markman34 Posted April 10, 2011 Share Posted April 10, 2011 (edited) Hello all, I just had new tires installed on my 88 gl wagon and I think my speedo is whacked. I used to have 175 70's on there and replaced them with the spec size tire 185 70's on 13" rims. I expected to have a more accurate measure of my speed with the spec sized tires on there but after a couple hundred km's on the highway today, I think the speedo is out even more than before. I was reading way over the limit(20-30km) but was getting passed like a pylon all day. Along with quite a few stink eyes! I think the speedo is reading WAY faster than actual speed. Even with the larger tires????? Am I out to lunch on the size? I read it off the door so thought I had it right. Any help? How can I figure this out. Might I have a altered speedo great ratio? The car only has to last the rest of the summer then im going to part it out. The body is falling off.....I don't want the parts going to the wrecker. I'll have a couple of EA82's a dual range 4x4 tranny and LSD dif. Any help with the sped question? Takers on the parts? Thx. Edited April 10, 2011 by markman34 Quote Link to comment Share on other sites More sharing options...
Dallasooby Posted April 10, 2011 Share Posted April 10, 2011 if you change the tire size it will change the speedo... for sure. There probably is no computer on the car so you would have to change the gears in the speedo head or something... not sure how to do that exactly... lol Quote Link to comment Share on other sites More sharing options...
Dallasooby Posted April 10, 2011 Share Posted April 10, 2011 The speedo on my car (92 Loyale wagon) works on a speed sensor, computer controlled) not sure if it is scan tool adjustable or not... the 88 was cable controlled i think Quote Link to comment Share on other sites More sharing options...
Dallasooby Posted April 10, 2011 Share Posted April 10, 2011 If i was closer i would probably be intersested in the motor and tranny:banana: Quote Link to comment Share on other sites More sharing options...
edrach Posted April 10, 2011 Share Posted April 10, 2011 (edited) Hello all, I just had new tires installed on my 88 gl wagon and I think my speedo is whacked. I used to have 175 70's on there and replaced them with the spec size tire 185 70's on 13" rims. I expected to have a more accurate measure of my speed with the spec sized tires on there but after a couple hundred km's on the highway today, I think the speedo is out even more than before. I was reading way over the limit(20-30km) but was getting passed like a pylon all day. Along with quite a few stink eyes! I think the speedo is reading WAY faster than actual speed. Even with the larger tires????? Am I out to lunch on the size? I read it off the door so thought I had it right. Any help? How can I figure this out. Might I have a altered speedo great ratio? The car only has to last the rest of the summer then im going to part it out. The body is falling off.....I don't want the parts going to the wrecker. I'll have a couple of EA82's a dual range 4x4 tranny and LSD dif. Any help with the sped question? Takers on the parts? Thx. Firstly, try this speed/tire size calculator: http://www.miata.net/garage/tirecalc.html . I suspect there shouldn't be much difference between the two tire sizes you have. By the way, your tires are wider; not necessarily larger diameter, at least not enough to make much of a difference. Usually the reason for a pretty large difference in speed reading like you have is that speedometer cable grease has migrated up the cable and into the speedometer head and causes the magnet to drag the speedo needle to a much higher speed than actual. Your odometer will still read correctly which will mis-lead you into thinking that the speedometer is correct and you're going faster than you really are. There's a seal in the transmission's cable drive which is supposed to keep the gear oil where it belongs; when the seal fails the gearoil migrates up the cable and causes this condition. Replacing the cable won't help since the oil/grease is already inside the speedometer head. Only correct fix is to replace the speedometer section, cable and install a new seal at the transmission to prevent that in the future. What to do? Hopefully, you have a tach in your instrument cluster. Have a friend follow you in another car and relay what speed you're actually travelling and write down the rpm and gear for the common speeds you normally travel at. Generate a table that tells you what speed you're actually travelling at what rpm in each gear. Speeds at the normal speed limits in each gear is usually all you need. Write the rpm for each speed on an index card and use that and your tach reading as a guide to how fast you're actually travelling. By the way, your LSD rear diff should sell for around $250 here on the Board. Please don't sell it to a Datsun fanatic who will offer you more. Please keep it in the Subaru family. Edited April 10, 2011 by edrach Quote Link to comment Share on other sites More sharing options...
markman34 Posted April 10, 2011 Author Share Posted April 10, 2011 Thanks for the replies guys! I'M very impressed with the explanation of the speedo cable and lube contamination. Never heard of that one before nor would I have ever thought of it myself! As for the friend helping me with calculating my speed, don't get me started. Lol. I had my wife helping me with that one. She just didn't get idea god bless her. I asked her to drive exactly the speed limit on a variety of roads during our drive home so that I could record the speeds and revs accordingly. After an hour drive home I was no closer to understanding my status. That's when I got on here. I did know however that the difference in circumfrance would be minimal though closer to the actual tire/speedo ratio. It just seems so much worse with the rubber. I do love the explanation though. Thanks for the help again. Over the years I've really gotten so much help with my soobies. I really do appreciate it. Mark Quote Link to comment Share on other sites More sharing options...
keltik Posted April 10, 2011 Share Posted April 10, 2011 (edited) 185/70 will have a larger diameter than 175/70 as the second number denotes tire profile as a percentage of width. Thus 70% of 185mm > 70% of 175mm. So following this, your new tires should read a slower speed than the old ones as they dont need to spin as fast to travel the same distance. Also, different tire manufacturers have different standards for measuring tread width. So one manufactuers 185 might be closer to 195mm wide and therefore have a different profile again. If your speedo is electronic - there are home build kits available which let you calibrate the speedo by wiring it in line with the sensor. I currently use one of these in my Legacy to compensate for using completely the wrong transmission. One theory to solve your problem: I have heard of oil/grease/lube of whatever sort seeping out of the top of the speedo cable and gumming up the internals of the cluster. This just gets progressively worse until your speedo reads waaaaay higher than what it should. Could be worth popping the cable out of the back of the cluster and making sure that hasnt happened. Edit: Crap just re-read edrachs post and it seems i wasnt very original with my solution Edited April 10, 2011 by keltik Quote Link to comment Share on other sites More sharing options...
edrach Posted April 10, 2011 Share Posted April 10, 2011 Edit: Crap just re-read edrachs post and it seems i wasnt very original with my solution Originality doesn't matter. It's the thought that counts. I just happened to post before you did. It's not a common problem but I've had it happen twice to me over the last 15 years and it's very memorable; especially after changing the speedometer and having the problem come back after a couple of weeks. Fortunately, pull a part had plenty of spares at the time for real cheap. Quote Link to comment Share on other sites More sharing options...
MilesFox Posted April 10, 2011 Share Posted April 10, 2011 The speedo on my car (92 Loyale wagon) works on a speed sensor, computer controlled) not sure if it is scan tool adjustable or not... the 88 was cable controlled i think your 92 loyale works the same as any other GL, cable operated for the speedo, although there is a VSS in the speedo head for the cruise and ecu, but not for the speedo itself, purely mechanical 185-70 should be the proper size tire for a 5mt 4wd. Quote Link to comment Share on other sites More sharing options...
markman34 Posted April 11, 2011 Author Share Posted April 11, 2011 Thanks for the reply. So, if it's cable driven the whole way through the oil contamination theory doesn't explain it. Now I'm even more confused. The ratio should be maintained at all times. Maybe the speedo has been corrected in the past by a previous owner. More to think of..... Quote Link to comment Share on other sites More sharing options...
dennyt Posted April 11, 2011 Share Posted April 11, 2011 Use the odometer and mileposts on the highway to check your calibration. Reset your trip-meter (or remember the number down to the tenth) at your first milepost, then check it again 10 miles later. If you're off by 0.1 miles in 10, that's 1%. My '94 Loyale reads 3% high with 185-70/13's. Quote Link to comment Share on other sites More sharing options...
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