what is a quarter tank off the gauge. It's not accurate enough for that.

 

[Jay_G]

Trying to work out on the fuel gauge is a bad plan though. Go by what you fill at the pump not try and guess what is a quarter tank off the gauge. It's not accurate enough for that.

I have noticed this a lot lately. If I park it on a certain place of my road, with a quarter in the tank, when I get in it in the morning, the lights on, and generally won't go back up until I start it again.

I was thinking that speed isn't used on the ignition maps, which is why I created this thread.

Seems to be behaving it self again now. The last quarter of the tank is lasting an age, and iv done loads of town driving.

Anyway, where is the best place to get a speed sensor?

 

[DannyR]

Renault

 

[Chip-mk1]

TBH the 13 quid on ebay ones work fine if you want a cheap and easy fix, Ive used them before and had no problems, its not like its a safety critical compenent so the sort of place you can risk it if you want.